1

Medium Access Control

2

Medium Access Control (1)

The Network

H1

H2

H3

H4

Broadcast networks have

possibility ofmultiple access (MA) to a channel

medium access control describes how we resolve the conflict

assume only one channel available for communication

additional channels would also be the subject of MAC

3

Medium Access Control (2)

The Network

H1

H2

H3

H4

4

Medium Access Control (3)

The Network

H1

H2

H3

H4

5

Medium Access Control (4)

The Network

H1

H2

H3

H4

assume when two frames overlaps at the Rx then both are lost, and thus both must

be retransmitted

assumption always be true in LANs

in broadcast WANs might not be true

6

ALOHA protocol

You can do nothing for MAC . . .

The Network

H1

H2

H3

H4The Network

H1

H2

H3

H4The Network

H1

H2

H3

H4The Network

H1

H2

H3

H4

ALOHA is contention based: a host may broadcast whenever necessary

higher layers spot errors caused by collisions, and do retransmission

7

Performance of ALOHA

H1

H2

t1 + t2

vulnerable period forH2 start-�

vulnerable period forH1 start

t1 + t2-�

t2 -�t1

-�

Let t1 = t2 = T

µS successfully sent per second

µG sent (including failures) per second

p is probability frame has no collisions

p =µS

µG

Pn(t) =(λt)n

n!e−λt

To findp from Poisson Equation

sett = 2T, λ = µG, n = 0:

p =(µG2T )

0

0!e−µG2T

p = e−µG2T

µS

µG= e−µG2T

µS = µGe−µG2T

µST frames are delivered inT

seconds

ρ = µST = µGe−µG2T T

8

Is ALOHA good?

-µGT

6ρ

0.0 0.5 1.0 1.5 2.0 2.5 3.00.0

0.1

0.2

0.3

0.4

load of 50% gives maximum efficiency of 18%

not a very satisfactory performance

no way of assuring that even this maximum efficiency is reached

9

Improving basic ALOHA

1. slotted transmission: there are certain specific times when a host maybroadcast.

2. carrier sensing: a broadcast is allowed only when the channel is idle.

3. token passing: a host may only broadcast when it holds some sort of tokenpermitting it to do so.

4. distributed queueing: the hosts collaborate to form a queue of hosts ready tosend data.

10

Slotted ALOHA

H1

H2

t-�

vulnerable period forH2 start-�

vulnerable period forH1 start-�

µS successfully sent per second

µG sent (including failures) per second

p is probability frame has no collisions

p =µS

µG

Pn(t) =(λt)n

n!e−λt

To findp from Poisson Equation

sett = T, λ = µG, n = 0:

p =(µGT )

0

0!e−µGT

p = e−µGT

µS

µG= e−µGT

µS = µGe−µGT

µST frames are delivered inT

seconds

ρ = µST = µGe−µGT T

11

Is Slotted ALOHA good?

-µGT

6ρ

0.0 0.5 1.0 1.5 2.0 2.5 3.00.0

0.1

0.2

0.3

0.4

now load of 100% loading gives maximum efficiency of 36%

still not a very satisfactory performance

small basic problems as ALOHA

12

Carrier Sensing

The Network

H1

H2

H3

H4

The Network

H1

H2

H3

H4

The Network

H1

H2

H3

H4

carrier sensing involves

checking that channel is idle

before transmission

calledCSMA

probability TITI+TP

of avoiding

a collision

H1

H2

TI -�

U

TP

-� TCS-�

13

Collision Detection

H3 causes collisionH2 causes collision

H1 H2 H3 H2H3 H1

transmission-�

contention-�

transmission-�

idle-�transmission

-�

host senses the medium to know that its frame is OK

transmission stopped as soon as collision occurs→ collision detection (CD)

MAC protocol calledCSMA/CD

host must transmit for long enough so as to know frame is OK

thus minimum frame length is2η whereη is end-to-end transmission time

14

Worksheet: CSMA/CD

A bus based network is being built using CSMA/CD for MAC, a cabling system

with signal propagation speeds of200× 106ms−1, and a bit rate 1Mbs−1.

1. If two hostsH1, H2 are separated by 2,000m, how long does it take for a

signal to travel between them?

2. If H1 has started to broadcast, andH2 starts to broadcast just before the signal

from H1 arrives atH2, what will happen?

3. How long, from the timeH1 starts transmitting, will it takeH1 to find out

about the event in question 2?

