Basic ModelMEDIUMDistributed SourcesFig.13 Generic Model for Media Access Systems

FrequencyMultiplexed Channel

l

l

l

1

2

N

Guard Band

BW

Time

FDMA

Message Delay - FDMA

Example FDMA50 kbps line - 1000 bit frames to be transmitted from four sourcesTwo Approaches 1) All traffic on 50 kps line

2) 12.5 kbps dedicated to each sourceMUXMUXMUXMUXMUX

Four 12.5 kbps linesOne 50 kbps linesAverage delay(sec)Load

L=1L=4L=16L=64LoadAverage delay(sec)

Continuation FDMA

l

TimeMultiplexed Channel

l

l

l

1

2

N

S1

S2

SN

Time

S1

S2

SN

Frame

Guard space

TDMA

TF

Ts

Message Delay - TDMA

L=64L=16L=4

Polling Models

CentralProcessor

Broadcast Polls

Trees

Bridge

Cycle timeDelay

exhaustive -

(44a)

gated -

(44b)

limited

_954856824.unknown

_954924024.unknown

_933775958.unknown

Normalized Average Delay

Load

Limited Service

Exhaustive andGated Service

32 StationsW/M=1

Limited service

Exhaustive andGated Service

W/M=1

Number of Stations

Normalized Delay

Load=0.4

Limited service

Exhaustive andGated Service

Normalized Delay

W/M

Number of Stations =32

Load=0.4

Random Access-ALOHA

Common Channel

Uncontrolled access

Collision

...

RetransmissionsAfter Random Time-out

M

Potentialcollidingpackets

Pure ALOHA

Collision Window

Fig. 20 Pure ALOHA Collision Mechanism

P(Collision)=Total trafficNew trafficRetransmitted trafficAssumption: Total lineflow , L, PoissonDefine R=LM, total load and r=lM, load due to new traffic

Slotted ALOHA

Pure ALOHA

r - Throughput

R -Total line traffic

M

Potentialcollidingpackets

Transmitted innext slot

Slotted ALOHA

Fig. 22 Slotted ALOHA

CollisionWindow

P(Collision)=Total trafficNew trafficRetransmitted trafficDefine R=LM, total load and r=lM, load due to new traffic

Delay Analysis

rAverage Delay/Retransmission Interval Pure ALOHASlotted ALOHA

Load

Average Delay

Polling

SlottedALOHA

Pure ALOHA

Half slot time

Walk-time

0 0.18 0.36

Carrier Sense Multiple AccessCollision avoided by sensing line for other users

Three strategies:

1-persistent - Transmit when line sensed idlep-persistent - Transmit with probability p when line sensed idle nonpersistent - random timeout before new attemptProblem: Two terminals transmit within t seconds of one another - collision results

Propagation delay - t secondsTerminal AtransmitsTerminal Btransmits

e2t-eContentionIntervalDuration of Conflict

Arrival RateThroughputa=1a=.1a=.01a=.001a=0nonpersistent CSMAa=round trip delaymessage transmission time

Ethernet Protocol

CSMA/CD Carrier Sense Multiple Accesswith Collision Detection

line length 1 km t = 5 m sec a = .01 and 10 Mbps rateB=5000Same technique with optical fiber?R=1 Gbps a=t/(B/R)=1 Reduced efficiency

Polling model appropriate

CentralProcessorBroadcast PollsBridgePhysical TreeLogical Tree000 001 010 011100 101 110 1110 10 0 01 10 11

0 10 0 01 10 11Terminalwithmessage011ExamplePolling - eight stepsProbing - four stepsGeneral result for light loadingPolling - N stepsProbing - log N steps

0 10 0 01 10 11011Contention Resolutionin Random Access101

Code Division Multiple Access (pages 271-275)User data#1User data#2

User data#N

Code seq #1Code seq #2Code seq #NCHANNELCode seq #1Code seq #2Code seq #NUser data#1User data#2User data#N

Two kinds of code sequencesDirect sequenceTCTB1/TB =Data rate1/TC =Chip rateProcessing gain= TB /TC Hard to generatefast sequences

Frequency HoppingTCTB1/TB =Data rate1/TC =Chip rateProcessing gain= TB /TC

Orthogonality of Code SequencesCode sequence ACode sequence B

Average delay(sec)

r

FDMA

TDMA

Polling

Star CouplerWavelength Division Multiple Access(WDMA)Tuneable Transmittersand/or ReceiversAll opticalno electronicbottleneck

802.1

Introduction and Interface Primatives

802.2

Upper Part Data Link Layer

802.3

CSMA/CD

802.4

Token Bus

802.5

Token Ring

802.6

DQDB Distributed Queue Dual Bus

802.3 - Derived from Ethernet

Name

Cable

Max seg

Nodes/Seg

Advantages

10 base5

Thick

Coax

500m

100

Good for backbones

10base10

Thin

Coax

200m

30

Cheapest

10 baseT

Twisted

Pair

100m

1024

Easy mainentance

10 baseF

Optical

Fiber

2000m

1024

Good between buildings

10base10

CoreVapire tapCore10base5Connector10baseTHubPhysicalArrangementsTwisted pairscoax

TopologiesCable snakingthrough offices

Backbone up an elevator shaft

TreeRepeaterGreatest length=2.5 km and four repeatersroundtrip delay=51.2 m seconds = 512 bit times

