DataLink Layer 1

Topic 2Data Link LayerPart C

The majority of the slides in this course are adapted from the accompanying slides to the books by Larry Peterson and Bruce Davie and by Jim Kurose and Keith Ross Additional slides andor figures from other sources and from Vasos Vassiliou are also included in this presentation

DataLink Layer 2

Media Access Control Where

Centralbull Greater controlbull Simple access logic at stationbull Avoids problems of co-ordinationbull Single point of failurebull Potential bottleneck

Distributed How

Synchronousbull Specific capacity dedicated to connection

Asynchronousbull In response to demand

DataLink Layer 3

Asynchronous Systems Round robin

Good if many stations have data to transmit over extended period

Reservation Good for stream traffic

Contention Good for bursty traffic All stations contend for time Distributed Simple to implement Efficient under moderate load Tend to collapse under heavy load

DataLink Layer 4

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point

PPP for dial-up access point-to-point link between Ethernet switch and host

broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN

DataLink Layer 5

Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

interference collision if node receives two or more signals at the same

timemultiple access protocol distributed algorithm that determines how nodes

share channel ie determine when node can transmit

communication about channel sharing must use channel itself no out-of-band channel for coordination

DataLink Layer 6

Ideal Mulitple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM3 Fully decentralized

no special node to coordinate transmissions no synchronization of clocks slots

4 Simple

DataLink Layer 7

MAC Protocols a taxonomyThree broad classes Channel Partitioning

divide channel into smaller ldquopiecesrdquo (time slots frequency code)

allocate piece to node for exclusive use Random Access

channel not divided allow collisions ldquorecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can

take longer turns

DataLink Layer 8

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

DataLink Layer 9

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

frequ

ency

ban

ds

time

DataLink Layer 10

Random Access Protocols When node has packet to send

transmit at full channel data rate R no a priori coordination among nodes

two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

how to detect collisions how to recover from collisions (eg via delayed

retransmissions) Examples of random access MAC protocols

slotted ALOHA ALOHA CSMA CSMACD CSMACA

DataLink Layer 11

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot all nodes detect collision

Operation when node obtains fresh

frame it transmits in next slot

no collision node can send new frame in next slot

if collision node retransmits frame in each subsequent slot with prob p until success

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 2

Media Access Control Where

Centralbull Greater controlbull Simple access logic at stationbull Avoids problems of co-ordinationbull Single point of failurebull Potential bottleneck

Distributed How

Synchronousbull Specific capacity dedicated to connection

Asynchronousbull In response to demand

DataLink Layer 3

Asynchronous Systems Round robin

Good if many stations have data to transmit over extended period

Reservation Good for stream traffic

Contention Good for bursty traffic All stations contend for time Distributed Simple to implement Efficient under moderate load Tend to collapse under heavy load

DataLink Layer 4

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point

PPP for dial-up access point-to-point link between Ethernet switch and host

broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN

DataLink Layer 5

Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

interference collision if node receives two or more signals at the same

timemultiple access protocol distributed algorithm that determines how nodes

share channel ie determine when node can transmit

communication about channel sharing must use channel itself no out-of-band channel for coordination

DataLink Layer 6

Ideal Mulitple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM3 Fully decentralized

no special node to coordinate transmissions no synchronization of clocks slots

4 Simple

DataLink Layer 7

MAC Protocols a taxonomyThree broad classes Channel Partitioning

divide channel into smaller ldquopiecesrdquo (time slots frequency code)

allocate piece to node for exclusive use Random Access

channel not divided allow collisions ldquorecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can

take longer turns

DataLink Layer 8

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

DataLink Layer 9

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

frequ

ency

ban

ds

time

DataLink Layer 10

Random Access Protocols When node has packet to send

transmit at full channel data rate R no a priori coordination among nodes

two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

how to detect collisions how to recover from collisions (eg via delayed

retransmissions) Examples of random access MAC protocols

slotted ALOHA ALOHA CSMA CSMACD CSMACA

DataLink Layer 11

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot all nodes detect collision

