GPON 关键技术——测距

GPON关键技术---测距OLT通过Ranging测距过程获取ONU的往返延迟RTD（RoundTripDelay），从而指定合适的均衡延时参数EqD（EqualizationDelay），保证每个ONU发送数据时不会在分光器上产生冲突。Ranging的过程需要开窗，即QuietZone，暂停其他ONU的上行发送通道。OLT开窗通过将BWmap设置为空，不授权任何时隙来实现。ForinstanceONTOLT(inCO)20km3km12kmMaincontent PhaserelationspecificationbetweenD/SandU/S Configurationofthephasedelaypoints Definitions RTDmeasurementprocessMaincontent PhaserelationspecificationbetweenD/SandU/S Configurationofthephasedelaypoints Definitions RTDmeasurementprocessPhaserelationspecificationbetweenD/SandU/SDuetothePONcharacteristic,theupstreamdata,receivedintheOLTisbasedonthesumofallONUstransmitteddata.Inordertoavoidsuchcollisions,atransmissionwithintheupstreamframeisassignedtoeachONU,whereonlythisspecificONUisallowedtosenddata.Inaddition,allONUsmustappearequidistantfromtheOLTforupstreamframing,i.e.,thebeginningofallupstreamframesfromallONUsshouldreachtheOLTatthesametime.Inordertoachievethis,anEqualization-DelayisassignedtoeachONU.TheONUdelaystheupstreamphase,inreferencetothedownstreamphase,basedonthevalueoftheassignedEqualization-Delay. TheRangingprocessdealswiththeEqualization-Delaycalculation.TheEqualization-DelaycalculationisbasedonmeasuringtheroundtripdelaybetweentheOLTandeachONU.Maincontent PhaserelationspecificationbetweenD/SandU/S Configurationofthephasedelaypoints Definitions RTDmeasurementprocessConfigurationofthephasedelaypointsMaincontent PhaserelationspecificationbetweenD/SandU/S Configurationofthephasedelaypoints Definitions RTDmeasurementprocessDefinition（一）Opticalfibrepropagationdelay(Tpd) TheOpticalfibrepropagationdelay(Tpd)isduetotheopticalfibrelength/distancebetweentheOLTandONUBasictransmissiondelay(Ts) TheBasictransmissiondelay(Ts)isduetothePONsignalprocessingintheONUOpticaldelay TheOpticaldelay(TiO1,TiO2,TiS1,TiS2)isduetooptoelectricalandelectro-opticalconversionintheONUandOLTEqualization-Delay(EqD) TheEqualizationdelayisaninternaldelayintheONU,setandcontrolledbytheOLT.Thepurposeofthisparameteristodelaytheupstreamtransmission,soitarrivesattheOLTatthecorrectphaseDefinition（二）Measuredroundtripdelay Whenmeasuringtheroundtripdelay(RTD)toaspecificONU,usingtherangingprocedure,theEqualization-delayisconventionallysettozero;however,thismaybesettosomepre-definedvalue.Therefore,theresultwillbebasedonthesumofthefollowingdelays:RTD=2*Tpd+Ts+TiO1+TiO2+TiS1+TiS2+EqD ItisestimatedthatassufficientsignalprocessingtimeintheOLTandONU,thevalueofTs+TiO1+TiO2+TiS1+TiS2shouldbelowerthan50μs.Inaddition,2*Tpdisequalto: Definition（三）Equalizedroundtripdelay DuetothedifferentdistancesoftheONUsfromtheOLT,itisrequiredthattheupstreamphaseofallONUswillbethesame.Therefore,theONU'sEqualizationdelay(EqD)issetinsuchawaythatallONUswillhavethesameconstantequalizedroundtripdelay(Teqd).Theequalizedroundtripdelay(Teqd),isdefinedas: Teqd=RTD(n)+EqD(n)Accordingly,theONU'sEqualizationdelay(forONUn)iscalculatedby: EqD(n)=Teqd–RTD(n)PhasemonitoringandupdatingRTDOncetheONUissuppliedwithitsEqualization-Delayfactor,itissynchronizedtothebeginningoftheupstreamframe.TheactualupstreamdataistransmittedinaspecificONUtransmission(agroupofTimeSlots)insidetheupstreamframeaccordingtothereceiveddownstreampointers.ThebeginningoftheupstreamframewillbedelayedinreferencetothebeginningofthereceiveddownstreamframebytheEqualization-Delayfactor.TheONU'supstreamtransmissionisexpectedtoarriveinafixedtimeduringtheupstreamframe.ThearrivalphaseoftheONUtransmissionmaydriftduetoagingandtemperaturechanges,etc.Inthosecases,theEqualization-Delaycanberecalculated/updatedfromthedriftoftheupstreamtransmission.ThiswaynoadditionalRangingprocessisrequired.ThechangeintheEqualization-Delaywillbeequaltothedrifttimewithoppositesign.Therefore,iftheframeisearly,thedrifttimewillbeaddedtotheEqualization-Delay.Iftheframeislate,thedrifttimewillbesubtractedfromtheEqualization-Delay.ThenewEqualization-DelayvaluewillbecalculatedbytheOLTandwillbeupdatedattheONU,usingtheRanging_TimePLOAMmessage.Maincontent PhaserelationspecificationbetweenD/SandU/S Configurationofthephasedelaypoints Definitions RTDmeasurementprocessRTDmeasurementprocessRound-Trip-Delay[RTD]isthetimefromfirstbit/byteoftheRanging-requestinthedownstreamframetillthereceptionoftheRanging-transmission'slastbit/byte.ItisusedforthecalculationoftheEqualization-Delay. CriteriaforsuccessfulorfailedRTDmeasurement SuccessfulRTDmeasurementprocedureCriteriaforsuccessfulorfailedRTDmeasurementAvalidRangingtransmissionwithamatchingONU-IDandSerialNumberisreceivedbytheOLT.TheRangingtransmissionisreceivedwithintheexpectedtime-limitsbasedonthemaximumlengthofthePON.ThemeasuredRTDiswithinatimerange,whichisbasedonanestimateddistancevaluebetweentheONUandtheOLT.TheOpSSystemsuppliestheestimateddistancevaluetotheOLT.Ifnodistancevaluewassupplied,thisconditionisignored.TheRTDiswithinAllowed-Ranging-Variance,whichisNbitsaccordingtotheupstreambitrate,comparedwiththelastsuccessfulRTDmeasurement.ThisconditionisignoreduntilthefirstsuccessfulRTD.1.244Gbit/s–8bits.622Mbit/s–4bits.155Mbit/s–1bit.SuccessfulRTDmeasurementprocedureTheRTDmeasurementprocedureisconsideredcompletedfollowingtwosuccessfulortwofailedmeasurements.ForsuccessfulRTDmeasurementprocedure,theRTDwillbetheaverageofthetwosuccessfulmeasurements.ForfailedRTDmeasurementprocedure,aSUFialarmwillbeassertedandtheONUwillbedeactivated,usingtheDeactivate_ONU-IDmessage(thusmovingtoInitial-state(O1)).Ranging测距原理图OLTONU处于距离修正状态StartofD/SFrameReceptionofD/SFrameTransmissionofS/NResponseActualReceptionofS/NResonseDesiredStartofU/SFrame分配EqDPre-AssignedEqD零距离'EqDDesiredReceptionofSNResponseSstartU/SBWMapcontainingRangingRequestONUResponseTimePre-AssignedEqDSstartStartofU/SFrameforPre-rangedONU时间