04/20/23

Chapter 4

20 % syllabus

Medium Access Sublayer

2

04/20/23

One problem with Broadcast Networks

• Who Goes Next

• The central theme of this chapter is – “how to allocate a single broadcast channel

among competing users “

• To General Schemes– Static– Dynamic

3

04/20/23

Static Channel Allocation(FDM or TDM)

FDM

4

04/20/23

Static Channel Allocation

• If there are N users, the bandwidth is divided into N equal-sized portions

• Application Area:-– a small and constant number of users, – each of which has a heavy (buffered) load of

traffic

• Not Applicable:-– varying no of users – fixed users with low load

5

04/20/23

Static Channel Allocation

• T = Mean Time Delay

• 1/µ = bits/frame

• C = Channel Capacity in bps

• λ = No of Frames/Second

(Without Static Allocation)

6

04/20/23

(With Static Allocation)

Static Channel Allocation

7

04/20/23

Dynamic channel allocation

• Carrier Sense Multiple Access Protocols (CSMA)

• Wireless LAN Protocols

8

04/20/23

Multiple Access Protocols

Different types:

• Contention protocols resolve a collision after it occurs. These protocols execute a collision resolution protocol after each collision

• Collision-free protocols (e.g., a bit-map protocol and binary countdown) ensure that a collision can never occur.

9

04/20/23

Nonpersistent/x-persistent CSMA Protocols

• Nonpersistent CSMA Protocol: Step 1: If the medium is idle, transmit immediately Step 2: If the medium is busy, wait a random amount of time and

repeat Step 1– Waste idle time if the backoff time is too long

• 1-persistent CSMA Protocol: Step 1: If the medium is idle, transmit immediately

- the host transmits with probability of 1 Step 2: If the medium is busy, continue to listen until medium

becomes idle, and then transmit immediately– There will always be a collision if two nodes want to retransmit

Human analogy: don’t interrupt others!

10

04/20/23

Nonpersistent/x-persistent CSMA Protocols

• p-persistent CSMA Protocol: Step 1: If the medium is idle, transmit with probability p

In case the transmission did not happen (the probability of this event is 1-p) the sender waits until the next available time slot and transmits again with the same probability p. Step 2: If the medium is busy, continue to listen until medium becomes idle, then go to Step 1– A good tradeoff between nonpersistent and 1-persistent CSMA

– Assume channels are slotted– One slot = contention period (i.e., one round trip propagation delay)

11

04/20/23

CSMA/CD (CSMA with Collision Detection)

• In CSMA, if 2 terminals begin sending packet at the same time, each will transmit its complete packet (although collision is taking place).

• Wasting medium for an entire packet time.

• CSMA/CD

Step 1: If the medium is idle, transmit

Step 2: If the medium is busy, continue to listen until the channel is idle then transmit

Step 3: If a collision is detected during transmission, cease transmitting

Step 4: Wait a random amount of time and repeats the same algorithm

12

04/20/23

CSMA with Collision Detection

• Stations abort their transmissions as soon as they detect a collision

13

04/20/23

Wireless LAN Protocols

• Hidden station problem. – A is transmitting to B– C wants to transmit it sense the medium and

falsely conclude that no one is transmitting

14

04/20/23

Wireless LAN Protocols

• Exposed station problem. – B is transmitting to A– C wants to transmit it sense the medium and

falsely conclude that I can’t send to D

15

04/20/23

MACA

• Multiple Access with Collision Avoidance

16

04/20/23

MACA

• A starts by sending an RTS (Request To Send) frame to B

• Size = 30 bytes

• Content = Length of data to be sent

• Then B replies with a CTS (Clear to Send) frame which also contains length of data

• A starts transmission

17

04/20/23

MACAW

• Multiple Access with Collision Avoidance for Wireless

• Improvements over MACA– Acknowledgement after each data frame– Carrier sensing before starting transmission

(i.e. before sending RTS frame to start transmission)

18

04/20/23

ETHERNET(In More Detail)

• Ethernet is a family of frame-based computer networking technologies for local area networks (LAN)

– LAN technology• Ethernet Cabling • 10 – 10Mbps• Base – Baseband

19

04/20/23

Transceiver and Hub

10Base5 10Base2 10BaseTTransceiver: Carrier detection and collision detectionController: transmits and receives frame, assembling of data and calculating checksums.

20

04/20/23

10Base5 10Base2

21

04/20/23

10BaseT

22

04/20/23

Time Domain Reflectometry • Deals with detecting cable breaks, or loose

connectors • A pulse of known shape is injected into the cable.• If the pulse hits an obstacle or the end of the

cable, an echo will be generated and sent back. • By carefully timing the interval between sending

the pulse and receiving the echo, it is possible to localize the origin of the echo.

