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附录1英文及其译文ComputerNetworksNetworkGoalsSomereasonsarecausingcentralizedcomputersystemstogivewaytonetworks.Thefirstoneisthatmanyorganizationsalreadyhaveasubstantialnumberofcomputersinoperation,oftenlocatedfarapart.Initially,eachofthesecomputersmayhaveworkedinisolationfromtheotherones,butatacertaintime,andmanagementmayhavedecidedtoconnectthemtobeabletocorrelateinformationabouttheentireorganization.Generallyspeaking,thegoalistomakeallprograms,data,andotherresourcesavailabletoanyoneonthenetworkwithoutregardtothephysicallocationoftheresourceandtheuser.Thesecondoneistoproviderhighreliabilitybyhavingalternativesourcesofsupply.Withanetwork,thetemporarylossofasinglecomputerismuchlessserious,becauseitsuserscanoftenbeaccommodatedelsewhereuntiltheserviceisrestored.Anotherimportantreasonfordistributingcomputingpowerhastodowiththerelativepriceofcomputingversuscommunication.Nowthecostofasmallcomputerisnegligible,soitbecomesattractivetoanalyzethedataatwhereitiscaptured,andonlytosendoccasionalsummariesbacktothecomputercenter,toreducethecommunicationcost,whichnowrepresentsalargerpercentageofthetotalcostthanitusedto.Yetanotherreasonofsettingupacomputernetworkisthatacomputernetworkcanproviderapowerfulcommunicationmediumamongwidelyseparatedpeople.ApplicationofNetworksOneofthemainareasofpotentialnetworkuseisaccesstoremotedatabases.Itmaysomedaybeeasyforpeoplesittingattheirterminalsathometomakereservationsforairplanes,trains,bused,boats,restaurants,theaters,hotels,andsoon,atanywhereintheworldwithinstantconfirmation.Homebanking,automatednewspaperandfullyautomatedlibraryalsofallinthiscategory.Computeraidededucationisanotherpossiblefieldforusingnetwork,withmanydifferentcoursedbeingoffered.Teleconferencingisawholenewformofcommunication.Withit,widelyseparatedpeoplecanconductameetingbytypingmessagesattheirterminals.Attendeesmayleaveatwill,andfindoutwhattheymissedwhentheycomeback.Internationalcontactsbyhumanbeingmaybegreatlyenhancedbynetworkbasedcommunicationfacilities.NetworkStructureInanynetworkexistsacollectionofmachinesintendedtorunninguser(i.e.,application)program.Thesemachinesarecalledhosts.Thehostsareconnectedbythecommunicationsubnet.Thejobofthesubnetistocarrymessagesfromhosttohost.Thesubnetconsistsoftwobasiccomponents:nodesandchannels.Nodesarespecializedcomputers.Alltraffictoorfromthehostgoesviaitsnode.Channelsaretransmissionlines.Broadlyspeaking,therearetwogeneraltypesofdesignsforthecommunicationsubnet:(1)Point-to-pointchannels(2BroadcastchannelsInthefirstone,thenetworkcontainsnumerouscablesorleasedtelephonelines,eachoneconnectingapairofnodes.Iftwonodesthatdonotshareacablewishtocommunicate,theymustdothisindirectlyviaothernodes.Whenamessageissentfromonenodetoanotherviaoneormoreintermediatenodes,eachintermediatenodewillreceivethemessageandstoreituntiltherequiredoutputlineisfreesothatitcantransmitthemessageforward.Thesubnetusingthisprincipleiscalledapoint-to-pointorstore-and-forwardsubnet.Whenapoint-to-pointsubsetisused,theimportantproblemishowtodesigntheinterconnectedtopologybetweenthenodes.Thesecondkindofcommunicationarchitectureusesbroadcasting.Inthisdesignthereisasinglecommunicationchannelsharedbyallnodes.Theinherenceinbroadcastsystemisthatmessagessentbyanynodearereceivedbyallothernodes.ProtocolHierarchiesToreducetheirdesigncomplexity,mostnetworksareorganizedasaseriesoflayersorlevels,eachonebuiltuponitspredecessor.Layernononemachinecarriesonaconversationwithlayernonanothermachine.Therulesandconventionsusedinthisconversationarecollectivelycalledthelayernprotocol.Inreality,nodataaredirectlytransferredfromlayernononemachinetolayernonanothermachine(exceptinthelowestlayer).Instead,eachlayerpassesdataandcontrolinformationtothefollowinglayerimmediately,untilthelowestlayerisreached.Atthelowestlayerthereisonephysicalcommunicationwiththeothermachine,asopposedtothevirtualcommunicationusedbythehigherlayers.Betweeneachpairofadjacentlayersthereisaninterface.Theinterfacedefineswhichprimitiveoperationsandservicesthelowerofferstotheupperone.Thesetoflayersandprotocolsiscallednetworkarchitecture.DesignIssuesfortheLayersOnesetofdesigndecisionsaretherulesfordatatransfer.Dodataonlytravelinonedirection,calledsimplexcommunication,ortravelineitherdirection,butnotsimultaneously,calledhalf-duplexcommunication,ortravelinbothdirectionsatonce,callfull-duplexcommunicative?Errorcontrolisanimportantissuewhenthephysicalcommunicationcircuitsarenotperfect.Manyerror-detectinganderror-correctingcodesareknown,butbothendsoftheconnectionmustagreeonwhatkindofcodeisbeingused.Inaddition,thereceivermusthavesomewayoftellingthesenderwhichmessageshavebeencorrectlyreceivedandwhichhasnot.Whentherearemultiplepathsbetweensourceanddestination,atsomepointsinthehierarchy,aroutingdecisionmustbemade.TheISOReferenceModelTheReferenceModelofOpenSystemInterconnection(OSI),asISOcallsit,hassevenlayers.Themajoronesoftheprinciples,fromwhichISOappliedtogetthesevenlayers,areasfollows:(1)Alayershouldbecreatedwhereadifferentlevelofabstractionsisneeded.(2)Eachlayershouldperformawelldefinedfunction.