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ThePTPTelecomProfileforFrequency
Synchronization
ConfidentialCopyright2012
TimFrost(presentedbyLeeCosart)
May23,2012
PTP(PrecisionTimeProtocol)Defined
TheNatureofPacketTiming
Agenda
FrequencySync ron zat onArc tecture
G.803ReferenceChain
PacketTimingArchitecture
ThePTPTelecomProfile(G.8265.1)
ObjectivesandDesignFeatures
SourceTraceabilit
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Multicastvs.
Unicast messages
RateofTimingMessages
MasterSelectionandProtection
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PTP(PrecisionTimeProtocol)Defined
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WhatisthePrecisionTimeProtocol(PTP)?
ProtocolSpecificationfordistributingprecisetimeandfrequencyoverpacketnetworks
Firstversion(2002)targetedLANapplications
Secondversion(2008)expandedapplicabilitytocovertelecommunicationsnetworks
TimeiscarriedineventmessagestransmittedfromaGrandmasterClocktoa
SlaveClock
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Runs
over
Ethernet
and/or
IP
networksCommonlyreferredtoas:
PTP(PrecisionTimeProtocol)orPTPv.2
IEEE15882008orIEEE1588v.2
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WhatisaPTPProfile?
Whatisaprofile?
ProfileswereintroducedinIEEE15882008,toallowotherstandards
Profilescontainadefinedcombinationofoptionsandattributevalues,aimedatsupportingagivenapplication
Allowsinteroperabilitybetweenequipmentdesignedforthatpurpose
PTPTelecomProfileforFrequency(G.8265.1)publishedOct.2010
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uppor s requencysync ron za onover e ecomne wor s
Main
use
case
is
the
synchronization
of
cellular
base
stations
TheG.8265.1PTPTelecomProfileenablesthedeploymentof
PTPbasedsynchronizationbytelecomoperators
TheNatureofPacketTiming
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Conventionaltimingsignal:
Anominallyperiodicsignal,generatedbyaclock:
WhatisaPacketTimingSignal?
Conventionaltimingsignal:
Anominallyperiodicsignal,generatedbyaclock:Si nificant instants
Packettimingsignal:
A nominall eriodic si nal enerated b a acket master clock:
Timingjitterandwander
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PacketDelay
Variation
Payload4 HPayload4 H Payload4 HPayload4 H Payload4 HPayload4 H Payload4 HPayload4 H
Packets(header,payloadandfooter)
Payload4 HF Payload3 HF Payload2 HF Payload1 HF
Significantinstants
UseofTimestamps
Packettimingsignal:
Asequenceoftimedevents,generatedbyapacketmasterclock:
TS=8 HF TS=4 HF TS=1 HF
gn cant nstants
Packetscontainingtimestamps
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123456789
Reconstructedfrequency:
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PrecisionTimeProtocol(PTP)
PTPdefinesanexchangeoftimedmessagesoverapacketnetwork
Each event message flow (sync,Master Clock Time Slave Clock Time
Sync message
t2
t1
Delay_Req message t3
_
Master frequency determined bycomparison of timestamps in the eventmessage flows
e.g. comparison oft1 to t2 over multiplesync messages, or t3 to t4 indelay_reqmessages
Time offset calculation requires all fourtimestamps:
Follow_Up messagecontaining accuratevalue oft1(if required)
Data atSlave Clock
(t1), t2
t1, t2, t3
t1, t2
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t4
Slave time o set =(t1 t2) + (t4 t3)
assumes symmetrical delays(i.e. the forward path delay is equal to the
reverse path delay)
Time offseterror=fwd. delay rev. delay
2
2time
Delay_Resp messagecontaining value oft4
t1, t2, t3, t4
FrequencySynchronizationArchitecture
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FrequencySynchronizationArchitecture
SSUk1PRC SSU1 SSUkChainofupto
*
Chainofupto*
Chainofupto*
NE
G.803SynchronizationReferenceChain:
s s s
SSUk1PRC SSU1 SSUkChainofupto
20EECs*MixedChainof upto
20SECs&EECs*MixedChainofupto
20SECs&EECs*NE
G.8261ModifiedReferenceChaintoincludeSynchronousEthernetClocks(EEC):
G.8261ModifiedReferenceChaintoincludePacketBasedTiming:
11ConfidentialCopyright2012* MaximumnumberofSECsorEECsintotalchain=60 MaximumnumberofSSUs(k)intotalchain =10
SSUk1PRC SSU1 SSUk
SM
Chainof
up
to
20EECs*Mixed
Chain
of
up
to20SECs&EECs*NE
Packet
Network
PTPMaster
and
Slave
connected
bypacketnetwork
SSUk1PRC SSU1 SSUk
SM
Chainofupto
20EECs*MixedChainof up
to20SECs&EECs*NE
Packet
Network
PTPMasterandSlaveconnected
bypacketnetwork
PRCPRC
GeneralPacketTimingArchitecture
PacketTimingArchitecture(G.8265)