4. How many bits would be sent in that time?

5. If the data rate were increased to 10Mbs−1, how many bits would have been

sent?

15

Length of bus network using CSMA/CD

A bus-based network transmits 64 bit frames at 10 Mbs−1, propagation speed

c = 200× 106ms−1

What is the maximum end-to-end lengthl on the bus?

We know that the end-to-end propagation delayη = l200×106

Also, the time to transmit a framet = 6410×106

= 6.4× 10−6

Since the time to transmit a frame must be greater than2η, we have:

6.4× 10−6 >2l

200× 106

l < 640m

16

Persistence

Used slotN N + 1 N + 2 N + 3

1-persistent 1 0 0

p-persistent p (1− p)p (1 − p)(1 − p)p

non-persistent p p p

once idle, it broadcasts with probabilityp→ p-persistent CSMA

if p = 1→ simply waiting for the channel to be free before broadcasting

if p < 1 then wait with probability1− p for one frame, before broadcasting

with probabilityp

17

Binary Exponential Back-off

The basis of the MAC in Ethernet is 1-persistent CSMA/CD

to avoid the poor performance of this protocol when network loads are high, a

variation calledbinary exponential back-off is introduced.

minimum frame length is treated as the slot length

when the net is idle, a host may attempt to broadcast

if a collision occurs, wait either 0 or 1 slots before attempting to broadcast

again

if another collision occurs waits 0, 1, 2 or 3 slots

afterc collisions we choose a slot in the range 0 to2c − 1 for the next attempt

upper limit of 1023 is placed on what this range

18

Binary Exponential Back-off (Light Load)

H2

H1

0 1Slot

H2

H1

0 1Slot

H2

H1

0 1Slot

H2

H1

0 1Slot

chances of collisions occurring are slight

during transmission only a few hosts waiting to transmit

probably only have a few hosts in contention

19

Binary Exponential Back-off (Heavy Load)

H5

H4

H3

H2

H1

0 1Slot

H5

H4

H3

H2

H1

0 1Slot

H5

H4

H3

H2

H1

0 1Slot

H5

H4

H3

H2

H1

0 1Slot

H5

H4

H3

H2

H1

0 1Slot

H5

H4

H3

H2

H1

0 1Slot

H5

H4

H3

H2

H1

0 1Slot

at any one time a number of hosts are likely to be in contention

waiting only a few slots would mean a repeat collision is verylikely

binary exponential back-off algorithm to quickly adapts

20

Problems with CSMA/CD

No notion of authority to broadcast

collisions are inevitable

in the worst case, a particular host may be delayed indefinitely

Acceptable for many applications, such as office information systems

Unacceptable for real time systems used in applications such as CAM

time that a host waits must have a fixed upper bound.

bandwidth available to each host must have a fixed lower bound.

21

Token Passing

H1

H2 H3

H4

H5

directionof

token6

token

logicalring ofhosts

node outsidelogical ring

Hosts must posses atoken in order to broadcast

token passed from one host to another

host has nothing to send→ pass token on immediately

host has something to send→ sets a timer, and transmits until

the timer expires or,

it has no more data to send

22

Token Passing Performance

No losses due to collisions, and therefore most of bandwidthavailable for data.

η is the average delay between hosts

B bits per second network

timeout at each host isT

The maximum frame sizef obeys

f = TB

The maximum delayd given by

d = N (η + T )

23

IEEE 802 standard for LANs

PhysicalLayer

Data LinkLayer

NetworkLayer

OSI

. . .802.3

PhysicalLayer

802.3MAC

Sub-layer802.4

PhysicalLayer

802.4MAC

Sub-layer802.17

PhysicalLayer

802.17MAC

Sub-layer

802.1D Bridging Sub-layer

802.2 LLC Sub-layer

OSI Network Layer

IEEE

802 standard sets out a framework:

logical link control (LLC) sub-layer performs ARQ (802.2)

Bridging might be present to link LANs (802.1D)

