毕业设计电子信息科学与技术专业（外文翻译材料）TelecommunicationModernOperationTelephoneInananaloguetelephonenetwork,thecallerisconnectedtothepersonhewantstotalktobyswitchesatvarioustelephoneexchanges.Theswitchesformanelectricalconnectionbetweenthetwousersandthesettingoftheseswitchesisdeterminedelectronicallywhenthecallerdialsthenumber.Oncetheconnectionismade,thecaller'svoiceistransformedtoanelectricalsignalusingasmallmicrophoneinthecaller'shandset.Thiselectricalsignalisthensentthroughthenetworktotheuserattheotherendwhereittransformedbackintosoundbyasmallspeakerinthatperson'shandset.Thereisaseparateelectricalconnectionthatworksinreverse,allowingtheuserstoconverse.Thefixed-linetelephonesinmostresidentialhomesareanalogue—thatis,thespeaker'svoicedirectlydeterminesthesignal'svoltage.Althoughshort-distancecallsmaybehandledfromend-to-endasanaloguesignals,increasinglytelephoneserviceprovidersaretransparentlyconvertingthesignalstodigitalfortransmissionbeforeconvertingthembacktoanalogueforreception.Theadvantageofthisisthatdigitizedvoicedatacantravelside-by-sidewithdatafromtheInternetandcanbeperfectlyreproducedinlongdistancecommunication(asopposedtoanaloguesignalsthatareinevitablyimpactedbynoise).Mobilephoneshavehadasignificantimpactontelephonenetworks.Mobilephonesubscriptionsnowoutnumberfixed-linesubscriptionsinmanymarkets.Salesofmobilephonesin2005totalled816.6millionwiththatfigurebeingalmostequallysharedamongstthemarketsofAsia/Pacific(204m),WesternEurope(164m),CEMEA(CentralEurope,theMiddleEastandAfrica)(153.5m),NorthAmerica(148m)andLatinAmerica(102m).Intermsofnewsubscriptionsoverthefiveyearsfrom1999,Africahasoutpacedothermarketswith58.2%growth.IncreasinglythesephonesarebeingservicedbysystemswherethevoicecontentistransmitteddigitallysuchasGSMorW-CDMAwithmanymarketschoosingtodepreciateanaloguesystemssuchasAMPS.Therehavealsobeendramaticchangesintelephonecommunicationbehindthescenes.StartingwiththeoperationofTAT-8in1988,the1990ssawthewidespreadadoptionofsystemsbasedonopticfibres.Thebenefitofcommunicatingwithopticfibresisthattheyofferadrasticincreaseindatacapacity.TAT-8itselfwasabletocarry10timesasmanytelephonecallsasthelastcoppercablelaidatthattimeandtoday'sopticfibrecablesareabletocarry25timesasmanytelephonecallsasTAT-8.Thisincreaseindatacapacityisduetoseveralfactors:First,opticfibresarephysicallymuchsmallerthancompetingtechnologies.Second,theydonotsufferfromcrosstalkwhichmeansseveralhundredofthemcanbeeasilybundledtogetherinasinglecable.Lastly,improvementsinmultiplexinghaveledtoanexponentialgrowthinthedatacapacityofasinglefibre.AssistingcommunicationacrossmanymodernopticfibrenetworksisaprotocolknownasAsynchronousTransferMode(ATM).TheATMprotocolallowsfortheside-by-sidedatatransmissionmentionedinthesecondparagraph.Itissuitableforpublictelephonenetworksbecauseitestablishesapathwayfordatathroughthenetworkandassociatesatrafficcontractwiththatpathway.Thetrafficcontractisessentiallyanagreementbetweentheclientandthenetworkabouthowthenetworkistohandlethedata;ifthenetworkcannotmeettheconditionsofthetrafficcontractitdoesnotaccepttheconnection.Thisisimportantbecausetelephonecallscannegotiateacontractsoastoguaranteethemselvesaconstantbitrate,somethingthatwillensureacaller'svoiceisnotdelayedinpartsorcut-offcompletely.TherearecompetitorstoATM,suchasMultiprotocolLabelSwitching(MPLS),thatperformas...
