计算机网络英文版4

Chapter4TheMediumAccessControlSublayerLi-pingShen申丽萍Email:[e-mail]《ComputerNetworkv4》cs.sjtu2004-11-24TheMediumAccessControlSublayerzBroadcastchannels(multiaccesschannelsorrandomaccesschannels):anumberofstationsthatsharethesame“wire.”Ifonestationsends,alltheothersgettohearit.zProblem:ifyou’resharingachannel,thentwoormorestationsmaydecidetostartframetransmissionatthesametime,itwillleadtoframecollision,whichmeansrubbishonthewire.zMediumAccessControl:Allocatethebroadcastchanneltooneofthecompetingstations《ComputerNetworkv4》cs.sjtu2004-11-24TopicszTheChannelAllocationProblemzMultipleAccessProtocolszEthernetzDataLinkLayerSwitching《ComputerNetworkv4》cs.sjtu2004-11-24TheChannelAllocationProblemzStaticChannelAllocationinLANsandMANszDynamicChannelAllocationinLANsandMANs《ComputerNetworkv4》cs.sjtu2004-11-24StaticChannelAllocationzFDM/TDMwitheachuserallocatedequal-sizedfrequency/timeportionissimpleandefficientwhenthenumberofusersissmallandconstant,andeachuserhasaheavytraffic.zItisinefficientifnumberofuserisvariableorwhenthetrafficisbursty.zThemeandelayusingFDMisNtimesworsethanifalltheframeswerearrangedorderlyinabigcentralqueue.interpretfromqueuingtheorycalculation《ComputerNetworkv4》cs.sjtu2004-11-24DynamicChannelAllocationzAllthedynamicchannelallocationmethodstobediscussedarebasedonfivekeyassumptions.zStationModel.zSingleChannelAssumption.zCollisionAssumption.z(a)ContinuousTime.(b)SlottedTime.z(a)CarrierSense.(b)NoCarrierSense.《ComputerNetworkv4》cs.sjtu2004-11-24StationModelzThemodelconsistsofNindependentstations(e.g.,computers,telephones,orpersonalcommunicators)zEachstationhasaprogramoruserthatgeneratesframesfortransmission.zTheprobabilityofaframebeinggeneratedinanintervaloflengthΔtisλΔt,whereλisaconstant(thearrivalrateofnewframes).zOnceaframehasbeengenerated,thestationisblockedanddoesnothinguntiltheframehasbeensuccessfullytransmitted.《ComputerNetworkv4》cs.sjtu2004-11-24SingleChannelAssumptionzAsinglechannelisavailableforallcommunication.zAllstationscantransmitonitandallcanreceivefromit.zAsfarasthehardwareisconcerned,allstationsareequivalent,althoughprotocolsoftwaremayassignprioritiestothem.《ComputerNetworkv4》cs.sjtu2004-11-24CollisionAssumptionzIftwoframesaretransmittedsimultaneously,theywillresultinacollision.zAllstationscandetectcollisions.zAcollidedframemustbetransmittedagainlater.Therearenoerrorsotherthanthosegeneratedbycollisions.《ComputerNetworkv4》cs.sjtu2004-11-24TimeAssumptionzContinuousTime.Frametransmissioncanbeginatanyinstant.Thereisnomasterclockdividingtimeintodiscreteintervals.zSlottedTime.Timeisdividedintodiscreteintervals(slots).Frametransmissionsalwaysbeginatthestartofaslot.Aslotmaycontain0,1,ormoreframes,correspondingtoanidleslot,asuccessfultransmission,oracollision,respectively.《ComputerNetworkv4》cs.sjtu2004-11-24CarrierSenseAssumptionzCarrierSense.Stationscantellifthechannelisinusebeforetryingtouseit.Ifthechannelissensedasbusy,nostationwillattempttouseituntilitgoesidle.zNoCarrierSense.Stationscannotsensethechannelbeforetryingtouseit.Theyjustgoaheadandtransmit.Onlylatercantheydeterminewhetherthetransmissionwassuccessful.《ComputerNetworkv4》cs.sjtu2004-11-24MultipleAccessProtocolszALOHAzCarrierSenseMultipleAccessProtocolszCollision-FreeProtocolszLimited-ContentionProtocols《ComputerNetworkv4》cs.sjtu2004-11-24MultipleAccessMethodszThreestrategiesforchannelallocation:zExercisenocontrolatall:simplyletastationtrytousethechannel,anddosomethingwhenacollisionhappens.Appliedincontentionsystems.zEmployaround-robintechnique:eachstationinturnisallowedtousethechannel.Appliedintoken-basedsystems–thestationthathasthetokenmayusethechannel.zLetastationplaceareservationforthechannel.Usedinslottedsystems.Theproblemishowtomakeareservation.《ComputerNetworkv4》cs.sjtu2004-11-24PureALOHAzPrinciple:letuserstransmitwhenevertheyhavedatatobesent.Therewillbecollisions,ofcourse.Ifacollisionoccurs,finishyourcurrenttransmissionandretryafterarandomamountoftime.zContentionSystems:multipleusersshareacommonchannelinawaythatcanleadtoconflictsInpureALOHA,framesaretransmittedatcompletelyarbitrarytimes.Collisions《ComputerNetworkv4》cs.sjtu2004-11-24ThroughputofPureALOHAzLetNbetheaveragenumberofnewframessubmittedduringaframetimeTframe(timeneededtotransmitafullframe).Poissondistributed.zLetGbetheaveragenumberofoldandnewframesubmissionsduringaframetime(G=N),AlsoPoissondistributed:zLetP0betheprobabilitythatframedoesnotsufferfromcollision=thethroughputS=G*P0zAframewillnotsufferacollisionifnootherframesaresentwithinoneframetimebeforeandafteritsstart(totallytwoframetimeslong)zProbabilitythataframewillnotbedamagedisz《ComputerNetworkv4》cs.sjtu2004-11-24ThroughputofPureALOHAThroughputversusofferedtrafficforALOHAsystems《ComputerNetworkv4》cs.sjtu2004-11-24SlottedALOHAzPrinciple:dataarealwayssentatthebeginningofatimeslot.Ifacollisionoccurs,finishthecurrenttransmissionandretryafterarandomamountoftimeslots.zAframewillnotsufferacollisionifnootherframesaresentatthebeginningofthesameframetime,theprobabilityofnoothertrafficduringthesameslotisp0=