宽带接入—“最后一公里”技术

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1、BROADBANDACCESS—“LASTKILOMETRE”TECHNOLOGY   　　AlthoughbroadbandnetworktechniqueshavebeensucceededbothinWANandinLAN，abottleneckofcommunicationstillexistsbetweentelecommunicationofficesandsubscribers．Thisissocalled“LastKilometre”high-speedtelecommunication．Causeofthebottleneckisthatthebro

2、adbandnetworksalmostuseopticalfiberastransmissionmediumwhiletheusersusecopperwireandthelatercannottransmithigh-bandwidthsignals．　　Discussionsaboutwhattosolvethisproblemtendtofocusontwosolutions[1]．Thestraightforwardone—runningafiberfromtheendofficeintoeveryone’shouseiscalledFTTH（fiberto

3、thehome）．Thissolutionfitsinwellwiththecurrentsystembutwillnotbeeconomicallyfeasiblefordecades．Itissimplytooexpensive．   ThesecondsolutionthatismuchcheaperisFTTC(fihertothecurb)．Inthismodel，thetelephonecompanyrunsanopticalfiberfromeachendofficeintoeachneighborhood（thecurb）thatisserves．Th

4、efiberisterminatedinajunctionboxthatallthelocalloopsenter．Sincethelocalloopsarenowmuchshorter（perhaps100metersinsteadof3km），theycanberunathigherspeeds，probablyaround1Mbps，whichisjustenoughforcompressedvideo．ThisdesignisshownFig.15-1．　　Inthismanner，multiplevideos（orotherinformationchanne

5、ls）canpourdownthefiberathighspeedandbesplitoverthetwistedpairsattheend[2]．Bysharinga1-Gbpsfiberover100to1000customers，thecostpercustomercanbereduced，andconsiderablyhigherbandwidthcanbeprovidedthannow．　　AtypicalapplicationofthisFTTCisADSL（asymmetricaldigitalsubscriberline）．Itisemergingas

6、thetechnologyforhome-andsmall-officeInternetconnectivity．Itprovideseither1．5Mb/sor6Mb/sfromthenetworktotheuserand64Kb/sor640Kb/sinthereversedirectiondependingonthedistance，12000or18000feet[3]．Thedifferentspeedforeachdirectiongivesittheasymmetriclabel．ADSLisdesignedtotakeadvantageofthefa

7、ctthatvideo-ondemand，telecommuting，andInternetaccesstrafficareinherentlyasymmetrical．Theusersendsabriefmessageuptothenetworkandreceivesatonofdatacomingback，eitheramovieorapieceofdatadownload．Undersuchascenario，low-speedtraffictothenetworkisjustfine．　　ADSLdelivershighbandwidthwh