网络信息系统cha

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1、Chapter4TheMediumAccessControlSublayer(介质访问子层)ContentTheMediumAccessControlSublayerTheChannelAllocationProblemMultipleAccessProtocolsEthernetWirelessLANsBroadbandWirelessDataLinkLayerSwitching信道分配问题多路访问协议CSMA以太网数据链路层交换无线局域网AbstractNetworkscanbedivided

2、intotwocategoriesthoseusingpoint-to-pointconnectionsthoseusingbroadcastchannels.Thischapterdealswithbroadcastnetworksandtheirprotocols.TheChannelAllocationProblemInanybroadcastnetwork,thekeyissueishowtodeterminewhogetstousethechannelwhenthereiscompeti

3、tionforit.Whenonlyasinglechannelisavailable,determiningwhoshouldgonextismuchharder.Broadcastchannelsaresometimesreferredtoasmultiaccesschannelsorrandomaccesschannels.Wheredoestheissuesolve(1)Theprotocolsusedtodeterminewhogoesnextonamultiaccesschannelb

4、elongtoasublayerofthedatalinklayercalledtheMAC(MediumAccessControl)sublayer.TheMACsublayerisespeciallyimportantinLANs,manyofwhichuseamultiaccesschannelasthebasisforcommunication.Technically,theMACsublayeristhebottompartofthedatalinklayer.Wheredoesthei

5、ssuesolve(2)逻辑链路控制（LLC）TheTypesofChannelAllocationStaticChannelAllocationinLANsandMANsDynamicChannelAllocationinLANsandMANsThetraditionalwayofallocatingasinglechannel,suchasatelephonetrunk,amongmultiplecompetingusersisFrequencyDivisionMultiplexing(FDM

6、).Dividethebandwidthintoequalsizedportionssothateachusercanbeassignedoneportion.Whenthereisonlyasmallandconstantnumberofusers,eachofwhichhasaheavy(buffered)loadoftraffic(e.g.,carriers'switchingoffices),FDMisasimpleandefficientallocationmechanism.Advan

7、tageofStaticChannelAllocationDisadvantageofStaticChannelAllocationIffewerthanusersarecurrentlyinterestedincommunication,someportionsofspectrumwillbewasted.Ifmorethanuserswanttocommunicate,someofthemwillbedeniedpermissionevenifsomeuserswithallocatedfre

8、quencyhardlyevertransmitanything.Eventhenumberofusersisandconstant,whensomeusersarequiescent,nooneelsecanusetheirbandwidthsoitissimplywasted.Forburstydatatraffic(peaktraffictomeantrafficratioof1000:1),theallocatedsmallsubchannelwillbeidlemosto