24

IEEE MAC Addressing

I/G

U/L OUID OUID OUID

localid

localid

localid

byte0 byte1 byte2 byte3 byte4 byte5

Each NIC card conforming to IEEE Standards will have 48 bit number

written as six colon separated bytes in hex

〈byte〉:〈byte〉:〈byte〉:〈byte〉:〈byte〉:〈byte〉

I/G bit decides if individual or group address

U/L bit decides if universally or locally administered

host00:90:27:A3:32:05 individual globally unique

broadcast addressFF:FF:FF:FF:FF:FF group locally unique

allows for7× 1013 hosts

25

IEEE LAN standards

Number Common Name Area MAC Topology

802.3 Ethernet LAN 1-persistent CSMA/CD Bus/Tree

802.4 Token Bus LAN Token Passing Bus/Tree

802.5 Token Ring LAN Token Passing Ring

802.6 DQDB MAN Distributed Queue Bus

802.9 isoEthernet LAN Ethernet + ISDN Star/mesh

802.11 WiFi LAN CSMA/CA Cellular

802.12 100BaseVG LAN Handshaking from hub Star/tree

802.15 Bluetooth PAN Adaptive FHSS Cellular

802.16 WiMAX MAN Connection oriented Cellular

802.17 Resilient Packet RingLAN to WAN Distributed Queue Ring

26

802.3 Ethernet

The IEEE 802.3 works over various cables and speeds

Code Common Name Cable Len (m) Topology

10Base5 Thick Ethernet 12

inch coaxial cable 500 bus

10Base2 Thin Ethernet 75-ohm coaxial cable 180 bus

10BaseT Twisted Pair Ethernet Category 3 UTP 100 star

100Base-TX Fast Ethernet Category 5 UTP 100 star

100Base-FX Fast Ethernet Fibre optic 185 star

1000Base-T Gigabit Ethernet Category 6 UTP (4 pairs) 100 star

10GBase-T 10 Gigabit Ethernet Category 6a UTP (4 pairs) 100 star

10Mbs−1 standards have been in use for many years

100Mbs−1 common

1Gbs−1 common on new computers, hubs still a little expensive

27

10Mbs−1 Ethernet

Thick Ethernet:

provides longest lengths

hosts are attached via transceivers

length ofdrops from the main cable must not exceed 50m.

Thin Ethernet:

The cable length must not exceed 200m

broken at hosts, connected via BNC connectors and a T-piece.

Twisted Pair Ethernet:

The telephone cable links each host to a hub

RJ45 telephone connectors used at ends of cable

hub acts as a repeater between cables.

28

Internetworking 10Mbs−1 Ethernet

10Base-5 Thick Ethernet10Base-2 Thin Ethernet10Base-T Twisted Pair Ethernet

Hhost

H

H

H

H

H

H H

H

H

H H

H H

repeater repeater hub

repeater copies bits between subnets to run over another cable length.

max of four repeaters→max end-to-end length 2.5km

29

Ethernet NIC

30

Low Cost Hub

31

Ethernet frame format

preamble7

STX

1destination

address

6sourceaddress

6len/type

2data

0-1500pad46-0

CRC4

EXT

0+

real defining feature of ‘Ethernet’

CSMA/CD→ some minimum frame size

this has be set to 512 bits (64 bytes)data frame.

zero length data→ frame length 18 bytes?

pad field is added to make up difference

len< 0x600 or type≥ 0x600

32

Ethernet Going Faster

When designed, 10Mbs−1 Ethernet seemed like ‘infinite’ bandwidth

Modern PCs and applications can now handle much higher data rates

Need to keep existing Ethernet hardware, but allow additionof faster machines

f

B∝

2l

c

end-to-end length∝1

bit rate

if 10Mbs−1→ 2500m, then 100Mbs−1→ 250m, 1Gbs−1→ 25m

33

802.3u Fast Ethernet 100Base-T Half-Duplex

H

H

H H

H H

H H

H

H

H H

H H

hub hub hub

Most modern Ethernet installations 10BaseT

For standard Ethernet, data rate∝ length

10Mbs−1 Ethernet max=2,500m→ 100Mbs−1 Ethernet max=250m

10BaseT uses short lengths→ run 10BaseT cable at 100Mbs−1

Leave other Ethernet parameters the same

34

Gigabyte Ethernet 802.3z

Backbone of networks require even higher speeds than 100Mbs−1

Supports STP cabling (25m), or Fibre optic (550m), via hubs.

Want to mix with 10Mbs−1 and 100Mbs−1 Ethernet

64bytes frame size at 10Mbs−1→ 51µs time

end-to-end∝1

bit rate

if 10Mbs−1→ 2500m, then 1Gbs−1→ 25m

CSMA/CD unaltered would lead to maximum end-to-end lengthsof about 25m

35

Keeping CSMA/CD: Carrier Extension

51µs -�

Ethernet Frame Carrier

Any packet< 512 bytes is extended by the host sending a carrier

Keep carrier on to make packet last the time for 512 bytes.