Operation when node obtains fresh

frame it transmits in next slot

no collision node can send new frame in next slot

if collision node retransmits frame in each subsequent slot with prob p until success

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 3

Asynchronous Systems Round robin

Good if many stations have data to transmit over extended period

Reservation Good for stream traffic

Contention Good for bursty traffic All stations contend for time Distributed Simple to implement Efficient under moderate load Tend to collapse under heavy load

DataLink Layer 4

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point

PPP for dial-up access point-to-point link between Ethernet switch and host

broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN

DataLink Layer 5

Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

interference collision if node receives two or more signals at the same

timemultiple access protocol distributed algorithm that determines how nodes

share channel ie determine when node can transmit

communication about channel sharing must use channel itself no out-of-band channel for coordination

DataLink Layer 6

Ideal Mulitple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM3 Fully decentralized

no special node to coordinate transmissions no synchronization of clocks slots

4 Simple

DataLink Layer 7

MAC Protocols a taxonomyThree broad classes Channel Partitioning

divide channel into smaller ldquopiecesrdquo (time slots frequency code)

allocate piece to node for exclusive use Random Access

channel not divided allow collisions ldquorecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can

take longer turns

DataLink Layer 8

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

DataLink Layer 9

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

frequ

ency

ban

ds

time

DataLink Layer 10

Random Access Protocols When node has packet to send

transmit at full channel data rate R no a priori coordination among nodes

two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

how to detect collisions how to recover from collisions (eg via delayed

retransmissions) Examples of random access MAC protocols

slotted ALOHA ALOHA CSMA CSMACD CSMACA

DataLink Layer 11

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot all nodes detect collision

Operation when node obtains fresh

frame it transmits in next slot

no collision node can send new frame in next slot

if collision node retransmits frame in each subsequent slot with prob p until success

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 4

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquo point-to-point

PPP for dial-up access point-to-point link between Ethernet switch and host

broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN

DataLink Layer 5

Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

interference collision if node receives two or more signals at the same

timemultiple access protocol distributed algorithm that determines how nodes

share channel ie determine when node can transmit

communication about channel sharing must use channel itself no out-of-band channel for coordination

DataLink Layer 6

Ideal Mulitple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM3 Fully decentralized

no special node to coordinate transmissions no synchronization of clocks slots

4 Simple

DataLink Layer 7

MAC Protocols a taxonomyThree broad classes Channel Partitioning

divide channel into smaller ldquopiecesrdquo (time slots frequency code)

allocate piece to node for exclusive use Random Access

channel not divided allow collisions ldquorecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can

take longer turns

DataLink Layer 8

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

DataLink Layer 9

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

frequ

ency

ban

ds

time

DataLink Layer 10

Random Access Protocols When node has packet to send

transmit at full channel data rate R no a priori coordination among nodes

two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

how to detect collisions how to recover from collisions (eg via delayed

retransmissions) Examples of random access MAC protocols

slotted ALOHA ALOHA CSMA CSMACD CSMACA

DataLink Layer 11

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot all nodes detect collision

Operation when node obtains fresh

frame it transmits in next slot

no collision node can send new frame in next slot

if collision node retransmits frame in each subsequent slot with prob p until success

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 5

Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes

interference collision if node receives two or more signals at the same

timemultiple access protocol distributed algorithm that determines how nodes

share channel ie determine when node can transmit

communication about channel sharing must use channel itself no out-of-band channel for coordination

DataLink Layer 6

Ideal Mulitple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM3 Fully decentralized

no special node to coordinate transmissions no synchronization of clocks slots

4 Simple

DataLink Layer 7

MAC Protocols a taxonomyThree broad classes Channel Partitioning

divide channel into smaller ldquopiecesrdquo (time slots frequency code)

allocate piece to node for exclusive use Random Access

channel not divided allow collisions ldquorecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can

take longer turns

DataLink Layer 8

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

DataLink Layer 9

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

frequ

ency

ban

ds

time

DataLink Layer 10

Random Access Protocols When node has packet to send

transmit at full channel data rate R no a priori coordination among nodes

two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

how to detect collisions how to recover from collisions (eg via delayed

retransmissions) Examples of random access MAC protocols

slotted ALOHA ALOHA CSMA CSMACD CSMACA

DataLink Layer 11

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot all nodes detect collision