• This technique is called time domain reflectometry.

23

04/20/23

Cable Topologies

A linear topology single cable is snaked from room to room, with

each station tapping into it at the nearest point

A spine (backbone) topology a vertical spine runs from the basement to the

roof, with horizontal cables on each floor connected to the spine by special amplifiers (repeaters).

In some buildings, the horizontal cables are thin and the backbone is thick

24

04/20/23

Cable Topologies

A tree topology A segmented topology with repeaters

To allow larger networks, multiple cables can be connected by repeaters

25

04/20/23

Topologies(The way in which building is wired)

26

04/20/23

Manchester Encoding and Differential ME

27

04/20/23

ME Encoding• Each bit period is divided into two equal intervals. • A binary 1 bit is sent by having the voltage set high

during the first interval and low in the second one.• A binary 0 is just the reverse: first low and then high. • This scheme ensures that every bit period has a transition

in the middle, • Advantage: making it easy for the receiver to synchronize

with the sender. • Disadvantage: it requires twice as much bandwidth as

straight binary encoding because the pulses are half the width

28

04/20/23

Differential Manchester Encoding

• A 1 bit is indicated by the absence of a transition at the start of the interval

• A 0 bit is indicated by the presence of a transition at the start of the interval.

• In both cases, there is a transition in the middle • Disadvantage : The differential scheme requires more

complex equipment • Advantage: offers better noise immunity. • All Ethernet systems use Manchester encoding due to its

simplicity. • The high signal is + 0.85 volts and the low signal is - 0.85

volts

29

04/20/23

Frame Formats

(a) DIX Ethernet. (b) IEEE 802.3.

30

04/20/23

Fields Description

• Preamble– Length=8 bytes

– each containing the bit pattern 10101010

– to allow the receiver's clock to synchronize with the sender's

• Address– Length=6 bytes

– Group addresses : High order bit = 1

– Multicast: Sending to group of stations

– Broadcast: Address consists of all 1 bit

31

04/20/23

Fields Description

• Type– tells the receiver what to do with the frame

– The Type field specifies which process to give the frame to.

• Data– up to 1500 bytes

– Minimum length must be 46 bytes

• Pad– To achieve to minimum length of frame

• Checksum

32

04/20/23

Fields Description

• Two changes in DIX frame by 802.3 frame– reduce the preamble to 7 bytes and use the last

byte for a Start of Frame delimiter – Type field into a Length field

33

04/20/23

The Binary Exponential Back off Algorithm

• If more than one stations try to acquire the channel at same time collision occurs

• After the first collision, each station waits either 0 or 1 slot times

• After the second collision, each one picks either 0, 1, 2, or 3 slot times.

• If a third collision occurs then the next time the number of slots to wait is chosen at random from the interval 0 to 23 - 1.

34

04/20/23

The Binary Exponential Back off Algorithm

• In general, after i collisions, a random number between 0 and 2i - 1 is chosen,

• However, after ten collisions have been reached, the randomization interval is frozen at a maximum of 1023 slots.

• After 16 collisions, the controller reports failure back to the computer. Further recovery is up to higher layers.

35

04/20/23

Switched 802.3 LAN• Way to deal with increased load: switched

Ethernet • An Ethernet network that is controlled by a switch

instead of a shared hub. – For example, a 24-port 100Base-T hub shares the total

200 Mbps bandwidth with all 24 attached nodes. By replacing the hub with a switch, each of the 12 sender/receiver pairs has the full 200 Mbps capacity.

• Switch = a high-speed backplane + room for typically 4 to 32 plug-in line cards, each having one to eight connectors

36

04/20/23

A Switched 803.2 LAN

37

04/20/23

Fast Ethernet .3u

• It is called 802.3u,

• Reduce the bit time from 100 nsec to 10 nsec

• Not feasible to use Manchester encoding

38

04/20/23

• 100 Base T4– Uses four separate twisted pairs of Cat 3 UTP– One to the hub, one from the hub,the other two are switchable to the

current transmission direction– Three-level ternary code is used 8B/6T (8 bits map to 6 trits.)

• Prior to transmission each set of 8 bits is converted into 6 ternary symbols. (0,1,2)• Three signal levels : +V, 0, -V

• 100 Base TX– Uses two pair of twisted pair, one pair for transmission and one pair for

reception.– Uses Cat 5 UTP.– Uses 4B/5B encoding– Full duplex

• 100 Base FX– Multimode fibre– Full duplex– Distance between station and hub can be 2 km

39

04/20/23

Gigabit Ethernet .3z

•Supports full duplex and half duplex•Full duplex – uses switch (buffereing is present)•Half duplex – uses hub (no buffering)

40

04/20/23

(a) A two-station Ethernet. (b) A multistation Ethernet.

