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1、ComputerNetworksandDatabaseNetworksSomereasonsarecausingcentralizedcomputersystemstogivewaytonetworks.Thefirstoneisthatmanyorganizationsalreadyhaveasubstantialnumberofcomputersinoperation,oftenlocatedfarapart.Initially,eachofthesecomputersmayhavework
2、edinisolationfromtheotherones,butatacertaintime,managementmayhavedecidedtoconnectthemtobeabletocorrelateinformationabouttheentireorganization.Generallyspeaking,thisgoalistomakeallprograms,data,andotherresourcesavailabletoanyoneonthenetworkwithoutrega
3、rdtothephysicallocationoftheresourceandtheuser.Thesecondoneistoprovidehighreliabilitybyhavingalternativesourcesofsupply.Withanetwork,thetemporarylossofasinglecomputerismuchlessserious,becauseitsuserscanoftenbeaccommodatedelsewhereuntiltheserviceisres
4、tored.Yetanotherreasonofsettingupacomputernetworkiscomputernetworkcanprovideapowerfulcommunicationmediumamongwidelyseparatedpeople.ApplicationofNetworksOneofthemainareasofpotentialnetworksueisaccesstoremotedatabase.Itmaysomedaybeeasyforpeoplesittinga
5、ttheirterminalsathometomakereservationsforairplanestrains,buses,boats,restaurants,theaters,hotels,andsoon,atanywhereintheworldwithinstantconfirmation.Homebanking,automatednewspaperandfullyautomatedlibraryalsofallinthiscategory.Computeraidededucationi
6、sanotherpossiblefieldforusingnetwork,withmanydifferentcoursesbeingoffered.Teleconferencingisawholenewformcommunication.Withitwidelyseparatedpeoplecanconductameetingbytypingmessagesattheirterminals.Attendeesmayleaveatwillandfindoutwhattheymissedwhenth
7、eycomeback.Internationalcontactsbyhumanbeginmaybegreatlyenhancedbynetworkbasedcommunicationfacilities.NetworkStructureBroadlyspeaking,therearetwogeneraltypesofdesignsforthecommunicationsubnet:(1)Point–to–pointchannels(1)BroadcastchannelsInthefirstone
8、,thenetworkcontainsnumerouscablesorlesasedtelephonelines,eachoneconnectingapairofnodes.Iftwonodesthatdonotshareacablewishtocommunicate,theymustdothisindirectlyviaothernodes.Whenamessageissentfromnodetoanotherviaoneormoreintermediatemodes,eachintermed