PacketNetworkTimingProtection
PTP
GM
PTP
Primary
PTPGMPTP
GM
PTP
Primary
PTPGM
Physicallayer
syncnetwork
PTPTimingFlows PTPTimingFlows
Protection
Timin Flows
PTP
GM
PRC
Secondary
PTP GM
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Slave
PTP
SlavePTP
Slave
PacketNetwork
Slave
PTP
SlavePTP
Slave
PacketNetwork
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ThePTPTelecomProfileforFrequency(G.8265.1)
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PrimeObjectives
TopermitthedistributionoffrequencyusingPTPoverexistingmanaged,widearea,packetbasedtelecomnetworks
(suchasSyncE andSDH)
Todefinemessageratesandparametervaluesconsistentwithfrequencydistributiontotherequiredperformancefortelecom
applications
Toallowthesynchronizationnetworktobedesignedand
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con gure na xe arrangement
Toenableprotectionschemestobeconstructedinaccordancewithstandardtelecomnetworkpractices
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Keydesigndecisions
Noonpathsupport,(e.g.boundaryandtransparentclocks),becausethesearenotgenerallyavailableinexistingnetworks
,
thanoperationoverEthernetorotherlowerlayerprotocols
ThePTPAnnounce messagewasadaptedtocarrytheQualityLevel(QL)indicationsdefinedinG.781,forcontinuitywith
SONET/SDHandSyncE synchronizationstatusmessaging.
Unicast transmissionwasadoptedovermulticast,sinceitcould
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eguarantee towor overw eareate ecomsnetwor s
BMCA(Best
Master
Clock
Algorithm)
was
replaced
by
static
provisioning,allowingthesynchronizationflowtobeplanned,
ratherthandynamicallyadjustingitself
SourceTraceability
EncodesQLvaluesintheclockClass fieldoftheAnnounce message
Providesendtoendtraceabilityofthereferencesourcealongthe
Informstheslaveclock(andsubsequentdevices)ofthequalityofthetimingsource
Allowsthetimingchaintobemanagedinasimilarwaytoexistingsynchronizationnetworks
EndtoEndSourceTraceability
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SSM:QLPRC[0010] clockClass:QLPRC[84]
PRC
PTP
GM
End
Equipment
PTP
Grandmaster
PacketNetworkPhysicalLayer
Synchronization Network
PTPSlave
PTP
Slave
SSM:QLPRC
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SSMQLvaluetoPTPclockClass Mapping
SSMQLvalue
ITUTG.781Network OptionsPTPclockClass
valueOptionI(2Mhierarchy)
OptionII(1.5Mhierarchy)
OptionIII(6.3Mhierarchy)
0001 QLPRS 80
0000 QLSTU QLUNK 82
0010 QLPRC 84
0111 QLST2 86
0011 88
0100 QLSSUA QLTNC 90
0101 92
0110 94
1000 QLSSUB 96
decreasing
quality,
increasing
clockClass
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1 1
1101 QLST3E 100
1010 QLST3/QLEEC2 102
1011 QLSEC/QLEEC1 QLSEC 104
1100 QLSMC 106
1110 QLPROV 108
1111 QLDNU QLDUS 110
Multicastvs.Unicast
Unicast facilitatestheuseofdistributedmasters
EachmasterslavecommunicationpathbecomesaseparatePTP
Allowseasierplanningofthesynchronizationnetwork
Redundancystrategycanbecarefullymanaged
Unicast packetspropagateuniformlythroughthenetwork
Multicastrequirespacketreplicationateachswitchorrouter
Replicationprocessaddsvariabledelay
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Multicast
harder
to
provision
for
network
operators Upstreammulticastoftennotsupportedintelecomnetworks
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UnicastRegistration
MasteronlyprovidesUnicast service
Nomulticastannouncemessagessent
Slaveismanuall confi uredwiththeIP
Master Slave
addressofoneormoremasters
SlaverequestsMastertoprovideunicastserviceataspecifiedrate
RequestsAnnounce servicefirst,toverifyqualityofthemaster
Ifwithincapacitylimits,Masterrespondswithservicegrantacknowledgements
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SlaverequestsSyncandDelay_Request
serviceonly
ifmaster
quality
is
sufficient
Grantsarelimitedduration
Requestsmustbeperiodicallyrepeated
Freesupmasterresourcesifslavefails
RateofTimingMessages
Therateoftimingmessagesrequiredisdependentonseveralfactors
Amountofnoiseinthenetwork
Efficiencyofclockservoalgorithm
TheTelecomProfiledefinestherangeofmessageratesMastersandSlavesshouldsupport
Messagerates Minimum Maximum Default
Announce 1msg.every 16s 8messages/s 1msg.every2s
S nc 1ms .ever 16s 128messa es s Notdefined
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Itisnotexpectedthataslavewillachievetherequiredperformanceatallmessagerates
Slavemustrequestthemessageratesneededtomaintainperformance
Delay_Request 1msg.