41

04/20/23

Carrier ExtensionRRRRRRRRRRRRRFrame

Carrier Extension

512 bytes

• For 10BaseT : 2.5 km max; slot time = 64 bytes• For 1000BaseT: 200 m max; slot time = 512 bytes• Carrier Extension :: continue transmitting control

characters [R] • This permits minimum 64-byte frame to be handled.• Control characters discarded at destination.

42

04/20/23

Frame Bursting

512 bytes

ExtensionFrame Frame Frame Frame

Frame burst

• Source sends out burst of frames• Maximum frame burst is 8192 bytes• Increases the throughput

43

04/20/23

Wireless LAN Protocol Stack

44

04/20/23

Physical Layer

• Five permitted transmission techniques – Infrared

– FHSS (Frequency Hopping Spread Spectrum)

– DSSS (Direct Sequence Spread Spectrum),

– OFDM (Orthogonal Frequency Division Multiplexing)

– HR-DSSS (High Rate Direct Sequence Spread Spectrum)

– An enhanced version of HR-DSSS

• They differ in the technology used and speeds achievable

45

04/20/23

Infrared Technique

• Two speeds are permitted: 1 Mbps and 2 Mbps • At 1 Mbps, an encoding scheme is used in which

a group of 4 bits is encoded as a 16-bit (15 0’s and single 1)

• At 2 Mbps, an encoding scheme is used in which a group of 2 bits is encoded as a 4-bit

• Cannot penetrate through wall.• Due to the low bandwidth and sunlight swamps

infrared signals), this is not a popular option.

46

04/20/23

FHSS (Frequency Hopping Spread Spectrum) • Is a method of transmitting radio signals by rapidly

switching a carrier among many frequency channels, – using a pseudorandom sequence known to both transmitter and

receiver.

• uses 79 channels each 1-MHz wide, starting at the low end of the 2.4-GHz ISM band

• A pseudorandom number generator is used to produce the sequence of frequencies hopped to

• Dwell time: The amount of time spent at each frequency (must be < 400 msec )

• Advantage : Good security• Disadvantage : Low Bandwidth

47

04/20/23

DSSS (Direct Sequence Spread Spectrum) • The scheme used has similarities to the

CDMA system.

• Each bit is transmitted as 11 chips (Barker Sequence)

• 1 or 2 Mbps speed

48

04/20/23

OFDM (Orthogonal Frequency Division Multiplexing)

• the high-speed wireless LAN

• deliver up to 54 Mbps in 5GHz ISM band

• different frequencies are used—52 of them, 48 for data and 4 for synchronization

49

04/20/23

HR-DSSS (High Rate Direct Sequence Spread Spectrum)

• uses 11 million chips/sec to achieve 11 Mbps in the 2.4-GHz band

• Known as 802.11b

• Data rates supported by 802.11b are 1, 2, 5.5, and 11 Mbps

• This technique was enhanced by IEEE to achieve higher speed. (called 802.11g)– In theory operates at 54MBps.

50

04/20/23

The 802.11 MAC Sub layer Protocol

• 802.11 supports two modes of operation – DCF (Distributed Coordination Function),

• does not use any kind of central control

– PCF (Point Coordination Function), • uses the base station to control all activity in its cell

• 802.11 uses a protocol called CSMA/CA and MACAW

51

04/20/23

CSMA/CA• In DCF, CSMA/CA is used• CSMA/CA has two mode of operation:1) Based on Physical Channel Sensing

– Station senses the channel. If it is idle, it just starts transmitting.

– It does not sense the channel while transmitting but emits its entire frame (half duplex)

– If a collision occurs, the colliding stations wait a random time, using the Ethernet binary exponential back off algorithm

2) Based on Virtual channel sensing (MACAW)

52

04/20/23

MACAW

(Network Allocation Vector)

53

04/20/23

To deal with the problem of noisy channels, 802.11 allows frames to be

fragmented into smaller pieces

54

04/20/23

PCF

• The base station polls the other stations, asking them if they have any frames to send

• Base station broadcasts a beacon frame periodically (10 to 100 times per second).

55

04/20/23

PCF and DCF can coexist within one cell

• It works by carefully defining the interframe time interval

• After a frame has been sent, a certain amount of dead time is required before any station may send a frame

• Four different intervals are defined – SIFS (Short InterFrame Spacing).

