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DATECOMMUNICATIONS

Oneofthefunctionsofthehostcomputeristostoretheapplicationsprogramsforthevarioussecondaries.

Theendequipmentwhicheithergeneratesthedigitalinformationfortransmissionorusesthereceiveddigitaldatacanbecomputer,printers,keyboards,CRTs,andsoon.Thisequipmentgenerallymanipulatesdigitalinformationinternallyinwordunits—allthebitsthatmakeupawordinaparticularpieceofequipmentaretransferredinparallel.Digitaldata,whentransmitted,areinserialform.Paralleltransmissionofan8-bitwordrequireeightpairsoftransmissionlines—notatallcost-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.Themostcommonofmodulationbymodemsarefrequencyshiftkeying（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.

ARCHITECTUREOFCOMPUTERNETWORKS

Computernetworkisacomplexconsistingoftwoormoreconnedcomputingunits,itisusedforthepurposeofdatacommunicationandresourceresourcesharing.Designofanetworkanditslogicalstructureshouldcomplywithasetofdesignprinciples,includingtheorganizationoffunctionsandthedescriptionofdataformatsandprocedure.Thisisthenetworkarchitectureandsocalledasasetoflayersandprotocols,becausethearchitectureisalayer-based.

Inthenexttwosectionswewilldiscusstwoimportantnetworkarchitectures,theOSIreferencemodelandtheTCP/IPreferencemodel.

1.TheOSIReferenceModel

TheOSImodelisshowninFig.14-2（minusthephysicalmedium）.ThismodelisbasedonaproposaldevelopedbytheInternationalStandardsOrganizations（OSI）asthefirststeptowardinternationalstandardizationoftheprotocolsusedinthevariouslayers.ThemodeliscalledtheISOOSI（OpenSystemInterconnection）ReferenceModelbecauseitdealswithconnectingopensystems--thatis,systemsthatareopenforcommunicationwithothersystems,WewillusuallyjustcallittheOSImodelforshort.

TheOSImodelhassevenhassevenlayers.NotethattheOSImodelitselfisnotanetworkarchitecturebecauseitdoesnotspecifytheexactservicesandprotocolstobeusedineachlayer.Itjusttellswhateachlayershoulddo.However,However,ISOhasalsoproducedstandardsforallthelayers,althoughthesearenotpartofthereferencemodelitself.Eachonehasbeenpublishedasaseparateinternationalstandard.

2.TheTCP/IPReferenceModel

TheTCP/IPreferencemodelisanearlytransportprotocolwhichwasdesignedbytheUSDepartmentofDefence（DOD）aroundin1978.ItisoftenclaimedthatitgaverisetheOSI“connectionless”modeofoperation.TCP/IPisstillusedextensivelyandiscalledasaindustrialstandardofinternetworkinfact,TCP/IPhastwoparts:

TCPandIP.TCPmeansitisonthetransportlayerandIPmeansitisonthenetworklayerseparately.

（1）Therearetwoend-to-endprotocolsinthetransportlayer,oneofwhichisTCP（TransmissionControlProtocol）,anotherisUDP（UserDatagramProtocol）.TCPisaconnection-orientedprotocolthatallowsabytestreamoriginatingononemachinetobedeliveredwithouterroronanyothermachineintheinternet.UDPisanunreliable,connectionlessprotocolforapplicationthatdonotwantTCP’ssequencingofflowscontrolflowcontrolandwishtoprovidetheirown.

（2）ThenetworklayerdefinesanofficialpacketformatandprotocolcalledIP（Internetprotocol）.ThejobofthenetworklayeristodeliverIPpacketswheretheyaresupposedtogo.

TheTCP/IPReferenceModelisshowninFig.14.3.Ontopofthetransportlayeristheapplicationlayer,Itcontainsallthehigher-levelprotocols.Theearlyonesincludedvirtualterminal（TELNET）,filetransfer（FTP）,electronicmail（SMTP）anddomainnameservice（DNS）.

