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
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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