第1页共13页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第1页共13页课题：TelecommunicationStepIwhatistelecommunicationThewordtelecommunicationderivesfromtheGreekwordtele,meaningoveradistance,plusthewordcommunication.Thus,puttingthetwopartstogether,yougettelecommunication-communicationthatspansadistance.Anancientexampleoftelecommunicationistheuseoffiresignalsonhilltopsthosemilesapart.Thesedays,moderntelecommunicationsisavastfieldthatencompasseseverythingfrombroadcastingtotelephonetechnologytocomputernetworks.TelecommunicationVoiceVideoDataWirelinewirelesswirelinewirelesswirelinewirelessLocal+longcellulartelephonyvideobroadcastTVcomputersRFnetworksDistanceconferencingTelephonydigitalcableradiobroadcastingdesktopvideoovernetworkssatellitefeedsLANspagingIntercomwirelesslocal+longdistancetelephonycableTVWANspoint-to-pointservicesclosedcircuitTVTheinternetFigure1.TelecommunicationservicesThreecategoriesoftelecommunicationare:voice,videoanddata.Voicetelecommunicationreferstoanymeansofusingelectrical第2页共13页第1页共13页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第2页共13页signalstotransmithumanvoiceacrossadistance,suchastelephoneandaudiobroadcasts.Videotelecommunicationreferstotheelectrically-basedtransmissionofmovingpicturesandsoundacrossadistance,suchasTVbroadcastingordistributinglivefeedsofaneventtothescreenofnetworkedcomputers.Datatelecommunicationreferstouseofelectricalsignalstoexchangeencodedinformationbetweencomputerizeddevicesacrossadistance.Sendingane-mailtoacolleague,retrievingaspread-sheetfromyourcomputer’sharddisk,andsendingamessagetoyourspouse’spagerareallexamplesofdatatelecommunications.Voice,videoanddatatelecommunicationsallbelongtothelarger“datatelecommunication”categoriesthattheyrelyonelectricalsignals.StepIIEvolutionoftelecommunicationnetworksThefirstphase※Thefirstphaseintheevolutionofcommunicationnetworksisdominatedbythewidespreadoftelephonyandcoversover90years.However,thetelegraphprecededthetelephonebymorethan30yearsandtheteletypewriterbyhalfacentury.In1844,SamuelMorse,thetelegraph’sinventor,sentthefirsttelegraphmessagefromWashingtonD.C,tohiscolleagueAlfredVailin第3页共13页第2页共13页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第3页共13页Baltimore,Maryland.ShortlyaftertheinventionofthetelephonebyAlexanderGrahanBellin1876,itwasrealizedthattelephonewireshadtoconvergeoncentralpointswheretelephone-to-telephoneconnectionscouldbemade.Thesepointsweremanualswitch-boards,staffedbyoperators.1889,AlmondB.Strowgerinventedthefirsttwo-motionstep-by-stepswitch.1932,thefirstcrossbarswitchwasusedinSweden.1938,theBellSystemintroducedtheNO.1Crossbarswitch.Inthemid1930s,analogpoint-to-pointradiorelaytransmissionwaspossiblebyextendinghigh-frequency(HF)communicationtechnologyintothevery-high-frequencyrange.DuringtheWorldWar2,theoperatingbandsoftheanalogueradiorelaysystemswereextendedintotheultra-high-frequency(UHF)range.Thefirstlinkwithover100repeaterstationwasplacedintocommercialservicein1951betweenNewYorkandSanFrancisco,operatinginthe4GHzbandwitha20MHzband.Bythe1980s,quadratureamplitudemodulation(QAM)methodswerewidelyusedasthemodulationschemefordigitalradio-relaysystems.Thesecondphase※第4页共13页第3页共13页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第4页共13页Thesecondphaseofnetworking,whichoccursinthe196...
