外文翻译--计算机网络.doc

平安实业有限公司网络设计方案第1页共6页COMPUTERNETWORKSDATECOMMUNICATIONSOneofthefunctionsofthehostcomputeristostoretheapplicationsprogramsforthevarioussecondaries.Theendequipmentwhicheithergeneratesthedigitalinformationfortransmissionorusesthereceiveddigitaldatacanbecomputer,printers,keyboards,CRTs,andsoon.Thisequipmentgenerallymanipulatesdigitalinformationinternallyinwordunitsallthebitsthatmakeupawordinaparticularpieceofequipmentaretransferredinparallel.Digitaldata,whentransmitted,areinserialform.Paralleltransmissionofan8-bitwordrequireeightpairsoftransmissionlinesnotatallcost-effective.Dataterminal(DTE)isageneralphraseencompassingallofthecircuitrynecessarytoperformparallel-to-serialandserial-to-parallelconversionsfortransmissionandreceptionrespectivelyandfordatalinkmanagement.TheUART(UniversalAsynchronousReceiver/Transmitter)andUSART(UniversalAsynchronous/AsynchronousReceiver/Transmitter)arethedevicesthatperformtheparallel-to-serialandserial-to-parallelconversions.TheprimaryDTEincludesalinecontrolunit(LCUorLinCo)whichcontrolstheflowofinformationinamultipointdatalinksystem.Astationcontroller(STACO)isthecorrespondingthatbelongedtothesubscriberinadatalinksystem.BetweentheDTEs,startingwiththemodems,wascommunicationsequipmentownedandmaintainedbyTelcoproperty.Datacommunicationsequipment(DCE)acceptstheserialdatastreamfromtheDTEandconvertsittosomeformofanalogsignalsuitablefortransmissiononvoice-gradelined.Atthereceiveend,theDCEperformsthereversefunctionofconvertingthereceivedanalogsignaltoaserialdigitaldatastream.ThesimplestformofDCEisamodem(modulator/demodulator)ordataset.Atthetransmitend,themodemcanbeconsideredaformofdigital-to-analogconverter,whileatthereceiveend,itcanconsideredaformofanalog-to-digitalconverter.Themostcommonofmodulationbymodemsarefrequencyshift平安实业有限公司网络设计方案第2页共6页keying(FSK),phaseshiftkeying(PSK),andquadratureamplitudemodulation(QAM).Thisisatypicallydatatransmissionmodeusingtheanalogtelephonelines.Ifyoutransmitdatabydigitalchannel(sometimesitiscalled“DigitalT-carrier”),apulseCodeModulation(PCM)equipmentmustbeused.Amicrowavetransmissionsystemcanalsobeusedforthedatacommunication.Finally,youcanusethesatellitecommunicationsystemfordatatransmission.IfthecablesandsignallevelsusedtointerconnecttheDTEandDCEwereleftunregulated,thevariationsgeneratedwouldprobablybeproportionaltothenumberofmanufacturers.ElectronicsindustriesAssociation(EIA),anorganizationofmanufactureswithestablishingtheDTEandmodem.Thisisa25-pincablewhosepinshavedesignatedfunctionsandspecifiedsignallevels.TheRS-232Cisanticipatedtobereplacedbyanupdatestandard.ARCHITECTUREOFCOMPUTERNETWORKSComputernetworkisacomplexconsistingoftwoormoreconnedcomputingunits,itisusedforthepurposeofdatacommunicationandresourceresourcesharing.Designofanetworkanditslogicalstructureshouldcomplywithasetofdesignprinciples,includingtheorganizationoffunctionsandthedescriptionofdataformatsandprocedure.Thisisthenetworkarchitectureandsocalledasasetoflayersandprotocols,becausethearchitectureisalayer-based.Inthenexttwosectionswewilldiscusstwoimportantnetworkarchitectures,theOSIreferencemodelandtheTCP/IPreferencemodel.1.TheOSIReferenceModelTheOSImodelisshowninFig.14-2(minusthephysicalmedium).ThismodelisbasedonaproposaldevelopedbytheInternationalStandardsOrganizations(OSI)asthefirststeptowardinternationalstandardizationoftheprotocolsusedinthevariouslayers.ThemodeliscalledtheISOOSI(OpenSystemInterconnection)ReferenceModelbecauseitdealswithconnectingopensystems-thatis,systemsthatareopenforcommunicationwithothersystems,WewillusuallyjustcallittheOSImodelforshort.