計算機(jī)網(wǎng)絡(luò)專業(yè)英語 課件 L18-1 Text 1 OSI Reference Model

L18-1

Lesson18OSIReferenceModelandTCP/IPText1

OSIReferenceModelKeyPoints要點Forshort/beshort

for、inexistence網(wǎng)絡(luò)的七層Above/below、bottom/top、upper/under、up/downProvide/receive透明的理解課文TheOpenSystemsInterconnectionReferenceModel(OSIreferencemodelorOSImodelforshort),releasedin1984,isalayered,abstractdescriptionforcommunicationsandcomputernetworkprotocoldesign.TheOSIreferencemodelistheprimarymodelfornetworkcommunications.AprimaryobjectiveoftheOSIreferencemodelistoacceleratethedevelopmentoffuturenetworkingproducts.Althoughthereareothermodelsinexistence,mostnetworkvendorstodayrelatetheirproductstotheOSIreferencemodel,especiallywhentheywanttoeducateuserstousetheirproducts.Theyconsideritthebesttoolavailabletoteachpeopleaboutsendingandreceivingdataonanetwork.TheOSIreferencemodelincludessevenlayers.Thelayers,describedbelowandshowninFigure18-1are,frombottomtotop:PhysicalLayer,DataLinkLayer,NetworkLayer,TransportLayer,SessionLayer,PresentationLayerandApplicationLayer.Eachlayerillustrates說明aparticularnetworkfunction.Thisseparationofnetworkingfunctionsiscalledlayering.Alayerisacollectionofrelatedfunctions,whichprovidesservicestothelayeraboveitandreceivesservicefromthelayerbelowit.Dividingthenetworkintothesesevenlayersprovidesthefollowingadvantages:Itbreaksnetworkcommunicationintosmaller,simplerpartsthatareeasiertodevelop.Itfacilitatesstandardizationofnetworkcomponentstoallowmultiplevendors’developmentandsupport.Itallowsdifferenttypesofnetworkhardwareandsoftwaretocommunicatewitheachother.Itpreventschangesinonelayerfromaffectingtheotherlayers,sothattheycandevelopmorequickly.Itbreaksnetworkcommunicationintosmallerpartstomakelearningiteasiertounderstand.PhysicalLayer–Isconcernedwithtransmissionofunstructuredbitstreamoverthephysicallink,anditinvokes調(diào)用suchparametersassignalvoltageswingandbitduration.Itdefinesthemechanical,electrical,procedural時序的andfunctionalcharacteristicstoestablish,maintainanddeactivate釋放thephysicallinkbetweenendsystems.DataLinkLayer–Providesreliabletransferofdataacrossthephysicallink.Itsendsblocksofdata(frames)withthenecessarysynchronization,errorcontrolandflowcontrol.NetworkLayer–Providesupperlayerswithindependencefromthedatatransmissionandswitchingtechnologiesusedtoconnectsystems.Itisresponsibleforestablishing,maintainingandterminatingconnections.TransportLayer–Providesreliable,transparenttransferofdatabetweenendpoints端點.Itisresponsibleforend-to-enderrorrecoveryandflowcontrol.SessionLayer–Providesthecontrolstructureforcommunicationbetweenapplications.Itestablishes,managesandterminatesconnections(sessions)betweencooperatingapplications.PresentationLayer–Performstransformationsondatatoprovideastandardizedapplicationinterfaceandtoprovidecommoncommunicationss