Allows network length to be 200m end-to-end

36

Keeping CSMA/CD: Packet Bursting

51µs -�

Ethernet Frame Ethernet Frame Ethernet Frame

Short frames need padding to make then ‘long enough’

Allow hosts to send multiple frames

Several short frames can be used (with carrier in between) tomake one long

frame

37

Avoiding CSMA/CD: Full Duplex Mode

H

H

H H

H H

H H

H

H

H H

H H

switch switch switch

Many media are full duplex

10BaseT (i.e. UTP cables) has separate Tx and Rx wires

Switch ensures that collisions never occur

buffer frames

ignore frames for certain channels

Possible to having multiple hosts transmitting simultaneously

38

802.11 WiFi

BSS1

AP1

H1

H2

BSS2 BSS3

AP2

H3 H4AP3

H5

distribution system

portal

wired LAN

H6 H7 H8

In practice, portal is built into APs.

39

802.11 Frame Format

bytes 2

FC

2durID

6destination

address

6source

address

6

address 3

2

SC

6

address 4

≤ 2312

data

4

CRC

ver type subtype toDSfromDS

morefrag

retrypwrmgt

moredata WEP order

distribution system (DS)

toDS=1→ destination address is AP

fromDS=1→ source address is AP

APs may communicate wirelessly or via LAN

wired equivalent privacy (WEP) usesRC4 encryption

WiFi portected access (WPA) usesAES encryption

40

802.5 Token Ring

H1

H2 H3

H4

H5

directionof

token6

token

logicalring ofhosts

node outsidelogical ring

H1

H2 H3

H4

H5

directionof

token6

token

physicalring ofhosts

for token passing MAC, rings topology is natural implementation technique

physical ordering of hosts→ logical order for token passing.

in thetoken ring, each host may operate in two modes:listen mode ortransmit mode

41

Listen mode

host

CPU

buffer

→ F →F

Hn uses a single bit buffer to copy input bit stream fromHn−1 to Hn+1.

Host keeps copy of any frame addressed to it.

42

Transmit Mode

host

CPU

buffer

→ Fin →Fout

Hn reads the bit stream fromHn−1 into memory, and transmits a frame from

its memory toHn+1

The whole frame need not fit on the ring→ any frame length may be used

43

Token Ring: Token Passing (1)

H1

CPU

bufferH2

CPU

bufferH3

CPU

bufferF T→ →→

Only one host in Tx mode

Drains off frameF it send, and places tokenT on ring

early release mode means token placed on ring beforeF arrives back

All other hosts will be in listen mode

44

Token Ring: Token Passing (2)

H1

CPU

bufferH2

CPU

bufferH3

CPU

bufferTT →→ →→

After sending token, host switches to listen mode

Host not wanting to Tx can just pass token on

45

Token Ring: Token Passing (2)

H1

CPU

bufferH2

CPU

bufferH3

CPU

bufferT F

↑

→

After sending token, host switches to listen mode

Host wanting to Tx can spot token in buffer

Switches to transmit mode

drains offT

sends data frameF

46

Token Ring Frames

STX

1AC

1FC

1destination

address

6sourceaddress

6route

information

0-30data

0+ (timer limited)

CRC4

ETX

1FS

1IFG

1+

STX

1AC

1ETX

1

access control (AC) field at the start of all frames

a single bit to denote presence oftoken

priority bits: priority level of frame

reservation bits priority of the data waiting to be sent

Theframe status (FS)

47

Token Ring: Claiming the Token

H1

CPU

bufferH2

CPU

buffer

F1

H3

CPU

buffer

F2

T2 →→

Can only claim token if data of priority of data high enough

H2 has priority1 dataF1→ can not take priority 2 tokenT2

48

Token ring: Reserving the Token

H1

CPU

bufferH2

CPU

bufferH3

CPU

bufferF2→

F3

→F ′2

Host is listen mode might have high priority data to send

Can increase reservation priority

TokenT always generated with priority of reservation bits inF

This priority scheme may cause low priority data to be delayed indefinitely

49

Token Ring: Acknowledgement

H1

CPU

bufferH2

CPU

bufferH3

CPU

bufferF ′ F F F ′

↓

→→ →→

↑

Theframe status (FS) changed by receiver

A=1: the destination host is working

C=1: the destination host correctly read the frame

Acknowledgements part of the I-frame

50

Ring Maintenance

UseFC to generate different control frames

do not need any protocols for maintaining the logical ring, since it is directly

implemented by the physical ring

some station, called themonitor, must take responsibility for generating atoken and draining orphaned frames

since the monitor may fail, any host on the network must be able to take on

this function

contention protocols are necessary for deciding who is monitor.