平安实业有限公司网络设计方案第3页共6页TheOSImodelhassevenhassevenlayers.NotethattheOSImodelitselfisnotanetworkarchitecturebecauseitdoesnotspecifytheexactservicesandprotocolstobeusedineachlayer.Itjusttellswhateachlayershoulddo.However,However,ISOhasalsoproducedstandardsforallthelayers,althoughthesearenotpartofthereferencemodelitself.Eachonehasbeenpublishedasaseparateinternationalstandard.2.TheTCP/IPReferenceModelTheTCP/IPreferencemodelisanearlytransportprotocolwhichwasdesignedbytheUSDepartmentofDefence(DOD)aroundin1978.ItisoftenclaimedthatitgaverisetheOSI“connectionless”modeofoperation.TCP/IPisstillusedextensivelyandiscalledasaindustrialstandardofinternetworkinfact,TCP/IPhastwoparts:TCPandIP.TCPmeansitisonthetransportlayerandIPmeansitisonthenetworklayerseparately.(1)Therearetwoend-to-endprotocolsinthetransportlayer,oneofwhichisTCP(TransmissionControlProtocol),anotherisUDP(UserDatagramProtocol).TCPisaconnection-orientedprotocolthatallowsabytestreamoriginatingononemachinetobedeliveredwithouterroronanyothermachineintheinternet.UDPisanunreliable,connectionlessprotocolforapplicationthatdonotwantTCPssequencingofflowscontrolflowcontrolandwishtoprovidetheirown.(2)ThenetworklayerdefinesanofficialpacketformatandprotocolcalledIP(Internetprotocol).ThejobofthenetworklayeristodeliverIPpacketswheretheyaresupposedtogo.TheTCP/IPReferenceModelisshowninFig.14.3.Ontopofthetransportlayeristheapplicationlayer,Itcontainsallthehigher-levelprotocols.Theearlyonesincludedvirtualterminal(TELNET),filetransfer(FTP),electronicmail(SMTP)anddomainnameservice(DNS).WIDEAREANETWORKAwideareanetwork,orWAN,spansalargegeographicalarea,oftenacountryorcontinent.Itcontainsacollectionofmachinesintendedforrunninguser(i.e.,application)programs.Wewillfollowtraditionalusageandcallthesemachineshosts.Bya平安实业有限公司网络设计方案第4页共6页communicationsubnet,orjustsubnetforshort.Thejobofthesubnetistocarrymessagesfromhosttohost,justasthetelephonesystemcarrieswordsfromspeakertolistener.Byseparatingthepurecommunicationaspectsofthenetwork(thesubnet)fromtheapplicationaspects(thehosts),thecompletenetworkdesignisgreatlysimplified.RelationbetweenhostsandthesubnetisshowninFig.14-4.Oneofmanymethodsthatcanbeusedtocategorizewideareanetworksiswithrespecttotheflowofinformationonatransmissionfacility.Ifweusethismethodtocategorizewideareanetworks,wecangroupthemintothreebasictypes:circuitswitched,leasedlineandpacketswitched.1.CIRCUITSWITCHEDNETWORKSThemostpopulartypeofnetworkandtheonealmostallreadersuseonadailybasisisacircuitswitchednetwork.Thepublicswitchedtelephonenetwork,however,isnotlimitedtothetelephonecompany,Bypurchasingappropriateswitchingequipment,anyorganizationcanconstructtheirowninternalcircuitswitchednetworkand,ifdesired,provideoneormoreinterfacestothepublicswitchednetworktoallowvoiceanddatatransmissiontoflowbetweenthepublicnetworkandtheirprivateinternalnetwork2LEASEDLINENETWORKSThisisadedicatednetworkconnectedbyleasedlines.Leasedlineisacommunicationslinereservedforth