Operation when node obtains fresh

frame it transmits in next slot

no collision node can send new frame in next slot

if collision node retransmits frame in each subsequent slot with prob p until success

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 6

Ideal Mulitple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM3 Fully decentralized

no special node to coordinate transmissions no synchronization of clocks slots

4 Simple

DataLink Layer 7

MAC Protocols a taxonomyThree broad classes Channel Partitioning

divide channel into smaller ldquopiecesrdquo (time slots frequency code)

allocate piece to node for exclusive use Random Access

channel not divided allow collisions ldquorecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can

take longer turns

DataLink Layer 8

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

DataLink Layer 9

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

frequ

ency

ban

ds

time

DataLink Layer 10

Random Access Protocols When node has packet to send

transmit at full channel data rate R no a priori coordination among nodes

two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

how to detect collisions how to recover from collisions (eg via delayed

retransmissions) Examples of random access MAC protocols

slotted ALOHA ALOHA CSMA CSMACD CSMACA

DataLink Layer 11

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot all nodes detect collision

Operation when node obtains fresh

frame it transmits in next slot

no collision node can send new frame in next slot

if collision node retransmits frame in each subsequent slot with prob p until success

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 7

MAC Protocols a taxonomyThree broad classes Channel Partitioning

divide channel into smaller ldquopiecesrdquo (time slots frequency code)

allocate piece to node for exclusive use Random Access

channel not divided allow collisions ldquorecoverrdquo from collisions

ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can

take longer turns

DataLink Layer 8

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

DataLink Layer 9

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

frequ

ency

ban

ds

time

DataLink Layer 10

Random Access Protocols When node has packet to send

transmit at full channel data rate R no a priori coordination among nodes

two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

how to detect collisions how to recover from collisions (eg via delayed

retransmissions) Examples of random access MAC protocols

slotted ALOHA ALOHA CSMA CSMACD CSMACA

DataLink Layer 11

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot all nodes detect collision

Operation when node obtains fresh

frame it transmits in next slot

no collision node can send new frame in next slot

if collision node retransmits frame in each subsequent slot with prob p until success

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 8

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

DataLink Layer 9

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

frequ

ency

ban

ds

time

DataLink Layer 10

Random Access Protocols When node has packet to send

transmit at full channel data rate R no a priori coordination among nodes

two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

how to detect collisions how to recover from collisions (eg via delayed

retransmissions) Examples of random access MAC protocols

slotted ALOHA ALOHA CSMA CSMACD CSMACA

DataLink Layer 11

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot all nodes detect collision

Operation when node obtains fresh

frame it transmits in next slot

no collision node can send new frame in next slot

if collision node retransmits frame in each subsequent slot with prob p until success

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 9

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle

TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load

FDM (Frequency Division Multiplexing) frequency subdivided

frequ

ency

ban

ds

time

DataLink Layer 10

Random Access Protocols When node has packet to send

transmit at full channel data rate R no a priori coordination among nodes

two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

how to detect collisions how to recover from collisions (eg via delayed

retransmissions) Examples of random access MAC protocols

slotted ALOHA ALOHA CSMA CSMACD CSMACA

DataLink Layer 11

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot all nodes detect collision

Operation when node obtains fresh

frame it transmits in next slot

no collision node can send new frame in next slot

if collision node retransmits frame in each subsequent slot with prob p until success

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 10

Random Access Protocols When node has packet to send

transmit at full channel data rate R no a priori coordination among nodes

two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies

how to detect collisions how to recover from collisions (eg via delayed

retransmissions) Examples of random access MAC protocols

slotted ALOHA ALOHA CSMA CSMACD CSMACA

DataLink Layer 11

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot all nodes detect collision