– PIFS (PCF InterFrame Spacing)

– DIFS (DCF InterFrame Spacing)

– EIFS (Extended InterFrame Spacing),

56

04/20/23

PCF and DCF can coexist within one cell

57

04/20/23

The 802.11 Frame Structure

• Three different classes of frames – Data,– Control, and – Management.

58

04/20/23

The 802.11 data frame

59

04/20/23

The 802.11 data frame• Frame Control field

– 11 subfields • Protocol version • Type (data, control, or management) • Subtype fields (e.g., RTS or CTS) • To DS (Distribution system)• From DS • MF bit means that more fragments will follow • Retry bit marks a retransmission of a frame sent earlier • Power management bit is used by the base station to put the receiver

into sleep state or take it out of sleep state • The More bit indicates that the sender has additional frames for the

receiver • The W bit specifies that the frame body has been encrypted using the

WEP (Wired Equivalent Privacy) algorithm • the O bit tells the receiver that a sequence of frames with this bit on

must be processed strictly in order.

60

04/20/23

The 802.11 data frame• the Duration field,

– tells how long the frame and its acknowledgement will occupy the channel

– Used to manage NAV mechanism• The source and destination are obviously needed,• The other two addresses are used for the source

and destination base stations for inter cell traffic. • Sequence field allows fragments to be numbered.

Of the 16 bits available, 12 identify the frame and 4 identify the fragment.

• The Data field contains the payload, up to 2312 bytes, followed by the usual Checksum

61

04/20/23

The 802.11 data frame

• Management frames have a format similar to that of data frames, except without one of the base station addresses, – because management frames are restricted to a

single cell.

• Control frames are shorter still, having only one or two addresses, – No data field and no sequence field– Key information used is subtype (RTS,CTS,

and ACK)

62

04/20/23

Services

• Wireless LAN must provide nine services

• divided into two categories: – five distribution services and

• relate to managing cell membership and interacting with stations outside the cell

– four station services • relate to activity within a single cell.

63

04/20/23

Five Distribution Services1. Association. This service is used by mobile stations to

connect themselves to base stations 2. Disassociation. Either the station or the base station may

disassociate, thus breaking the relationship 3. Re association. A station may change its preferred base

station using this service 4. Distribution. This service determines how to route

frames sent to the base station5. Integration. If a frame needs to be sent through a non-

802.11 network with a different addressing scheme or frame format, this service handles the translation from the 802.11 format to the format required by the destination network.

64

04/20/23

Four Station Services

• Authentication. Allow new station to get into network

• Deauthentication. When a previously authenticated station wants to leave the network, it is deauthenticated. After deauthentication, it may no longer use the network

• Privacy. information sent over a wireless LAN must be encrypted

• Data delivery

65

04/20/23

Broadband Wireless

• Erecting a big antenna on a hill just outside of town and installing antennas directed at it on customers' roofs

• Some people prefer to call it a wireless MAN (Metropolitan Area Network) or a wireless local loop.

• IEEE calls 802.16• Commercialized under the name “WiMAX”

(from "Worldwide Interoperability for Microwave Access")

66

04/20/23

Broadband Wireless

67

04/20/23

Comparison of 802.11 with 802.16

• 802.16 runs over part of a city, the distances involved can be several kilometers,

• Each cell is likely to have many more users than will a typical 802.11 cell

• No ISM bands • 802.11 was designed to be mobile Ethernet,

whereas 802.16 was designed to be wireless, but stationary, cable television

68

04/20/23

The 802.16 Protocol Stack

69

04/20/23

The 802.16 Protocol Stack

• Physical layer – convergence sub layer to hide the different

technologies from the data link layer

• Data link layer– consists of three sub layers

• The bottom one deals with privacy and security • MAC sub layer main protocols, such as channel

management (Connection Oriented)• Service-specific convergence sub layer (LLC) –

interface with NL

70

04/20/23

The 802.16 Physical Layer • 802.16 employs three different modulation schemes• 10-66 GHz range • Signal strength decreases as distance from base station

increases.• Depending on how far the subscriber station is from the

base station. – For close-in subscribers, QAM-64 is used, with 6 bits/baud.– For medium-distance subscribers, QAM-16 is used, with 4

bits/baud. – For distant subscribers, QPSK is used, with 2 bits/baud

• Bit Rate (bps) = Baud per second * bits per baud.• Ex 25MHz spectrum

– For QAM-64 => 25*6 = 150 Mbps– QAM-16 => 25*4 = 100 Mbps– QPSK => 25 * 2 = 50 Mbps

71

04/20/23

The 802.16 MAC Sub layer Protocol

• Four classes of service are defined 1. Constant bit rate service.