51

Wiring Concentrators

concentrator

H1 H2 H3

↑ ↓ ↑ ↓ ↑ ↓

→ →

concentrator

H1 H2 H3

↑ ↓ ↑ ↓

→ → →

Rings are unreliable — one break means no ring

Wiring concentrators can switch out fault hosts/breaks

52

Fibre Distributed Data Interface (FDDI)

A

A

A

A

B B

B

←

→

Class A hosts attached to both rings

Class B hosts attached only to one

ring based network, token passing

optical fibre cabling→ supports high data transmission rates over longdistances

125MHz clock over 100km

4B5B synchronous coding→ effective bit rate is 100Mbs−1

53

Faulty Class B

A

A

A

A

B

B

←

Any faulty class B host will only affect one ring

54

Breaks in Ring/Faulty Class A

A

A

A

B B

B

←

→

Class A hosts may ‘short circuit’ the two rings together

creates new single ring almost twice as long as original

all Class A hosts will still be connected

rings up to 100km long→ FDDI must operate with length up to 200km

55

FDDI frame format

preamble≥ 8

STX

1FC

1destination

address

6sourceaddress

6data

timer limitedCRC

4ETX

1FS

1.5

preamble≥ 8

STX

1FC

1ETX

1

unlike Token Ring, no priority bits in theFC

the hosts time delay in receiving token→ how busy ring is

FC→ synchronous or asynchronous

56

Ring Size Means Must Release Token Early

H1

CPU

buffer

H2

CPU

buffer

H3

CPU

buffer

Fa →→

H1

CPU

buffer

H2

CPU

buffer

H3

CPU

bufferT →→ Fa →→

H1

CPU

buffer

H2

CPU

buffer

H3

CPU

buffer

Fa

↑

→Fb →→

long rings (about 4200 bytes fits into 100km)

draining off the frame is more complicated

57

FDDI-II

supports the transmission of synchronous data

Channels of 6.144Mbs−1 may be allocated to synchronous traffic

adequate to carry 96 ISDN B-channels

four US or three European primary rate services

synchronous frames generated at 8000Hz

pairs of hosts will be allocated a slot (i.e. 64kbs−1)

58

802.4 Token Bus

set up in response to the Ethernet standard by users unhappy with CSMA/CD

use of token passing, the variety of speeds and signal modulation techniques

used, make the standard much more complicated than Ethernet.

The physical layer uses 75 ohm coaxial cable in a bus topology

MAC sub-layer uses token passing; forming logical rings between the hosts.

59

A Token Bus Host

Hn

H0n H2n H

4n H

6n

� Token����

transmissions given a priority level of 0 (lowest), 2, 4, or 6(highest)

in effect, eachHn contains four separate hostsH0n, H2n, H

4n, H

6n

whenHn receives token, passes it toH6n, so that highest priority Tx sent first

each of ‘sub-hosts’ given timer to divide bandwidth different priorities.

60

Assigning Priorities to Transmissions

A token bus is to be used at 10Mbs−1to implement ISDN B-channel (64kbs−1)

voice traffic. Calculate the proportion of the maximum framesizef (f = TB,

whereT is the host timeout andB the channel bit rate) that should be assigned to

priority 6 traffic if this carries the voice traffic on a 25 hostnetwork.

Total voice traffic from all hosts =25× 64× 103 = 1.6× 106bs−1

Priority 6 traffic must have proportion off = 1.6×106

10×106= 0.16

61

Token bus frame format

preamble

7STX

1FC

1destination

address

6sourceaddress

6

data

0-8174

CRC

4ETX

1Bytes in each field

a frame control (FC)→ type of frame

no padding of frames is required, since CSMA/CD.

a much longer maximum frame is permitted

62

Logical Ring Maintenance

Hn−2

Hn−1 Hn Hn+1

Hn+2token

logical ringof hosts

physical bus

Each hostHn knows who is its successorHn+1

token: sent fromHn to Hn+1; transfer the permission to broadcast frames.

who follows: sent byHn when it realisesHn+1 has failed, to findHn+2

solicit successor: sent byHn to find hosts betweenHn andHn+1 wantingto join ring

claim token: sent by a host during initialisation

set successor: sent byHn when to leave the logical ring, giving the addressof Hn+1. Hn−1 will set Hn+1 to be its successor.