Operation when node obtains fresh

frame it transmits in next slot

no collision node can send new frame in next slot

if collision node retransmits frame in each subsequent slot with prob p until success

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 11

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot all nodes detect collision

Operation when node obtains fresh

frame it transmits in next slot

no collision node can send new frame in next slot

if collision node retransmits frame in each subsequent slot with prob p until success

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 12

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized only slots in nodes need to be in sync

simple

Cons collisions wasting

slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 13

Slotted Aloha efficiency

Suppose N nodes with many frames to send each transmits in slot with probability p

probability that node 1 has success in a slot = p(1-p)N-1

probability that any node has a success = Np(1-p)N-1

For max efficiency with N nodes find p that maximizes Np(1-p)N-1

For many nodes take limit of Np(1-p)N-1

as N goes to infinity gives 1e = 37

Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send

At best channelused for useful transmissions 37of time

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 15

Pure ALOHA (2)Vulnerable period for the shaded frame

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 16

Pure Aloha efficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18 Even worse

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 17

ALOHA EfficiencyThroughput versus offered traffic for

ALOHA systems

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 18

CSMA (Carrier Sense Multiple Access) Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If channel sensed busy defer transmission If channel sensed idle transmit entire frame

Wait reasonable time (round trip plus ACK contention) No ACK then retransmit

If two stations start at the same instant collision Max utilization depends on propagation time (medium length) and frame

length Longer frame and shorter propagation gives better utilization

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 19

CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

noterole of distance amp propagation delay in determining collision probability

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 20

Nonpersistent CSMA1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn

from probability distribution (retransmission delay) and repeat 1

Random delays reduces probability of collisions Consider two stations become ready to transmit at

same time bull While another transmission is in progress

If both stations delay same time before retrying both will attempt to transmit at same time

Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

Nonpersistent stations deferential

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 21

1-persistent CSMA To avoid idle channel time 1-persistent protocol

used Station wishing to transmit listens and obeys

following 1 If medium idle transmit otherwise go to step 22 If medium busy listen until idle then transmit

immediately 1-persistent stations selfish If two or more stations waiting collision

guaranteed Gets sorted out after collision

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 22

P-persistent CSMA Compromise that attempts to reduce collisions

Like nonpersistent And reduce idle time

Like1-persistent Rules1 If medium idle transmit with probability p and

delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay

2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat

step 1 What is an effective value of p

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 23

Value of p Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations attempting

to transmit is number of stations ready times probability of transmitting n x p

If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more

collisions Retries compete with new transmissions Eventually all stations trying to send

Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 24

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various

random access protocols

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 25

Which Persistence Algorithm IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have

performance problems 1-persistent (p = 1) seems more unstable

than p-persistent Greed of the stations But wasted time due to collisions is short (if

frames long relative to propagation delay With random backoff unlikely to collide on next

tries To ensure backoff maintains stability IEEE 8023

and Ethernet use binary exponential backoff

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 26

CSMACD (Collision Detection) With CSMA collision occupies medium

for duration of transmission Stations listen whilst transmitting

1 If medium idle transmit otherwise step 2

2 If busy listen for idle then transmit3 If collision detected jam then cease

transmission4 After jam wait random time then start

from step 1

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 27

CSMACDCSMACD carrier sensing deferral as in CSMA

collisions detected within short time colliding transmissions aborted reducing channel

wastage collision detection

easy in wired LANs measure signal strengths compare transmitted received signals

difficult in wireless LANs receiver shut off while transmitting

human analogy the polite conversationalist

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 28

CSMACD collision detection

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 29

CSMACD

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 30

CSMACDOperation

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 31

ldquoTaking Turnsrdquo MAC protocolsPolling master node

ldquoinvitesrdquo slave nodes to transmit in turn

concerns polling overhead latency single point of

failure (master)

Token passing control token passed

from one node to next sequentially

token message concerns

token overhead latency single point of failure

(token)