WIDEAREANETWORK

Awideareanetwork,orWAN,spansalargegeographicalarea,oftenacountryorcontinent.Itcontainsacollectionofmachinesintendedforrunninguser（i.e.,application）programs.Wewillfollowtraditionalusageandcallthesemachineshosts.Byacommunicationsubnet,orjustsubnetforshort.Thejobofthesubnetistocarrymessagesfromhosttohost,justasthetelephonesystemcarrieswordsfromspeakertolistener.Byseparatingthepurecommunicationaspectsofthenetwork（thesubnet）fromtheapplicationaspects（thehosts）,thecompletenetworkdesignisgreatlysimplified.RelationbetweenhostsandthesubnetisshowninFig.14-4.

Oneofmanymethodsthatcanbeusedtocategorizewideareanetworksiswithrespecttotheflowofinformationonatransmissionfacility.Ifweusethismethodtocategorizewideareanetworks,wecangroupthemintothreebasictypes:

circuitswitched,leasedlineandpacketswitched.

1.CIRCUITSWITCHEDNETWORKS

Themostpopulartypeofnetworkandtheonealmostallreadersuseonadailybasisisacircuitswitchednetwork.Thepublicswitchedtelephonenetwork,however,isnotlimitedtothetelephonecompany,Bypurchasingappropriateswitchingequipment,anyorganizationcanconstructtheirowninternalcircuitswitchednetworkand,ifdesired,provideoneormoreinterfacestothepublicswitchednetworktoallowvoiceanddatatransmissiontoflowbetweenthepublicnetworkandtheirprivateinternalnetwork

2．LEASEDLINENETWORKS

Thisisadedicatednetworkconnectedbyleasedlines.Leasedlineisacommunicationslinereservedfortheexclusiveuseofaleasingcustomerwithoutinter-exchangeswitchingarrangements.Leasedorprivatelinesarededicatedtotheuser.Thisadvantageisthattheterminalorcomputerisaalwaysphysicallyconnectedtotheline.Veryshortresponsetimesaremetwithservice.

3.PACKETSWITCHINGNETWORKS

Apacketnetworkisconstructedthroughtheuseofequipmentthatassemblesanddisassemblespackets,equipmentthatroutespacket,andtransmissionfacilitiesusedtoroutepacketsfromtheoriginatortothedestinationdevice.Sometypesofdataterminalequipment（DTE）cancreatetheirownpackets,whileothertypesofDTErequiretheconversionoftheirprotocolintopacketsthroughtheuseofapacketassembler/disassemble（PAD）.Packetsareroutedthroughthenetworkbypacketswitches.Packetswitchesexaminethedestinationofpacketsastheyflowthroughthenetworkandtransferthepacketsontotrunksinterconnectingswitchesbaseduponthepacketdestinationdestinationandnetworkactivity.

ManyolderpubicnetworksfollowastandardcalledX.25.Itwasdevelopedduring1970sbyCCITTtoprovideaninterfacebetweenpublicpacket-switchednetworkandtheircustomers.

CCITTRecommendationX.25controlstheaccessfromapacketmodeDTE,suchasaterminaldeviceorcomputersystemcapableofformingpackets,totheDCEatapacketmode.CCITTRecommendationX.28controlstheinterfacebetweennon-packetmodedevicesthatcannotinterfacebetweenthePADandthehostcomputer.CCITTRecommendationX.3specifiestheparametersettingsonthePADandX.75specifiestheinterfacebetweenpacketnetwork.

LOCALAREANETWORK

Localareadatanetwork,normallyreferredtosimplyaslocalareanetworkorLANs,areusedtointerconnectdistributedcommunitiesofcomputer-basedDTEslocatedwithinabuildingorlocalizedgroupofbuilding.Forexample,aLANmaybeusedtointerconnectworkstationsdistributedaroundofficeswithinasinglebuildingoragroupofbuildingssuchasauniversitycampus.Alternatively,itmaybecomplex.Sincealltheequipmentislocatedwithinasingleestablishment,however,LANsarenormallyinstalledandmaintainedbytheorganization.Hencetheyarealsoreferredtoasprivatedatanetworks.

ThemaindifferencebetweenacommunicationpathestablishedusingaLANandaconnect