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Увод
HybridfiberCoaxialcable(HybridFiberCoax,referredtoasHFC)isabroadbandaccessnetworkthatcombinesopticalfiberandcoaxialcable.Opticalfiberreplacesordinarycable.AsthebackboneofthecableTVnetwork,itcanprovide30Mbpssharedbandwidthforhigh-speedInternet.Access,VODandanalogordigitalcableTVanddigitalvideoprograms.ThetransmissionmediumfromtheHeadEndcomputerroomtothefibernodeneartheuserisopticalfiber,andtheterminalequipmentfromtheopticalfibernodetotheuserisRG-62cable,soitiscalledhybridfibercoaxial.cable.
ФеатуресофХФЦ
ThefrequencybandwidthofHFCnetworkcanmeettheneedsofintegratedservicesandhigh-speeddatatransmission.Becausethebandwidthofthecoaxialcablecanreach1GHz,inadditiontotheoriginalanalogvideosignal,therearealsoabundantspectrumresourcesthatcanbeusedtotransmitdigitalvideosignalsandtwo-waydata.
ThetransmissionrateofHFCnetworkishigh.WhentheCableModemisusedfortwo-waycommunicationontheHFCnetwork,thedownlinkratecanreach30Mbit/s,andtheuplinkratecanreach10Mbit/s.
TheflexibilityandscalabilityoftheHFCnetworkisbetter.TheHFCnetworkcanbecompatiblewithtraditionaltelephoneservicesandanalogvideoservicesinbusiness,whilesupportingInternetaccess,digitalvideo,VOD,andotherfutureinteractiveservices.Intermsofstructure,HFCnetworkhasstrongflexibilityandcansmoothlytransitiontoFTTH.
Мрежна структура ХФЦ мреже
TheHFCnetworkevolvedfromtheCATVnetworkhastwotypes:one-waynetworkandtwo-waynetwork.Amongthem,theone-wayHFCnetworkonlyreplacesthetraditionalCATVnetworkbackbonecableswithopticalfibers,expandingthecoverageoftheaccessnetwork,andbasicallyonlysupportsCATVservices.Thetwo-wayHFCnetworkistransformedonthebasisoftheone-wayHFCnetwork,anditsstructureisshowninFigure1.Thetwo-wayHFCnetworkcanprovideCATV,voice,dataandotherinteractiveservices,andrealizethe"tripleplay"oftelecommunicationnetwork,cabletelevisionnetworkandcomputernetwork.
Thetwo-wayHFCnetworkiscomposedofalocalareanetworkdigitalterminal(HDTorgateway),anopticalfiberfeedernetwork,anopticalnode,acoaxialcabletransmissionanddistributionnetwork,andanintegratedserviceunit.Amongthem,thelocaldigitalterminalhasfourmainfunctions:First,itcompletestheinterfacefunctionsofvariousbusinessnodeequipment,suchasthelocalswitchofPSTN,theprogramsourceofCATV,theVODserver,andtheaccesstothebroadbandnetworkthroughtheedgeswitchofthebroadbandnetwork.Internetserviceprovider’saccessserverinterface,etc.;second,HFCusessub-carriermultiplexingtransmissiontechnology,andthemodulationanddemodulationpartofthecentraldigitalterminalmodulatesvariousservicesignalsontheirrespectivesub-carrierstoachievefrequencyDivisionmultiplexing(FDM),sharingthetransmissionmedium;third,completingthephotoelectricconversionfunction;fourth,providingthemonitoringinterfacefunction.
Opticalfiberfeedernetworkandcoaxialcabletransmissionanddistributionnetworkarethebackbonetransmissionnetworkanduser-sidedistributionnetworkofHFCnetworkrespectively.
Theopticalnodeistheconnectionpointbetweentheopticalfiberbackbonetransmissionnetworkandtheuser-sidecoaxialdistributionnetworkintheHFCnetwork.Itmainlycompletesoptical/electricconversion,multiplexing/demultiplexingofelectricalsignals,andmodulation/demodulation.Inaddition,theopticalnodecontrolsthefrequencybandallocationtorealizemultipleservices,andmanagesthecompetitionontheuplinkchanneltorealizemulti-usersharingofthetransmissionmedium.
Интегрисана услужна јединица (ИСУ) обезбеђује интерфејс са терминалом телекомуникационих услуга и кабловским ТВ терминалом. ИСУ је подељен на једну јединицу за више корисника (Х-ИСУ) и вишекорисничку јединицу (М-ИСУ), а структура мреже може имати оба Х-ИСУ и М-ИСУ. /синтерфацеандотхер хигх-спеединтерфацес.Додатак, такође обезбеђује интерфејс за праћење.Корисник приступа ХФЦ мрежи преко корисничке терминалске опреме, која укључује корисничко сучеље мреже (УНИ), сет-топбок (СИБ) и кабловски модем (ЦаблеМодем).
кабловски модем
Cablemodemisamodemsuitableforcabletransmissionsystem.Itsmainfunctionistomodulatedigitalsignaltoradiofrequencysignalanddemodulatethedigitalinformationinradiofrequencysignal..Inaddition,theкабловски модемalsoprovidesastandardEthernetinterface,whichcancompletepartofthefunctionsofbridges,routers,networkcardsandhubs.Therefore,itsstructureismuchmorecomplicatedthantraditionalmodems.
Cablemodemsarebasicallythesameinprincipleastraditionalmodems.Theybothmodulatedigitalsignalsintoanalogsignalsfortransmissioninafrequencyrangeofthecable,andthendemodulatethemintodigitalsignalswhenreceiving.Thedifferenceisthatкабловски модемdemodulatesthroughacertaintransmissionfrequencybandofcableTVratherthanthroughtelephonelines.Inaddition,themediumusedbyordinarymodemsisexclusivelyusedbyusers,whileкабловски модемsbelongtoasharedmediumsystem,andtheremainingidlefrequencybandscanstillbeusedtotransmitcableTVsignals.
CablemodemisalsosimilartoADSL,providingasymmetrictwo-waychannels.Thecarrierfrequencyrangeadoptedbytheuplinkchannelisbetween5MHzand42MHz,andatransmissionrateof128Kbpsto10Mbpscanbeachieved.Thecarrierfrequencyofthedownlinkchannelrangesfrom42to750MHz,andcanachieveatransmissionrateof27to36Mbps.
Cablemodemhasthecharacteristicsofhighcostperformance,asymmetricprivatelineconnection,notlimitedbyconnectiondistance,usuallydoesnotoccupybandwidth(onlythebandwidthisoccupiedwhendownloadingandsendingdata),anditisnotwrongtowatchTVonline..
Thetransmissionrateofкабловски модемinonechannelisupto27～36Mbps.ThebandwidthofeachcableTVchannelis8MHz,andthebandwidthoftheHFCnetworkis750MHz,sotheentirebandwidthcansupportnearly90channels.IntheHFCnetwork,therearecurrentlyabout33channels(550-750MHzrange)reservedfordatatransmission,andtheentirebandwidthisconsiderable.