TheabovecontentisreviewedandannouncedbytheNationalScienceandTechnologyTermReviewCommittee
Úvod
HybridfiberCoaxialcable(HybridFiberCoax,referredtoasHFC)isabroadbandaccessnetworkthatcombinesopticalfiberandcoaxialcable.Opticalfiberreplacesordinarycable.AsthebackboneofthecableTVnetwork,itcanprovide30Mbpssharedbandwidthforhigh-speedInternet.Access,VODandanalogordigitalcableTVanddigitalvideoprograms.ThetransmissionmediumfromtheHeadEndcomputerroomtothefibernodeneartheuserisopticalfiber,andtheterminalequipmentfromtheopticalfibernodetotheuserisRG-62cable,soitiscalledhybridfibercoaxial.cable.
Vlastnosti HFC
ThefrequencybandwidthofHFCnetworkcanmeettheneedsofintegratedservicesandhigh-speeddatatransmission.Becausethebandwidthofthecoaxialcablecanreach1GHz,inadditiontotheoriginalanalogvideosignal,therearealsoabundantspectrumresourcesthatcanbeusedtotransmitdigitalvideosignalsandtwo-waydata.
ThetransmissionrateofHFCnetworkishigh.WhentheCableModemisusedfortwo-waycommunicationontheHFCnetwork,thedownlinkratecanreach30Mbit/s,andtheuplinkratecanreach10Mbit/s.
TheflexibilityandscalabilityoftheHFCnetworkisbetter.TheHFCnetworkcanbecompatiblewithtraditionaltelephoneservicesandanalogvideoservicesinbusiness,whilesupportingInternetaccess,digitalvideo,VOD,andotherfutureinteractiveservices.Intermsofstructure,HFCnetworkhasstrongflexibilityandcansmoothlytransitiontoFTTH.
Síťová struktura HFCsítě
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.
Integrovaná servisní jednotka (ISU) poskytuje rozhraní s terminálem telekomunikačních služeb a terminálem pro kabelovou televizi. ISU je rozdělena na jednu uživatelskou jednotku (H-ISU) a víceuživatelskou jednotku (M-ISU) a struktura sítě může mít jak H-ISU, tak M-ISU. /rozhraníadalšívysokorychlostnírozhraní.Navíc také poskytuje rozhraní pro monitorování.Uživatel přistupuje k síti HFC prostřednictvím uživatelského terminálového zařízení, které zahrnuje uživatelské síťové rozhraní (UNI), set-topbox (SYB) a kabelový modem (kabelový modem).
kabelový modem
Cablemodemisamodemsuitableforcabletransmissionsystem.Itsmainfunctionistomodulatedigitalsignaltoradiofrequencysignalanddemodulatethedigitalinformationinradiofrequencysignal..Inaddition,thekabelový modemalsoprovidesastandardEthernetinterface,whichcancompletepartofthefunctionsofbridges,routers,networkcardsandhubs.Therefore,itsstructureismuchmorecomplicatedthantraditionalmodems.
Cablemodemsarebasicallythesameinprincipleastraditionalmodems.Theybothmodulatedigitalsignalsintoanalogsignalsfortransmissioninafrequencyrangeofthecable,andthendemodulatethemintodigitalsignalswhenreceiving.Thedifferenceisthatkabelový modemdemodulatesthroughacertaintransmissionfrequencybandofcableTVratherthanthroughtelephonelines.Inaddition,themediumusedbyordinarymodemsisexclusivelyusedbyusers,whilekabelový modemsbelongtoasharedmediumsystem,andtheremainingidlefrequencybandscanstillbeusedtotransmitcableTVsignals.
CablemodemisalsosimilartoADSL,providingasymmetrictwo-waychannels.Thecarrierfrequencyrangeadoptedbytheuplinkchannelisbetween5MHzand42MHz,andatransmissionrateof128Kbpsto10Mbpscanbeachieved.Thecarrierfrequencyofthedownlinkchannelrangesfrom42to750MHz,andcanachieveatransmissionrateof27to36Mbps.
Cablemodemhasthecharacteristicsofhighcostperformance,asymmetricprivatelineconnection,notlimitedbyconnectiondistance,usuallydoesnotoccupybandwidth(onlythebandwidthisoccupiedwhendownloadingandsendingdata),anditisnotwrongtowatchTVonline..
Thetransmissionrateofkabelový modeminonechannelisupto27～36Mbps.ThebandwidthofeachcableTVchannelis8MHz,andthebandwidthoftheHFCnetworkis750MHz,sotheentirebandwidthcansupportnearly90channels.IntheHFCnetwork,therearecurrentlyabout33channels(550-750MHzrange)reservedfordatatransmission,andtheentirebandwidthisconsiderable.