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1、4TheMediumAccessControlSublayerHighLightsChannelallocationalgorithmsALOHA,S-ALOHA,CSMA,CSMA/CD,Collision-FreeProtocols:bit-map,binarycountdownLimited-ContentionProtocolsTokenLAN:IEEE802.3,HIGH-SPEEDLANS(802.3u)GigabitEthernet(802.3z)IEEE802.11(WLAN)LLC(
2、802.2)BRIDGE:transparentbridgeREPEATER,HUB,BRIDGE,SWITCH,ROUTERVLAN4.1TheChannelAllocationProblemM/M/1SystemM/M/1SystemincomingnumberisKinperiodTλ:meanarrivalrate(frames/sec)1/μ:meanframelength(bits/frame)C:datatransmissionrate,channelcapacity(bits/sec)
3、,theservicerateisµCframes/secmeantimedelayM/M/1Exampleλ=200μC=250T=1/(250-200)=20msTheChannelAllocationProblemStaticChannelAllocationinLANsandMANsDynamicChannelAllocationinLANsandMANsStaticChannelAllocationStaticChannelAllocationDividethesinglechannelin
4、toN(TDMorFDM)independentsubchannels,eachchannelcapacitymeanarrivalratemeantimedelayStaticChannelAllocation(2)SinglechannelCis100MbpsMeanframelength,1/µ,is10,000bitsµC=10000frames/secFramearrivalrateis5000frames/secMeandelay=200µsecDividethechannelintoNR
5、eplacethe100-Mbpsnetworkwith10networksof10MbpseachandstaticallyallocateeachusertooneofthemMeandelay=2msecDynamicChannelAllocationClassificationofDynamicChannelAllocationControlledmultipleaccessCentralized:pollingDecentralized:tokenRandommultipleaccessAL
6、OHACSMA,CSMA/CD,CSMA/CAFiveKeyAssumptionsofDynamicChannelAllocationStationModelSingleChannelAssumption.CollisionAssumption(a)ContinuousTime.(b)SlottedTime.(a)CarrierSense.(b)NoCarrierSense.StationModel.Nindependentstations,generatesframesfortransmission
7、Theprobabilityofaframebeinggeneratedinanintervaloflengthtisλt,whereλisaconstant(thearrivalrateofnewframes).Onceaframehasbeengenerated,thestationisblockedanddoesnothinguntiltheframehasbeensuccessfullytransmittedSingleChannelAssumptionAsinglechannelisavai
8、lableforallcommunicationAllstationscantransmitonitandallcanreceivefromitAllstationsareequivalent,althoughprotocolsoftwaremayassignprioritiestothem.CollisionAssumptionIftwoframesaretransmittedsimultaneously,theyoverlapintimeandthe