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 32

Summary of MAC protocols What do you do with a shared media

Channel Partitioning by time frequency or code

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211

Taking Turnsbull polling from a central site token passing

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 33

Summary of MAC protocolsChannel partitioning MAC protocols

share channel efficiently and fairly at high load inefficient at low load delay in channel access

1N bandwidth allocated even if only 1 active node

Random access MAC protocols efficient at low load single node can fully

utilize channel high load collision overhead

ldquotaking turnsrdquo protocolslook for best of both worlds

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 34

Ethernet Overview History

developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox DEC and Intel in

1978 similar to IEEE 8023 standard

Metcalfersquos Ethernetsketch

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 35

Ethernetldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 36

Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)

hub orswitch

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 37

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock rates

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 38

Ethernet Frame Structure (more) Addresses 6 bytes

if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame Type indicates the higher layer protocol

(mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

Destaddr

64 48 32CRCPreamble Src

addr Type Body1648

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 39

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps gaps will be filled if app is using TCP otherwise app will see the gaps

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 40

Ethernet uses CSMACD No slots adapter doesnrsquot

transmit if it senses that some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 41

Ethernet CSMACD algorithm1 Adaptor receives datagram

from net layer amp creates frame

2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits

3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame

4 If adapter detects another transmission while transmitting aborts and sends jam signal

5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff Goal adapt retransmission

attempts to estimated current load heavy load random wait

will be longer first collision choose K

from 01 delay is K 512 bit transmission times

after second collision choose K from 0123hellip

after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 43

Collision Detection On baseband bus collision produces much

higher signal voltage than signal Collision detected if cable signal greater than

single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m

(10Base2) For twisted pair (star-topology) activity on

more than one port is collision Special collision presence signal

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 44

Algorithm (cont)

If collisionhellip jam for 32 bits then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes

of data) delay and try again

bull 1st time 0 or 512usbull 2nd time 0 512 or 1024usbull 3rd time512 1024 or 1536usbull nth time k x 512us for randomly selected

k=02n - 1bull give up after several tries (usually 16)bull exponential backoff

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 45

Binary Exponential Backoff Attempt to transmit repeatedly if repeated collisions First 10 attempts mean value of random delay doubled Value then remains same for 6 further attempts After 16 unsuccessful attempts station gives up and

reports error As congestion increases stations back off by larger

amounts to reduce the probability of collision 1-persistent algorithm with binary exponential backoff

efficient over wide range of loads Low loads 1-persistence guarantees station can

seize channel once idle High loads at least as stable as other techniques

Backoff algorithm gives last-in first-out effect Stations with few collisions transmit first

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 46

Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 47

Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with

512-bit slot times

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 48

CSMACD efficiency Tprop = max prop between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 511efficiency

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 49

Ethernet CablingThe most common kinds of Ethernet

cabling

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 50

Ethernet Cabling (2)Three kinds of Ethernet cabling

(a) 10Base5 (b) 10Base2 (c) 10Base-T

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 51

Ethernet Cabling (3)Cable topologies (a) Linear (b) Spine (c)

Tree (d) Segmented

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 52

10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100

m max distance between nodes and hub

twisted pair

hub

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 53

100Mbps Fast Ethernet Use IEEE 8023 MAC protocol and frame format 100BASE-X use physical medium specifications

from FDDI Two physical links between nodes

bull Transmission and reception 100BASE-TX uses STP or Cat 5 UTP

bull May require new cable 100BASE-FX uses optical fiber 100BASE-T4 can use Cat 3 voice-grade UTP

bull Uses four twisted-pair lines between nodesbull Data transmission uses three pairs in one direction at a

time Star-wire topology

Similar to 10BASE-T

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 54

Fast EthernetThe original fast Ethernet cabling

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 55

Gigabit EthernetGigabit Ethernet cabling

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 56

Ethernet (cont) Addresses

unique 48-bit unicast address assigned to each adapter example 80e4b12 broadcast all 1s multicast first bit is 1

