【计算机网络课程课件@中科大】01-2introduction

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1、COMPUTERNETWORKSChapter01Introduction(2)ShoubaoYangComputerScienceDepartmentTel:3601540E-mail:syang@ustc.edu.cnhttp://202.38.64.11/~syang2012-8-18ComputerNetworks-1-Part21/441.4ReferenceModels1.4.1OSIReferenceModel1983,InternationalStandardsOrganization(ISO)p

2、roposedthe“ISOOSI(OpenSystemsInterconnection)ReferenceModel”,OSI-rmTheprinciplesoflayeringAlayershouldbecreatedwhereadifferentabstractionisneededEachlayershouldperformawell-definedfunctionThefunctionofeachlayershouldbechosentowardsdefininginternationallyst

3、andardizedprotocolsThelayerboundariesshouldbechosentominimizetheinformationflowacrosstheinterfacesThenumberoflayersshouldbelargeenoughthatdistinctfunctionsandsmallenoughthatthearchitecturedoesnotbecomeunwieldy2012-8-18ComputerNetworks-1-Part22/44TheOSIModel2

4、012-8-18ComputerNetworks-1-Part23/44ThePhysicalLayerEssence:Describesthetransmissionofrawbitsintermsofmechanicalandelectricalissues:Example:Connecttwocomputersbymeansofawire:Setting-3Vto-12Vonthewirecorrespondstoabinary1;+3Vto+12Visabinary0Thewireisnottobe

5、longerthan15metersYoumaychangethevoltageatmost20,000timespersecond(Question:what’sthetransferrate?)2012-8-18ComputerNetworks-1-Part24/44TheDataLinkLayerDatalinklayeristotransformarawtransmissionfacilityintoalinethatappearsfreeofundetectedtransmissionerrorsto

6、thenetworklayerObservation:Weneedtoatleastdetectbittransmissionerrors,sendbitsinframesthataddredundancytodetectsomethingwentwrongExamples:Addaparitybittoevery7transmittedbits:1saystherewereoddnumberof1’s;0saystherewereanevennumberof1’sAddachecksum(cyclicredun

7、dancycheck)thatshouldmatchthebitsbeforeitAlso:Providethemechanismssothatfastsendersdon’toverwhelmslowreceivers(flowcontrol)2012-8-18ComputerNetworks-1-Part25/44OSIDataLinkLayerObservation:Wealsoneedtospecifyhowanumberofcomputerscanshareacommonchannel(e.g.wire

8、),thatismediumaccesscontrolsublayer(MAC):1.Specifieshowoneoutofseveralcompetingsenders,iseventuallyallowedexclusiveaccesstothewire2.Commonapproach1:listentoeachother;retreatwhenyou