OSI参考模型-英文ppt课件.ppt

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1、OpenSystemsInterconnectionReferenceModel(1984),orsimplytheOSImodel.通信网络ISO-OSI七层模型Analogy-2PhilosophersexampleOneEnglishspeakingphilosopherwishestosendamessagetoacolleagueinFrance(Peersinlayer3)Sincetheyhavenocommonlanguage,theybothhireatranslator.Thetranslators(Pee

2、rsinlayer2)decidewhichlanguagetheywilltranslateto.Thetranslatorssendtheirmessagetotheirsecretary(PeersinLayer1)whomaydecidetosendthemessagebyfax,oremailortelephone(Layer1protocol)Notethatlogicallyeachlayercommunicateswithitspeer.Inpractice,eachlayercommunicateswitht

3、helayersbelowthemAnalogy-ThingstorememberMessageispassedfromlayer3downthroughlayers2and1beforetransmissionAttheoppositeside,themessagetravelsupthroughlayer1and2beforearrivingatlayer3Layer3doesnotcarehowthemessageistransformedorhowitistransmittedbythelowerlayersLayer

4、2isonlyconcernedwithtransformingthemessage(i.e.translation)anddoesnotconcernitselfwithhowthemessageistransmittedLayer1isonlyconcernedwithtransmittingthemessageanddoesnotconcernitselfwiththecontentsofthemessageLayeringDivideataskintopiecesandthensolveeachpieceindepen

5、dently(ornearlyso).Establishingawelldefinedinterfacebetweenlayersmakesportingeasier.MajorAdvantages:CodeReuseExtensibilityProtocolsUsedbyNetworkmodellayersSetsofrulestodefinehowtocommunicateateachlayerandhowtointerfacewithadjacentlayersreceiversenderLayerNLayerN-1La

6、yerN+1LayerNLayerN-1LayerN+1KeyElementsofaProtocolSyntaxDataformatsSignallevelsSemanticsControlinformationErrorhandlingTimingSpeedmatchingSequencingProtocolArchitecturesandNetworksLayeringExample:FederalExpressLetterinenvelope,addressonoutsideFedXguyaddsaddressingin

7、formation,barcode.Localofficedrivestoairportanddeliverstohub.Sentviaairplanetonearestcity.DeliveredtorightofficeDeliveredtorightpersonLetterFedXLayersLetterAddressedEnvelopeAddressedEnvelopeLayeredSoftwareSystemsNetworksoftwareOperatingsystemsWindowingsystemsUnixisa

8、LayeredSystemApplicationsLibrariesSystemCallsKernelTheOSIReferenceModelPhysicalLayeristhelowestlayerandisconcernedwithwiringandelectricals