Preamble DestnAddrssSourceAddrssDataBytes 7 1 2 or 6 2 or 6 2 0-1500 0-46 4PadCksm802.3 Frame Format Start of frame delimiterLength ofdata fieldDotting patternAddressing - broadcast, multicast, local, globalMinimum frame length - 64 bytes

Binary Exponential Backoff AlgorithmInitial transmissioncollisionslot time= 51.2 m seconds = 512 bit timescollisionretransmissionretransmissionProcess continues to ten retransmissions-1023 slots

Efficiencyk statons with messages each transmits in a slot with prob Pslot = 2t=2line length(km)/propagation speed(km/sec)=2L/c

A=P(one successful)=P(only one transmits)=kp(1-p)k-1 If p=1/k A is maximum (dA/dP=0) Amax=e-1 for k>>1

P(j slot in contention interval)=A(1-A)j-1 Average number of slots in a contention interval=1/AAverage duration contention interval=2t/A=< 2t e=5.4 tP=average frame transmission time =average length(bits)/line rate(bps)=F/B

Efficiency=P/(P+ 2t/A)=1/(1+2BLe/cF)

802.4 Token BusLogical ringDirection of token motion75 ohm broadband coaxSpeeds - 1,5 and 10 Mbps

Four priority classes - 0(lowest), 2, 4, 6 (highest)Token passing sequenceClass 6 stationsClass 4 stationsClass 2 stationsClass 0 stationsStart

Preamble DestnAddrssSourceAddrssDataBytes 1 1 1 2 or 6 2 or 6 0-8182 4 1Cksm802.4 Frame Format Start of frame delimiterLength ofdata fieldframe controlend delimiter

802.5 Token RingStationInterface

Two Modes for InterfaceStationOne bit delayRingInterfaceStationOne bit delayRingInterfaceListenTransmit

Speeds-1, 4 and 16 Mbps

velocity = 1.8 x 105 km/secBits circulating in ringB=N+LR/V=No stations +Line length x bit rate/ Signal velocityPadding to transmit entire token

No restriction onmessage length

Wire CenterWire centerConnectorBypassrelayCable

802.5 Frame Format Starting delimiteraccess control(ACK)SD AC ED 1 1 110 msec frame holding timeToken formatDestnAddrssSourceAddrssDataBytes 1 1 1 2 or 6 2 or 6 no limit 4 1 1CksmLength ofdata fieldframe controlending delimiterSD AC FCED FSframe statusViolations of differentialManchester coding for controlData frame format

Comparison of 802 LANs

Advantages

Disadvantages

802.3

most widely used, simple protocol, easy installation

low delay for light loads

analog, probabilistic limited line length, no priorities

802.4

reliable, priorities, more deterministic, good perform at high load

complex protocol,

not suited to opticalfiber

802.5

any transmission medium, priorities, unlimited message length

control monitor,delay at low load

Load

Average Delay

Polling

SlottedALOHA

Pure ALOHA

Half slot time

Walk-time

0 0.18 0.36

Bus ABus BData flowHead endData flowMAN-Distributed Queue Dual Bus(DQDB)IEEE 802.6

Distributed Queue ProtocolStation with a message for station to right(left) sendsrequest left(right) and puts message on internal stack forright(left)Each station counts requests from right(left) and increases the right(left)stack by one for eachEach station counts empty frames from left(right) anddecreases right(left)stack by oneTransmit message when it comes to head of line

LLCMACNetworklayerDatalinklayerPhysicallayerPacketPacketLLCPacketLLCMACNetwork802.2-Logical link controlService options:unreliable datagramACKed datagramconnection-oriented

BBBBWorkstationsFileserversBackboneLANBridgeCluster on single LANBridges

Reasons for BridgesConnect different kinds of LANs in an organizationConnect dispersed LANsHandle growth by splitting LANsIncreasing distance constraintsReliabilitySecurity

Host AHigher levels

Network

LCC

MAC

PhysicalHost BHigher levels

Network

LCC

MAC

Physical802.3802.4MAC

PhysicalMAC

PhysicalBridgePktPath

802.3802.4802.5PreambleStart limiterAccess controlFrame controladdressesLengthDataPadChcksmEnd delimiterFrame statusProblem for Bridges - different formats

Transparent or Spanning tree Bridges

Flooding and backward learningSpanning tree topology prevents loopingsub-optimum use of bandwidth Source Routing Bridgesdiscovery framesDestinationaddresses

Comparison Transparent and Source Routing BridgesIssue Transparent Source Routing

Orientation

Connectionless

Connection

oriented

Transparency

Fully

Not transparent

Configuration

Automatic

Manual

Routing

Suboptimal

Optimal

Locating

Backward learning

Discovery frames

Failures

Handled by bridges

Handled by hosts

Complexity

In Bridges

In Hosts

High-speed LANsFDDI-Fiber Distributed Data InterfaceFast Ethernet(802.3u)HIPPI - High-Performance Parallel InterfaceFibre Channel

FDDIHigh-speed optical ring100 Mbps200 km1000 stationsMultimode fiberLEDs

FDDI - II PCM voiceISDN traffic

TopologyFailure modeAAABB

DestnAddrssSourceAddrssDataBytes >7 1 1 2 or 6 2 or 6 no limit 4 1 1CksmLength ofdata fieldframe controlending delimiterED FSframe statusStartdelimiterFDDI FrameBlocks of 5 bits16 data symbols3 delimiter symbols2 control symbols3 hardware signaling8 spareSynchronous frames every 125 m sec - 96 bytes of data each