Transmissionmethod
1.Spacedivisionmultiplexing(SDM)
SDMisacommunicationmethodthatusesoneopticalfiberineachdirectionoftwo-waycommunication,theso-calledSimplexmode.IntheSDMmode,thesignalsinthetwodirectionsaretransmittedintwocompletelyindependentopticalfibers,withoutaffectingeachother,thetransmissionperformanceisthebest,andthesystemdesignisthesimplest.However,apairofopticalfibersarerequiredtocompletethetaskoftwo-waytransmission.Itisnoteconomicalforalongtime.Forapplicationswhere0LTandONUareveryclose,duetothecontinuousdeclineinfiberprices,SDMisstillatwo-waytransmissionschemethatcanbeconsidered.Finally,sincethesignaltransmissionpathsinthetwodirectionsareindependentofeachother,thereisnospecialrequirementforthewavelengthofthelightsource,aslongasitisinthe1310nmwavelengthregion,itdoesnotmatterwhethertheyarethesame.
2.TimeCompressionMultiplexing(TCM)
TCMisoneoftheeffectivemeanstosolvetwo-waytransmission.Thismethodusesonlyoneopticalfiber,butcontinuouslyalternatesthetransmissiondirection,sothatthesignalsinthetwodirectionscanbetransmittedonthesameopticalfiberinturn,justlikeplayingtabletennis,soitisalsocalled"ping-pongmethod."TherearetwowaystorealizeTCMtransmission.Thefirstmethodistousealaserasbothalightsourceandadetector.Itisverysimple,aslongasthereisatransceivercontrolswitchtoaccuratelycontrolthetransceivertimesothatitdoesnotconflict.However,themethodofusingthislaserasadetectorhaspoorsensitivity,andwhenthespeedishigh,theavailableopticalbudgetoftheopticalchannelissmall.Thesecondmethodistousetwosetsofindependenttransceiverequipment,withanopticalcouplerateachendtoseparatetheupstreamanddownstreamsignals.Thesignaltransmissioninthetwodirectionsisseparatedintime,andeachtakesupdifferenttimeslotstotransmitinturn.Sinceonlyonedirectionisallowedtotransmitsignalsatthesametime,itiscalledahalf-duplexmodetodistinguishitfromthefull-duplexmodeofWDMandSCM.WhenusingtheTCMmethod,thesignalsinthetwodirectionsareallowedtoworkatthesamewavelength,butitmustbeinthe1310nmwavelengthregion.
Itshouldbenotedthatintheaccessnetworkenvironment,PONmainlyworksinapoint-to-multipointmode,sotheprocessingmethodsofupstreamanddownstreamsignalsaredifferent.ThesignalsinthedownstreamdirectionaresentinsequenceandsenttoeachONUinabroadcastmanner.EachONUreceivesallthesignalsbutcanonlytakeoutitsownsignalinitsowntimeslot.Theupstreamdirectionisdifferent.EachONUsendssignalsinbursts,andcanonlysendsignalsintheirowntimeslots,sothesignalsfromeachONUareindiscreteburstswithvaryingamplitudes.same.
UsingtheTCMmethod,oneopticalfibercanbeusedtocompletethetwo-waytransmissiontask,savingopticalfibers,splittersandmovableconnectors,andthenetworkmanagementsystemiseasiertojudgethefault,soithasbeenwidelyused.Thedisadvantageofthissystemisthatthecouplersatbothendshaveapowerlossof3dB,andthecircuitsofOLTandONUaremorecomplicated.
3.WavelengthDivisionMultiplexing(WDM)
Whenthetransmissionpowerofthelightsourcedoesnotexceedacertainthreshold,theopticalfiberworksinalineartransmissionstate.Atthistime,aslongasthereisacertainintervalbetweensignalsofdifferentwavelengths,theycanbeindependentlytransmittedonthesameopticalfiberwithoutmutualinterference.Thisisthebasicprincipleofwavelengthdivisionmultiplexing.Fortwo-waytransmission,thepurposeofsingle-fibertwo-waytransmissioncanberealizedbyadjustingthesignalsinthetwodirectionstodifferentwavelengths,whichiscalledthedifferentwavelengthduplexmode.Thismethodhasgreatpotentialforfutureupgradesandexpansion,anditiseasytoexpandtodozensofwavelengths,butthecostofWDMdevicesisstilltoohigh,sothetransmissiondistanceisnotlongenoughtobeeconomical.
