【计算机网络课程课件@中科大】3the data link layer

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1、COMPUTERNETWORKSChapter03TheDataLinkLayerShoubaoYangTel:3601540syang@ustc.edu.cnhttp://202.38.64.11/~syangSeptember20062012-8-18ComputerNetworks-031/77Chapter3TheDataLinkLayerPhysical:Describesthetransmissionofrawbitsintermsofmechanicalandelectricalissues.DataLink:Descr

2、ibeshowasharedcommunicationchannelcanbeaccessed,andhowadataframecanbereliablytransmitted.Network:Describeshowroutingistobedone.Mostlyneededinsubnets.2012-8-18ComputerNetworks-032/77Transport:Thehardestone,generallyoffersconnection-orientedaswellasconnectionlessservices,

3、andvaryingdegreesofreliability.Thislayerprovidestheactualnetworkinterfacetoapplications.Application:Containsthestuffthatuserssee:e-mail,remotelogins,theWeb’sexchangeprotocol,etc.Note:We’lljustconcentrateontransmissionissues.ChannelaccessisdiscussedinChapter42012-8-18Com

4、puterNetworks-033/773.1DataLinkLayerDesignIssuesBasicServicesNetworklayerpassesanumberofbits(frame)tothedatalinklayer.Datalinklayerisresponsiblefortransmittingtheframetothedestinationmachine.Receivinglayerpassesreceivedframetoitsnetworklayer.FunctionsoftheDataLinkLay

5、er•Providingawell-definedserviceinterfacetothenetworklayer•Dealingwithtransmissionerrors•Regulatingdataflowsothatslowreceiversarenotswampedbyfastsenders2012-8-18ComputerNetworks-034/77Relationshipamongpacketandframe•Datalinklayertakesthepacketsitgetsfromthenetworklayerand

6、encapsulatesthemintoframesfortransmission.Eachframecontainsaframeheader,apayloadfieldforholdingthepacket,andaframetrailer.Framemanagementformstheheartofwhatthedatalinklayerdoes.2012-8-18ComputerNetworks-035/773.1.1ServicesProvidedtotheNetworkLayer•Datalinklayerisresponsib

7、letotransmitbitsfromsourcetodestination•Threemainjobs:framing,errorcontrol,andflowcontrol(a)Virtualcommunication.(b)Actualcommunication.2012-8-18ComputerNetworks-036/77ServicesProvidedtotheNetworkLayerUnacknowledgedconnectionlessserviceSourcemachinesendsindependentframes

8、tothedestinationwithouthavingthedestinationacknowledgethem.Noconnectionisestablishedbeforehandor