Chapter 12.1: Media Access Control (MAC) sublayer

LLC is similar to HDLC (book 11.6)

12.2

Figure 12.2 Taxonomy of multiple-access protocols discussed in this chapter

Will only cover random-access protocols

Contention: Access to shared media Problem:

two (or more devices) want to transmit over a shared baseband medium at the same time.

Collision Transmissions collide, destroying the information

in each transmission

Aloha protocol University of Hawaii (where else) - packet switched

radio (mid 70s). Transmit a packet using a specified radio frequency wait for ack If the ack does not arrive send packet again because it

probably collided with another station sending a packet over the same frequency.

12.6

Figure 12.4 Procedure for pure ALOHA protocol

Don’t worry yet about how TB is calculated

12.7

Figure 12.3 Frames in a pure ALOHA network

However, time from the start of 3.1 to the end of 1.2 is wasted time.

wasted

12.8

Figure 12.5 Vulnerable time for pure ALOHA protocol

Keep in mind even if a small part of a packet collides, the entire packet is of no use.

Slotted Aloha

Time divided into intervals of length Tfr, the time required to send one frame.

Device begins sending only at the start of an interval Does not eliminate collisions, but

If two frames collide the wasted time is Tfr

With pure Aloha, it could be 2Tfr

12.10

Figure 12.6 Frames in a slotted ALOHA network

CSMA (Carrier Sense Multiple Access) Original Ethernet configured similar to that below Only one device could transmit at a time Refinement of Aloha protocols

If a device has something to send it will Listen: if medium is quiet then send if not quiet then wait for quiet.

Works most of the time Two waiting devices could detect quiet at nearly the same

time and their transmissions would collide

12.13

Figure 12.8 Space/time model of the collision in CSMA

I believe yellow represents B’s signal and blue represents C’s signal

P-persistence when quiet, transmit with probability p

(i.e. p=1/2 => 50% chance of transmitting Non-persistent: If busy, wait one time slot (time to

transmit one frame) and try again. Show examples showing all possible scenarios with

two or three stations waiting via p-persistence with p=0.5.

12.15

Figure 12.11 Flow diagram for three persistence methods

CSMA/CD: adds collision detection (CD). Transmit and listen for collision. If one occurs stop transmitting, wait random time

and try again. Reduces amount of time during which signals

collide.

Requires a minimum frame size for CSMA/CD to work.

Device must still be sending when collision detected or else it does not know if its frame was involved.

Early Ethernet: 10Mbps and 2 km cable signal propagation speed was 200 meters/msec Worst case: 10 msec to collision and 10 more to

detect it. Can send 200 bits in that 20 msec time. Must detect

collision while frame is still being sent.

12.19

Figure 12.12 Collision of the first bit in CSMA/CD

Binary exponential backoff algorithm 1st collisionwait 0/1 time slot (chosen randomly) 2nd collision wait 0-3 time slots (chosen randomly) 3rd collision wait 0-7 time slots (chosen randomly) in general wait 0-2n-1 –1 time slots (chosen randomly). After 16 collisions give up.

Section12.1 also discusses CSMA/CA (Collision Avoidance)

Somewhat different protocol used for 802.11 Wireless standards.

Covered later Does not really avoid collisions.

Token ring and token bus (brief mention in section 12.1) Tokens circulated among devices A device with something to send waiting for the

token NO collisions Needed management protocols if a problem occurred

with the token IEEE 802.4 and 802.5 are standards for token ring

and bus networks IEEE 802.3 Ethernet (actually its variations) is

dominant

Ethernet Chapter 13 There are numerous IEEE802.3 standards for

Ethernet. Standard Ethernet (10 Mbps) Fast Ethernet (100 Mbps) Gigabit Ethernet (1 Gbps) Ten-Gigabit Ethernet (10 Gbps) More than just bit rates - accounts for different

media, signaling techniques, frame format, collision handling, etc.

13.24

Figure 13.1 IEEE standard for LANs

Original IEEE 802.3 Ethernet design:

Nic: network interface card

13.26

Figure 13.4 802.3 MAC frame

Source and destination MAC (48-bit addresses) - groups assigned/controlled by IEEE

13.27

Figure 13.5 Minimum and maximum lengths

Computer executes network software and routes packet of info to memory and signals nic via internal bus

nic gets packet from memory, creates a frame, and waits for signal from transceiver

transceiver listens to cable. Quiet => signal nic to send data to transceiver;

transceiver transmits bits onto cable and listens for collision. If collision occurs, nic executes binary exponential backoff

algorithm. If too many collisions => send message to network software

other transceivers copy a frame and send it to the nic.

If address in frame corresponds to nic address then do crc check.

If OK, signal computer that packet arrived. Computer executes network software to determine

whether packet can be accepted as per flow control. If so, move packet to memory.