4.Subcarriermultiplexing(SCM)
TheprincipleofusingSCMtoachievetwo-waytransmissionisverysimple.YouonlyneedtoarrangethesignalsinthetwodirectionsindifferentfrequencybandstoachievesingleThepurposeoffiberbidirectionaltransmissionatthesamewavelength.IntheactualoANtransmissionsystem,thedownlinkdirectionoftenadoptstheTDMmodebasebandtransmissionform,sothefrequencycomponentsareconcentratedonthelowfrequencyend,whiletheuplinkdirectionadoptsthesubcarriermultipleaccess(SCMA)mode,thatis,thefrequencyofeachuserisadjustedinthehigherfrequencyband.,Separatedfromthespectrumofthedownlinksignal.Becausetheuplinkanddownlinksignalsoccupydifferentfrequencybandsrespectively,thesystemisnotsensitivetoreflection,andthereisnoneedforthecomplicateddelayadjustmentcircuitnecessaryfortheTDMAmode.Thetransmissiondelayissmallandthecircuitisrelativelysimple.
Frequencydivision
Thefrequencydivisiontwo-waytransmissionmethodusesdifferentcarrierfrequenciestotransmituplinkanddownlinksignalsrespectively.Dependingonthecontentoftheuplinkanddownlinktransmissions,differentuplinkfrequencybandsanddownlinkfrequencybandscanbeused,buttheremustbeaguardbandinbetween(anidlefrequencybandthatneithertransmitsuplinksignalsnordownlinksignals)toreducethefiltercharacteristics.Thefrequencybandcrossovereffectcausedbysteepness.
AccordingtotheGY/T106-1999regulations,5～65MHzfrequencybandisusedtotransmituplinksignals,110～1000MHzisusedtotransmitanalogTV,digitalTVanddigitalservices,and65～87MHzisthetransitionband.
Becausetheuplinksignalandthedownlinksignalarearrangedondifferentfrequencybands,thetransmissionofthesignalcanbetransmittedthroughatransmissionline,andtheseparationofthesignalcanbesolvedbyusingdifferentfilters.Thecharacteristicofthismethodisthatthetechnologyissimpleandmature.Aslongasthegeneralfilterisused,theuplinkanddownlinksignalscanbeseparatedeasily.
Accordingtodifferentapplications,thefrequencydivisioncanbedividedintodifferentlevelsoffrequencydivision:
Lowdivision:thedivisionfrequencyis30-40MHz;
Middlesplit:thesplitfrequencyisabout100MHz;
Highsplit:thesplitfrequencyisabout200MHz.
Theselectionofthehigh,medium,andlowsegmentationmethodsmainlydependsontheamountofinformationtransmitted.Ingeneral,low-segmentationmethodscanbeusedforapplicationsystemswithsmallnodescalesandlessuplinkinformation(suchason-demandTV,Internetaccess,dataretrieval,etc.).Thescaleofthedockingpointislarge,andtheapplicationswithmoreuplinkinformation(suchasvideophone,conferenceTV,etc.)adoptthemediumandhighsplitmode.
Spacedivision
Spacedivisionusesdifferentlinestotransmituplinkanddownlinksignalsrespectively.Inopticalfibertransmissionsystems,thismethodismostlyadopted,usingtwo-coreopticalfiberstotransmituplinkanddownlinksignalsrespectively.However,incabletransmissionsystems,thismethodisusuallynotusedbecausethecostistoohigh.Theadvantageofthismethodisthatthetechnologyissimple,andthereisnointerferenceproblembetweentheuplinksignalandthedownlinksignal,butthespacedivisiontransmissionactuallyusestwosetsofone-waytransmissionsystems,whichareacombinationoftwoone-waytransmissionsystems,andcannotbetrulycalledTwo-waysystem.
Timedivision
Thetimedivisionmethodistousetimedivisionmultiplexingtechnologytoseparatetheuplinkanddownlinksignals,thatis,dividethesystemtransmissionsignalintoseveraltimeperiodsandtransmitthemalternatelyintime.,Downlinksignal.
Indigitalcommunicationsystems,thismethodisused,thatis,thetimeoftransmittingtheuplinkordownlinksignaliscontrolledbyapulseswitch,andthedownlinksignalistransmittedinonepulseperiod,andthenextpulseperiodistransmittedinthenextpulseperiod.Uplinksignal.Theadvantageofthismethodisthattheuplinkanddownlinksignalsarecross-transmittedwithoutmutualinterference.However,itrequiresaccuratesynchronizationbetweentheswitchesofthesendingendandthereceivingendofthesignal,andthesignalprocessingismorecomplicated.