– for transmitting uncompressed voice

2. Real-time variable bit rate service.– for compressed multimedia and other soft real-time

applications

3. Non-real-time variable bit rate service.– for heavy transmissions that are not real time, such as

large file transfers

4. Best-efforts service.– for everything else

• All service in 802.16 is connection-oriented

72

04/20/23

The 802.16 Frame Structure

(a) A generic frame. (b) A bandwidth request frame.

73

04/20/23

The 802.16 Frame Structure• The EC bit tells whether the payload is encrypted • The Type field identifies the frame type, mostly telling

whether fragmentations are present.• The CI field indicates the presence or absence of the final

checksum. • The EK field tells which of the encryption keys is being

used (if any).• The Length field gives the complete length of the frame,

including the header. • The Connection identifier tells which connection this

frame belongs to. • Finally, the HeaderCRC field is a checksum over the

header only, using the polynomial x8 + x2 + x + 1.

74

04/20/23

For BW Req Frames

• Starts with 1 instead of 0

• Bytes needed field tell how much bytes to be transferred

75

04/20/23

Bluetooth

• Short Range, Low Power, Inexpensive wireless radio

• IEEE assigned designation 802.15

• Bluetooth Architecture:– Piconet – the basic unit of a Bluetooth

system.• 1 Master node

• 1 to 7 active slave nodes (upto distance of 10km)

• 0 to 255 parked nodes

76

04/20/23

Two Piconets connected to form a Scatternet

77

04/20/23

Bluetooth Architecture• Parked node

– a slave device in a low power state to conserve the drain on the devices batteries

– In this state the device can only respond to the beacon from the master node

• Communication– Only possible between master and slave nodes– Piconet uses centralized Time Division Multiplexing.– The master node controls the clock and determines

which device gets to communicate in which time slot.– All communication is between master and slave; direct

slave-slave communication is not possible

78

04/20/23

Bluetooth Applications

• The 13 applications, which are called profiles, are:

networking

telephony

Object exchange

79

04/20/23

The Bluetooth Frame Structure

• Access code : Identifies the master so that slaves within radio range of two masters can tell which traffic is for them.

• Data – 2744 bits

80

04/20/23

The Bluetooth Frame Structure• Header:

– Adr: identifies one of the eight active devices – Type: frame type (ACL, SCO, poll, or null), the type of error

correction used in the data field, and how many slots long the frame is.

– F: asserted by a slave when its buffer is full and cannot receive any more data.

– A: piggyback an ACK onto a frame – S: number the frames to detect retransmissions

• Protocol is stop and wait, so 1 bit is enough– Checksum

• Entire 18-bit header is repeated three times to form 54 bits header– On Receiver side, each copy is verified. If all three are same, the

bit is accepted.

81

04/20/23

DLL Switching

(a) Which device is in which layer. (b) Frames, packets, and headers

82

04/20/23

Repeaters

• Analog devices that are connected to two cable segments.

• A signal appearing on one of them is amplified and put out on the other.

• Repeaters do not understand frames, packets, or headers. – They understand volts.

83

04/20/23

Hubs• A hub has a number of input lines

that it joins electrically • Frames arriving on any of the lines

are broadcast on all the others port • If two frames arrive at the same

time, they will collide, just as on a coaxial cable

• Hubs differ from repeaters in that they do not (usually) amplify the incoming signals

• Also known as Multiport repeaters

84

04/20/23

Bridge

• A bridge connects two or more LANs

• When a frame arrives, software in the bridge extracts the destination address from the frame header and looks it up in a table to see where to send the frame.

85

04/20/23

Switches• Switches has buffer space

– If frames come in faster than they can be retransmitted, the switch may run out of buffer space and have to start discarding frames.

• Cut-through switches: – modern switches start forwarding

frames as soon as the destination header field has come in, but before the rest of the frame has arrived

– Do not use store-and-forward switching.

• Address is Ethernet address (48 bit)

86

04/20/23

Routers

• When a packet comes into a router, the frame header and trailer are stripped off and the packet located in the frame's payload field is passed to the routing software.

• Uses IP address– IPV4 – 32 bits– IPV6 – 128 bits

87

04/20/23

Gateways

• Transport Gateway : – These connect two computers that use different

connection-oriented transport protocols.

• Application Gateway :– Application gateways understand the format and

contents of the data and translate messages from one format to another

– Ex: An e-mail gateway could translate Internet messages into SMS messages for mobile phones.