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1、1INTRODUCTIONEachofthepastthreecenturieshasbeendominatedbyasingletechnology.The18thcenturywastheeraofthegreatmechanicalsystemsaccompanyingtheIndustrialRevolution.The19thcenturywastheageofthesteamengine.Duringthe20thcentury,thekeytechnologywasinformationgathering,pro
2、cessing,anddistribution.Amongotherdevelopments,wesawtheinstallationofworldwidetelephonenetworks,theinventionofradioandtelevision,thebirthandunprece-dentedgrowthofthecomputerindustry,andthelaunchingofcommunicationsatellites.Asaresultofrapidtechnologicalprogress,these
3、areasarerapidlyconvergingandthedifferencesbetweencollecting,transporting,storing,andprocessinginfor-mationarequicklydisappearing.Organizationswithhundredsofofficesspreadoverawidegeographicalarearoutinelyexpecttobeabletoexaminethecurrentstatusofeventheirmostremoteout
4、postatthepushofabutton.Asourabilitytogather,process,anddistributeinformationgrows,thedemandforevermoresophisticatedinformationprocessinggrowsevenfaster.Althoughthecomputerindustryisstillyoungcomparedtootherindustries(e.g.,automobilesandairtransportation),computersha
5、vemadespectacularpro-gressinashorttime.Duringthefirsttwodecadesoftheirexistence,computersystemswerehighlycentralized,usuallywithinasinglelargeroom.Notinfre-quently,thisroomhadglasswalls,throughwhichvisitorscouldgawkatthegreatelectronicwonderinside.Amedium-sizedcompa
6、nyoruniversitymighthavehadoneortwocomputers,whilelargeinstitutionshadatmostafewdozen.Theidea12INTRODUCTIONCHAP.1thatwithintwentyyearsequallypowerfulcomputerssmallerthanpostagestampswouldbemassproducedbythemillionswaspuresciencefiction.Themergingofcomputersandcommuni
7、cationshashadaprofoundinfluenceonthewaycomputersystemsareorganized.Theconceptofthe‘‘computercenter’’asaroomwithalargecomputertowhichusersbringtheirworkforproc-essingisnowtotallyobsolete.Theoldmodelofasinglecomputerservingalloftheorganization’scomputationalneedshasbe
8、enreplacedbyoneinwhichalargenumberofseparatebutinterconnectedcomputersdothejob.Thesesystemsarecalledcomputernetworks.Thedesignandorganizat
