电子通信专业 外文翻译 外文文献 英文文献 电信现代运营

毕业设计（外文翻译材料）TELECOMMUNICATIONMODERNOPERATIONTELEPHONEINANANALOGUETELEPHONENETWORK,THECALLERISCONNECTEDTOTHEPERSONHEWANTSTOTALKTOBYSWITCHESATVARIOUSTELEPHONEEXCHANGESTHESWITCHESFORMANELECTRICALCONNECTIONBETWEENTHETWOUSERSANDTHESETTINGOFTHESESWITCHESISDETERMINEDELECTRONICALLYWHENTHECALLERDIALSTHENUMBERONCETHECONNECTIONISMADE,THECALLERSVOICEISTRANSFORMEDTOANELECTRICALSIGNALUSINGASMALLMICROPHONEINTHECALLERSHANDSETTHISELECTRICALSIGNALISTHENSENTTHROUGHTHENETWORKTOTHEUSERATTHEOTHERENDWHEREITTRANSFORMEDBACKINTOSOUNDBYASMALLSPEAKERINTHATPERSONSHANDSETTHEREISASEPARATEELECTRICALCONNECTIONTHATWORKSINREVERSE,ALLOWINGTHEUSERSTOCONVERSETHEFIXEDLINETELEPHONESINMOSTRESIDENTIALHOMESAREANALOGUETHATIS,THESPEAKERSVOICEDIRECTLYDETERMINESTHESIGNALSVOLTAGEALTHOUGHSHORTDISTANCECALLSMAYBEHANDLEDFROMENDTOENDASANALOGUESIGNALS,INCREASINGLYTELEPHONESERVICEPROVIDERSARETRANSPARENTLYCONVERTINGTHESIGNALSTODIGITALFORTRANSMISSIONBEFORECONVERTINGTHEMBACKTOANALOGUEFORRECEPTIONTHEADVANTAGEOFTHISISTHATDIGITIZEDVOICEDATACANTRAVELSIDEBYSIDEWITHDATAFROMTHEINTERNETANDCANBEPERFECTLYREPRODUCEDINLONGDISTANCECOMMUNICATIONASOPPOSEDTOANALOGUESIGNALSTHATAREINEVITABLYIMPACTEDBYNOISEMOBILEPHONESHAVEHADASIGNIFICANTIMPACTONTELEPHONENETWORKSMOBILEPHONESUBSCRIPTIONSNOWOUTNUMBERFIXEDLINESUBSCRIPTIONSINMANYMARKETSSALESOFMOBILEPHONESIN2005TOTALLED8166MILLIONWITHTHATFIGUREBEINGALMOSTEQUALLYSHAREDAMONGSTTHEMARKETSOFASIA/PACIFIC204M,WESTERNEUROPE164M,CEMEACENTRALEUROPE,THEMIDDLEEASTANDAFRICA1535M,NORTHAMERICA148MANDLATINAMERICA102MINTERMSOFNEWSUBSCRIPTIONSOVERTHEFIVEYEARSFROM1999,AFRICAHASOUTPACEDOTHERMARKETSWITH582GROWTHINCREASINGLYTHESEPHONESAREBEINGSERVICEDBYSYSTEMSWHERETHEVOICECONTENTISTRANSMITTEDDIGITALLYSUCHASGSMORWCDMAWITHMANYMARKETSCHOOSINGTODEPRECIATEANALOGUESYSTEMSSUCHASAMPSTHEREHAVEALSOBEENDRAMATICCHANGESINTELEPHONECOMMUNICATIONBEHINDTHESCENESSTARTINGWITHTHEOPERATIONOFTAT8IN1988,THE1990SSAWTHEWIDESPREADADOPTIONOFSYSTEMSBASEDONOPTICFIBRESTHEBENEFITOFCOMMUNICATINGWITHOPTICFIBRESISTHATTHEYOFFERADRASTICINCREASEINDATACAPACITYTAT8ITSELFWASABLETOCARRY10TIMESASMANYTELEPHONECALLSASTHELASTCOPPERCABLELAIDATTHATTIMEANDTODAYSOPTICFIBRECABLESAREABLETOCARRY25TIMESASMANYTELEPHONECALLSASTAT8THISINCREASEINDATACAPACITYISDUETOSEVERALFACTORSFIRST,OPTICFIBRESAREPHYSICALLYMUCHSMALLERTHANCOMPETINGTECHNOLOGIESSECOND,THEYDONOTSUFFERFROMCROSSTALKWHICHMEANSSEVERALHUNDREDOFTHEMCANBEEASILYBUNDLEDTOGETHERINASINGLECABLELASTLY,IMPROVEMENTSINMULTIPLEXINGHAVELEDTOANEXPONENTIALGROWTHINTHEDATACAPACITYOFASINGLEFIBREASSISTINGCOMMUNICATIONACROSSMANYMODERNOPTICFIBRENETWORKSISAPROTOCOLKNOWNASASYNCHRONOUSTRANSFERMODEATMTHEATMPROTOCOLALLOWSFORTHESIDEBYSIDEDATATRANSMISSIONMENTIONEDINTHESECONDPARAGRAPHITISSUITABLEFORPUBLICTELEPHONENETWORKSBECAUSEITESTABLISHESAPATHWAYFORDATATHROUGHTHENETWORKANDASSOCIATESATRAFFICCONTRACTWITHTHATPATHWAYTHETRAFFICCONTRACTISESSENTIALLYANAGREEMENTBETWEENTHECLIENTANDTHENETWORKABOUTHOWTHENETWORKISTOHANDLETHEDATAIFTHENETWORKCANNOTMEETTHECONDITIONSOFTHETRAFFICCONTRACTITDOESNOTACCEPTTHECONNECTIONTHISISIMPORTANTBECAUSETELEPHONECALLSCANNEGOTIATEACONTRACTSOASTOGUARANTEETHEMSELVESACONSTANTBITRATE,SOMETHINGTHATWILLENSUREACALLERSVOICEISNOTDELAYEDINPARTSORCUTOFFCOMPLETELYTHEREARECOMPETITORSTOATM,SUCHASMULTIPROTOCOLLABELSWITCHINGMPLS,THATPERFORMASIMILARTASKANDAREEXPECTEDTOSUPPLANTATMINTHEFUTURERADIOANDTELEVISIONINABROADCASTSYSTEM,ACENTRALHIGHPOWEREDBROADCASTTOWERTRANSMITSAHIGHFREQUENCYELECTROMAGNETICWAVETONUMEROUSLOWPOWEREDRECEIVERSTHEHIGHFREQUENCYWAVESENTBYTHETOWERISMODULATEDWITHASIGNALCONTAININGVISUALORAUDIOINFORMATIONTHEANTENNAOFTHERECEIVERISTHENTUNEDSOASTOPICKUPTHEHIGHFREQUENCYWAVEANDADEMODULATORISUSEDTORETRIEVETHESIGNALCONTAININGTHEVISUALORAUDIOINFORMATIONTHEBROADCASTSIGNALCANBEEITHERANALOGUESIGNALISVARIEDCONTINUOUSLYWITHRESPECTTOTHEINFORMATIONORDIGITALINFORMATIONISENCODEDASASETOFDISCRETEVALUESTHEBROADCASTMEDIAINDUSTRYISATACRITICALTURNINGPOINTINITSDEVELOPMENT,WITHMANYCOUNTRIESMOVINGFROMANALOGUETODIGITALBROADCASTSTHISMOVEISMADEPOSSIBLEBYTHEPRODUCTIONOFCHEAPER,FASTERANDMORECAPABLEINTEGRATEDCIRCUITSTHECHIEFADVANTAGEOFDIGITALBROADCASTSISTHATTHEYPREVENTANUMBEROFCOMPLAINTSWITHTRADITIONALANALOGUEBROADCASTSFORTELEVISION,THISINCLUDESTHEELIMINATIONOFPROBLEMSSUCHASSNOWYPICTURES,GHOSTINGANDOTHERDISTORTIONTHESEOCCURBECAUSEOFTHENATUREOFANALOGUETRANSMISSION,WHICHMEANSTHATPERTURBATIONSDUETONOISEWILLBEEVIDENTINTHEFINALOUTPUTDIGITALTRANSMISSIONOVERCOMESTHISPROBLEMBECAUSEDIGITALSIGNALSAREREDUCEDTODISCRETEVALUESUPONRECEPTIONANDHENCESMALLPERTURBATIONSDONOTAFFECTTHEFINALOUTPUTINASIMPLIFIEDEXAMPLE,IFABINARYMESSAGE1011WASTRANSMITTEDWITHSIGNALAMPLITUDES10001010ANDRECEIVEDWITHSIGNALAMPLITUDES09021109ITWOULDSTILLDECODETOTHEBINARYMESSAGE1011APERFECTREPRODUCTIONOFWHATWASSENTFROMTHISEXAMPLE,APROBLEMWITHDIGITALTRANSMISSIONSCANALSOBESEENINTHATIFTHENOISEISGREATENOUGHITCANSIGNIFICANTLYALTERTHEDECODEDMESSAGEUSINGFORWARDERRORCORRECTIONARECEIVERCANCORRECTAHANDFULOFBITERRORSINTHERESULTINGMESSAGEBUTTOOMUCHNOISEWILLLEADTOINCOMPREHENSIBLEOUTPUTANDHENCEABREAKDOWNOFTHETRANSMISSIONINDIGITALTELEVISIONBROADCASTING,THEREARETHREEC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