10-11远程通信

湖南城市学院教师备课纸第次课题：TelecommunicationStepIwhatistelecommunicationThewordtelecommunicationderivesfromtheGreekwordtele,meaningoveradistance,plusthewordcommunication.Thus,puttingthetwopartstogether,yougettelecommunication-communicationthatspansadistance.Anancientexampleoftelecommunicationistheuseoffiresignalsonhilltopsthosemilesapart.Thesedays,moderntelecommunicationsisavastfieldthatencompasseseverythingfrombroadcastingtotelephonetechnologytocomputernetworks.TelecommunicationVoiceVideoDataWirelinewirelesswirelinewirelesswirelinewirelessLocal+longcellulartelephonyvideobroadcastTVcomputersRFnetworksDistanceconferencingTelephonydigitalcableradiobroadcastingdesktopvideoovernetworkssatellitefeedsLANspagingIntercomwirelesslocal+longdistancetelephonycableTVWANspoint-to-pointservicesclosedcircuitTVTheinternetFigure1.TelecommunicationservicesThreecategoriesoftelecommunicationare:voice,videoanddata.Voicetelecommunicationreferstoanymeansofusingelectricalsignalstotransmithumanvoiceacrossadistance,suchastelephoneandaudiobroadcasts.Videotelecommunicationreferstotheelectrically-based湖南城市学院教师备课纸第次transmissionofmovingpicturesandsoundacrossadistance,suchasTVbroadcastingordistributinglivefeedsofaneventtothescreenofnetworkedcomputers.Datatelecommunicationreferstouseofelectricalsignalstoexchangeencodedinformationbetweencomputerizeddevicesacrossadistance.Sendingane-mailtoacolleague,retrievingaspread-sheetfromyourcomputer’sharddisk,andsendingamessagetoyourspouse’spagerareallexamplesofdatatelecommunications.Voice,videoanddatatelecommunicationsallbelongtothelarger“datatelecommunication”categoriesthattheyrelyonelectricalsignals.StepIIEvolutionoftelecommunicationnetworks※ThefirstphaseThefirstphaseintheevolutionofcommunicationnetworksisdominatedbythewidespreadoftelephonyandcoversover90years.However,thetelegraphprecededthetelephonebymorethan30yearsandtheteletypewriterbyhalfacentury.In1844,SamuelMorse,thetelegraph’sinventor,sentthefirsttelegraphmessagefromWashingtonD.C,tohiscolleagueAlfredVailinBaltimore,Maryland.ShortlyaftertheinventionofthetelephonebyAlexanderGrahanBellin1876,itwasrealizedthattelephonewireshadtoconvergeoncentralpointswheretelephone-to-telephoneconnectionscouldbemade.湖南城市学院教师备课纸第次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.※ThesecondphaseThesecondphaseofnetworking,whichoccursinthe1960s,includedthreemajorsmilestones:softwareswitching,digitaltransmission,andsatellitedeployment.In1965,AmericanTelephoneandTelegraph(ATT)introducedthefirststored-programcontrol(SPC)localswitch,the1EESswitching湖南城市学院教师备课纸第次system.Theprincipleofconvertinganalogsignalstodigitalsignalsbecamepopularwiththeintroductionofpulsecodemodulation(PCM);consequently,theknownratefortelephone-qualityspeech(4kHzbandwidth)is64kbps.Satellitecommunicationwasfirstproposedin1945byBritishsciencefictionwriter,ArthurC.Clarke.SatellitecommunicationbecamearealitywiththelaunchingoftheRussiansatelliteSputnikandtheAmericansatelliteExplorer.TheATTTelstarwasthefirstexperimentalsatellitecapableofrelayingTVprogramsacrosstheAtlanticOcean.※ThethirdphaseThethirdphaseoftheevolutionofcommunicationnetworkoccurredinthe1970sandischaracterizedbytheintroductionofdatanetworksandpacketswitchingtechnology.Theconceptofpacketswitchingwasfirstpublishedina1964reportbyPaulBaranoftheRandCorporationfortheU.S.AirForce.In1971,ARPANETwassetupandwasputintoservice.TheAmericanStandardCodeforInformationInterchange(ASCII)wasthefirststandard,whichwasapprovedin1964andbecamethecommonmethodofcodingdataforcommunication.ThesecondstandardistheElectronicsIndustriesAssociation(EIA)recommendedstandard(RS)RS-232D.Itsfirstversion,issuedin1969,specifiedhow湖南城市学院教师备课纸第次theencodedinformationwouldbetransmittedbymodemoveratelephonenetwork.Toachievecompatibilitybetweencomputersandpacket-switchedNetworks,theInternationalTelegraphandTelephoneConsultativeCommittee(CCITT)establishedaworld-standardprotocol,calledX.25in1976.WorkingwithCCITT,theInternationalOrganizationofStandardization(ISO)in1978approveditsseven-layerframeworkofprotocolsfordatacommunication,calledtheOpenSystemInterconnection(OSI)referencemodel.Thisphaseofnetworkingisalsocharacterizedbytheintroductionandwideacceptanceoflocalareanetworking.(LANs).TheoldestandbestknownofallLANsiftheEthernet.TheinventionoftheLASER(whichstandsforlightamplificationbysimulatedemissionofradiation)in1959ledtomajor