Bandwidth 10Mbps 100Mbps 1Gbps Length 2500m (500m segments with 4 repeaters) Problem Distributed algorithm that provides fair

access

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 57

Wireless Link CharacteristicsDifferences from wired link hellip

decreased signal strength radio signal attenuates as it propagates through matter (path loss)

interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 58

Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problem B A hear each other B C hear each other A C can not hear each othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal fading B A hear each other B C hear each other A C can not hear each other

interferring at B

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 59

IEEE 80211 Wireless LAN 80211b

24-5 GHz unlicensed radio spectrum

up to 11 Mbps direct sequence

spread spectrum (DSSS) in physical layer

bull all hosts use same chipping code

widely deployed using base stations

80211a 5-6 GHz range up to 54 Mbps

80211g 24-5 GHz range up to 54 Mbps

All use CSMACA for multiple access

All have base-station and ad-hoc network versions

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 60

Figure 3-12ISM bands

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 61

80211 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

(aka ldquocellrdquo) in infrastructure mode contains wireless hosts access point (AP) base

station ad hoc mode hosts only

BSS 1

BSS 2

Internet

hub switchor routerAP

AP

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 62

80211 Channels association 80211b 24GHz-2485GHz spectrum divided

into 11 channels at different frequencies AP admin chooses frequency for AP interference possible channel can be same as

that chosen by neighboring AP host must associate with an AP

scans channels listening for beacon frames containing APrsquos name (SSID) and MAC address

selects AP to associate with may perform authentication [Chapter 8] will typically run DHCP to get IP address in

APrsquos subnet

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 63

IEEE 80211 multiple access avoid collisions 2+ nodes transmitting at same

time 80211 CSMA - sense before transmitting

donrsquot collide with ongoing transmission by other node 80211 no collision detection

difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

canrsquot sense all collisions in any case hidden terminal fading

goal avoid collisions CSMAC(ollision)A(voidance)

AB

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 64

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 65

Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random

access of data frames avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets

to BS using CSMA RTSs may still collide with each other (but theyrsquore short)

BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

Avoid data frame collisions completely using small reservation packets

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 66

Collision Avoidance RTS-CTS exchange

APA B

time

RTS(A) RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards

DataLink Layer 67

IEEE 802 Standards

The 802 working groups The important ones are marked with The ones marked with are hibernating The one marked with dagger gave up

• Slide 1
• Media Access Control
• Asynchronous Systems
• Multiple Access Links and Protocols
• Multiple Access protocols
• Ideal Mulitple Access Protocol
• MAC Protocols a taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slide 12
• Slotted Aloha efficiency
• Pure ALOHA (2)
• Pure Aloha efficiency
• ALOHA Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA collisions
• Nonpersistent CSMA
• 1-persistent CSMA
• P-persistent CSMA
• Value of p
• Persistent and Nonpersistent CSMA
• Which Persistence Algorithm
• CSMACD (Collision Detection)
• CSMACD
• CSMACD collision detection
• Slide 29
• CSMACD Operation
• ldquoTaking Turnsrdquo MAC protocols
• Summary of MAC protocols
• Summary of MAC protocols
• Ethernet Overview
• Ethernet
• Star topology
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Unreliable connectionless service
• Ethernet uses CSMACD
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• Collision Detection
• Algorithm (cont)
• Binary Exponential Backoff
• Ethernet MAC Sublayer Protocol (2)
• Ethernet Performance
• CSMACD efficiency
• Ethernet Cabling
• Ethernet Cabling (2)
• Ethernet Cabling (3)
• 10BaseT and 100BaseT
• 100Mbps Fast Ethernet
• Fast Ethernet
• Gigabit Ethernet
• Ethernet (cont)
• Wireless Link Characteristics
• Wireless network characteristics
• IEEE 80211 Wireless LAN
• Slide 60
• 80211 LAN architecture
• 80211 Channels association
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Avoiding collisions (more)
• Collision Avoidance RTS-CTS exchange
• IEEE 802 Standards