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1、UnitThreeDataCommunicationsPassageADataCommunicationsPassageBCharacteristicsofCommunicationChannelsPassageCCommunicationEquipmentandSoftwarePassageADataCommunicationsTherapidgrowthofdatacommunicationshasbeeninfluencedprimarilybytheincreasingneedtomoveinformationtoandfromcomputers.Mo
2、derntime-sharedcomputerscancommunicatewithmanydatastationssimultaneously.Otherformsofdatatransmissionwhichthetelephoneandtelegraphlinesarecalledupontohandlearebatchdatatransmission,real-timetransmission(asinthecaseofairlinereservationsystems),bankingandcreditdata,man-computerconvers
3、ationwiththeaidofgraphics,datacollectionsystemsandautomaticmeterreading.Thecombinationofcomputersanddatacommunicationplacessuchnewrequirementsonthesystems,whichmusthandlethem,thatonecanrefertothembetterasteleprocessingratherthantelecommunications.[1]Teleprocessingstartedwiththeairli
4、nereservationsystems,wherethedistributionofcomputerdataisnotasimportantasthemaintenanceofaninventoryofseatsforthewholesystemandtheabilitytoaccesssuchaninventoryrapidlyatanumberofdistantpoints.Anotherexampleisthatofthebanks,whodesiredtocentralizetheiraccountingandprovideaccesstoacent
5、ralfileforalltheirbranches.Basically,therearethreereasonsforthedevelopmentofteleprocessingsystems:therequirementforcentralizedfiles,theneedtodistributecomputerservicesandtheadvantagegainedbyhavingflexibilityinlocationoftheoperatingstaffwhichutilizesthesystem.Initially,boththeairline
6、andbankingoperationscanbeestablishedonaprivatebasis,whichwouldminimizetheneedforswitchingoperations.Butinbothcases,theneedtoexpandbeyondprivateoperationssoonarises.Inthecaseoftheairlines,theneedarisestoaccessthefacilitiesofotherairlines;andinthecaseofthebanks,certaincommondatafacili
7、tiesmaybesharedwithotherbanks.Eventhoughprivatenetworksmaysufficeformanyservices,theneedtoprovidedatacommunicationsbetweenurbancentersleadstoregionalandnationalnetworks.IntheU.S.,theBellSystem,theindependenttelephonecompaniesandspecializeddatacommunicationnetworksareinvolvedascarrie
8、rsofsuchdata.Themos