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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.Paralleltransmissionofan8bitwordrequireeightpairsoftransmissionlinesnotatallcosteffective.DataterminalDTEisageneralphraseencompassingallofthecircuitrynecessarytoperformparalleltoserialandserialtoparallelconversionsfortransmissionandreceptionrespectivelyandfordatalinkmanagement.TheUARTUniversalAsynchronousReceiver/TransmitterandUSARTUniversalAsynchronous/AsynchronousReceiver/Transmitterarethedevicesthatperformtheparalleltoserialandserialtoparallelconversions.TheprimaryDTEincludesalinecontrolunitLCUorLinCowhichcontrolstheflowofinformationinamultipointdatalinksystem.AstationcontrollerSTACOisthecorrespondingthatbelongedtothesubscriberinadatalinksystem.BetweentheDTEs,startingwiththemodems,wascommunicationsequipmentownedandmaintainedbyTelcoproperty.DatacommunicationsequipmentDCEacceptstheserialdatastreamfromtheDTEandconvertsittosomeformofanalogsignalsuitablefortransmissiononvoicegradelined.Atthereceiveend,theDCEperformsthereversefunctionofconvertingthereceivedanalogsignaltoaserialdigitaldatastream.ThesimplestformofDCEisamodemmodulator/demodulatorordataset.Atthetransmitend,themodemcanbeconsideredaformofdigitaltoanalogconverter,whileatthereceiveend,itcanconsideredaformofanalogtodigitalconverter.Themostcommonofmodulationbymodemsarefrequencyshift平安实业有限公司网络设计方案第2页共6页keyingFSK,phaseshiftkeyingPSK,andquadratureamplitudemodulationQAM.Thisisatypicallydatatransmissionmodeusingtheanalogtelephonelines.IfyoutransmitdatabydigitalchannelsometimesitiscalledDigitalTcarrier,apulseCodeModulationPCMequipmentmustbeused.Amicrowavetransmissionsystemcanalsobeusedforthedatacommunication.Finally,youcanusethesatellitecommunicationsystemfordatatransmission.IfthecablesandsignallevelsusedtointerconnecttheDTEandDCEwereleftunregulated,thevariationsgeneratedwouldprobablybeproportionaltothenumberofmanufacturers.ElectronicsindustriesAssociationEIA,anorganizationofmanufactureswithestablishingtheDTEandmodem.Thisisa25pincablewhosepinshavedesignatedfunctionsandspecifiedsignallevels.TheRS232Cisanticipatedtobereplacedbyanupdatestandard.ARCHITECTUREOFCOMPUTERNETWORKSComputernetworkisacomplexconsistingoftwoormoreconnedcomputingunits,itisusedforthepurposeofdatacommunicationandresourceresourcesharing.Designofanetworkanditslogicalstructureshouldcomplywithasetofdesignprinciples,includingtheorganizationoffunctionsandthedescriptionofdataformatsandprocedure.Thisisthenetworkarchitectureandsocalledasasetoflayersandprotocols,becausethearchitectureisalayerbased.Inthenexttwosectionswewilldiscusstwoimportantnetworkarchitectures,theOSIreferencemodelandtheTCP/IPreferencemodel.1.TheOSIReferenceModelTheOSImodelisshowninFig.142minusthephysicalmedium.ThismodelisbasedonaproposaldevelopedbytheInternationalStandardsOrganizationsOSIasthefirststeptowardinternationalstandardizationoftheprotocolsusedinthevariouslayers.ThemodeliscalledtheISOOSIOpenSystemInterconnectionReferenceModelbecauseitdealswithconnectingopensystemsthatis,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/IPreferencemodelisanearlytransportprotocolwhichwasdesignedbytheUSDepartmentofDefenceDODaroundin1978.ItisoftenclaimedthatitgaverisetheOSIconnectionlessmodeofoperation.TCP/IPisstillusedextensivelyandiscalledasaindustrialstandardofinternetworkinfact,TCP/IPhastwopartsTCPandIP.TCPmeansitisonthetransportlayerandIPmeansitisonthenetworklayerseparately.1Therearetwoendtoendprotocolsinthetransportlayer,oneofwhichisTCPTransmissionControlProtocol,anotherisUDPUserDatagramProtocol.TCPisaconnectionorientedprotocolthatallowsabytestreamoriginatingononemachinetobedeliveredwithouterroronanyothermachineintheinternet.UDPisanunreliable,connectionlessprotocolforapplicationthatdonotwantTCPssequencingofflowscontrolflowcontrolandwishtoprovidetheirown.2ThenetworklayerdefinesanofficialpacketformatandprotocolcalledIPInternetprotocol.ThejobofthenetworklayeristodeliverIPpacketswheretheyaresupposedtogo.TheTCP/IPReferenceModelisshowninFig.14.3.Ontopofthetransportlayeristheapplicationlayer,Itcontainsallthehigherlevelprotocols.TheearlyonesincludedvirtualterminalTELNET,filetransferFTP,electronicmailSMTPanddomainnameserviceDNS.WIDEAREANETWORKAwideareanetwork,orWAN,spansalargegeographicalarea,oftenacountryorcontinent.Itcontainsacollectionofmachinesintendedforrunninguseri.e.,applicationprograms.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.Byseparatingthepurecommunicationaspectsofthenetworkthesubnetfromtheapplicationaspectsthehosts,thecompletenetworkdesignisgreatlysimplified.RelationbetweenhostsandthesubnetisshowninFig.144.Oneofmanymethodsthatcanbeusedtocategorizewideareanetworksiswithrespecttotheflowofinformationonatransmissionfacility.Ifweusethismethodtocategorizewideareanetworks,wecangroupthemintothreebasictypescircuitswitched,leasedlineandpacketswitched.1.CIRCUITSWITCHEDNETWORKSThemostpopulartypeofnetworkandtheonealmostallreadersuseonadailybasisisacircuitswitchednetwork.Thepublicswitchedtelephonenetwork,however,isnotlimitedtothetelephonecompany,Bypurchasingappropriateswitchingequipment,anyorganizationcanconstructtheirowninternalcircuitswitchednetworkand,ifdesired,provideoneormoreinterfacestothepublicswitchednetworktoallowvoiceanddatatransmissiontoflowbetweenthepublicnetworkandtheirprivateinternalnetwork2.LEASEDLINENETWORKSThisisadedicatednetworkconnectedbyleasedlines.Leasedlineisacommunicationslinereservedfortheexclusiveuseofaleasingcustomerwithoutinterexchangeswitchingarrangements.Leasedorprivatelinesarededicatedtotheuser.Thisadvantageisthattheterminalorcomputerisaalwaysphysicallyconnectedtotheline.Veryshortresponsetimesaremetwithservice.3.PACKETSWITCHINGNETWORKSApacketnetworkisconstructedthroughtheuseofequipmentthatassemblesanddisassemblespackets,equipmentthatroutespacket,andtransmissionfacilitiesusedtoroutepacketsfromtheoriginatortothedestinationdevice.SometypesofdataterminalequipmentDTEcancreatetheirownpackets,whileothertypesofDTErequiretheconversionoftheirprotocolintopacketsthroughtheuseofapacketassembler/disassemblePAD.Packetsareroutedthroughthenetworkbypacketswitches.Packetswitchesexaminethedestinationof
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