Ethernet address: 48 bits - burned into network card A company that manufactures cards purchases a

group of 224 addresses (managed by IEEE) [http://standards.ieee.org/regauth/oui/index.shtml]

Look up your physical address (ipconfig /all). Search for the OUI (Organizationally Unique Identifier) in the previous link. Just enter the first 6 hex digits in the form xx-xx-xx. Example: 00-19-D1.

Unicast address: identify one device Multicast address: identify numerous (all -

broadcast) devices

802.3 flavors 802.3

10Base5, thick cable, 500-meter segments max of 5 segments (4 repeaters) in sequence collision domain (span in which a collision can occur) is

2500 m. Manchester coding, min. 8 ft between transceivers.

13.32

Figure 13.10 10Base5 implementation

802.3a 10Base2 (Thinnet, CheaperNet) thinner cable T-connectors, max of ~200 meter segments 5 segments, collision domain ~1 km. Cheaper & more flexible but shorter segments

802.3i 10BaseT CAT 3, 4, 5 UTP central hub – device to regenerate and send signals max UTP length is 100 m. collision domain ~500-2500 m

(depends on delay at hubs) Collisions still occur (in hub)

802.3j

10BaseF Fiber Requires a 10-BaseF hub max fiber length 2000 m.

13.36

Table 13.1 Summary of Standard Ethernet implementations

Bridged Ethernet Collisions can occur between any two devices – not

ideal. Bridge – layer 2 connector of two networks. Only forwards a frame if the destination is on the

other side Also adds a little extra security since each frame

does not go everywhere

13.38

Figure 13.16 Collision domains in an unbridged network and a bridged network

Switched Ethernet Switch is like a bridge but with more connections Otherwise, very similar Could connect multiple switches

No collisions assumes connections are full duplex – separate wire pairs used for transmission in opposite directions

Hierarchy of switches

Fast Ethernet (100 Mbps) - 802.3u, 100 BaseTX (over CAT 5 UTP) Collision domain around 200 m. (1/10 the size of

standard Ethernet since rate is 10 times as fast)

100 mbps rate w/ Manchester signal won’t work frequency too high for the CAT 5 UTP, produces too

much noise Fundamental is that higher frequency signals don’t

propagate as far. The square wave forms become deformed more

quickly and become unrecognizable. Thus, the limit on higher frequency signals.

Manchester was used to prevent long runs of 0s or 1s we need another way to do this such as 4B/5B encoding from Chapter 4.

Might use NRZ coding but there are still noise issues Multilevel Line transmission-3 level (MLT-3).

MLT-3 signal cycles through the states in the order –1, 0, +1, 0, -1, 0, +1, 0, -1,

If the bit is 1, the MLT-3 signal should progress to the next state in its sequence.

If the bit is 0, the MLT-3 signal remains at its current state.

4.46

Figure 4.13 Multitransition: MLT-3 scheme

100 BaseFX (Fiber) Uses 4B/5B encoding combined with NRZI encoding.

100 BaseT4 (over CAT 3 wire), Nice example of ingenuity used where it was too expensive to replace existing

wiring w/ CAT 5 How can you implement a new technology over old

wiring

8B/6T encoding associates every byte (8 bits) value with a unique

string of 6 ternary values, called trits See appendix D bytes are transmitted in just 6 consecutive time

intervals From chapter 4 below

Needed baud rate still to high to fall with acceptable noise levels for cat 3 UTP

So, data is transmitted through three wire pairs simultaneously

Cuts the needed rate by a factor of 3 translates to a 75 million (100 million bits) trits per

second rate Not common but was an interesting solution to a

problem.

Gigabit Ethernet optical fiber (1000BaseSX and 1000BaseLX) and

over copper (1000BaseT and 1000BaseCX) a device transmits a minimum size frame in 0.512

msec reduces the size of the collision domain to around 25

meters (real problem!)

Either use switches with full duplex mode to eliminate collisions or increase min frame size to increase size of the collision domain.

first option is common but second option standard still supports CSMA/CD by increasing the minimum frame size from 512 bits to 4096 bits

a carrier extension consists of an extra 448 bytes to meet the minimum of 512 bytes

frame bursting Better for a series of short frames transmit multiple frames without going through the

contention protocol for each one by creating a burst frame which consists of one MAC frame (including the carrier extension) followed by a sequence of additional MAC frames

802.3z (SX & LX for fiber, CX for copper). CX corresponds to shielded copper cable, is good

only for very short spans, used to connect devices in a rack.

Uses 8B/10B scheme or a 4D-PAM5 scheme discussed in chapter 4 but not here.

http://grouper.ieee.org/groups/802/3/tutorial/march98/mick_170398.pdf

http://www.ieee802.org/802_tutorials/99-July/mastut.pdf