every 16s 128
messages/s Not
defined
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PacketTimingSignalFail
Profiledefinesthreetypesofsignalfailure:
PTSFlossAnnounce,wherethePTPSlaveisnolongerreceiving
Thismeansthereisnotraceabilityinformationforthatmaster
SlaveshouldswitchtoanalternativeGMafterasuitabletimeoutperiod
PTSFlossSync,wherethePTPSlaveisnolongerreceivingtimingmessagesfromtheGM(i.e.SyncorDelay_Response messages)
Thismeansthereisnotiminginformationforthatmaster
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PTSFunusable,wherethePTPSlaveisreceivingtiming messagesfromtheGM,butisunabletorecovertheclockfrequency
Thismeansthereisnorecoverabletiminginformationforthatmaster
Actionisundefined
Telecomslaveclockconsistsofseverallogicalprotocolinstances,eachcommunicatingwithadifferentgrandmaster
Selection rocess follows G.781 selection rules:
MasterSelectionandProtection
Availability,Traceability,Priority
Telecom
SlaveClockPTPGM
1
Slave
Protocol
Instance1
Slave
G.781
based
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2
PTPGM
N
Protocol
Instance2
Slave
Protocol
InstanceN
Master
Selection
Process
Network
ListofN
Grandmasters
SeparatePTPdomains
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AdditionalProtectionFunctions
Nonreversionfunction
Bydefault,aslaveshouldswitchbacktotheoriginalmasteronce
Optionally,thisautomaticreversionfunctioncanbedisabled
WaittoRestoreTime
Followsaninitialprotectionswitch,e.g.duetolossoftraceabilityorsignalfailure
Timewaitedbeforeswitchingbacktotheoriginalhighestpriority
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mas er,once e a urecon on as eenrec e
Implies
slaves
must
continually
monitor
the
original
master
followingaprotectionswitch
AdditionalTraceabilityFunctions
Forcedtraceability
IfthePTPGMisconnectedtoareferencebyasignalwithnoSSMQLvalue,thein utcanbemanuall forcedtoasuitablevalue
OutputQLHoldOff
AppliestoslaveoutputtimingsignalsthatcarryanSSMQLvalue(e.g.SyncE)
ChangeofQLintheincomingPTPclockClass inputshouldbedelayedbeforebeingappliedtotheoutput
Allowstimeforsynchronizationtoanewreference
Avoidsanyunecessary switchingindownstreamequipment
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Output
Squelch OutputclocksignalofaPTPslaveshouldbesquelchedincaseofholdover
Preventsendequipmentattemptingtosynchronizetoaclockinholdover
OnlyappliestosignalsthatdonotcarryaQLvalue(e.g.a2.048MHz
unframedtimingsignal)
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ForFurtherReading
WhitePaper:
SynchronizationforNextGenerationNetworks ThePTPTelecomProfile,Symmetricom WhitePaper,June2011
IEEEStandardforaPrecisionClockSynchronizationProtocolforNetworkedMeasurementandControlSystems,IEEEStd.1588TM2008,24July2008
PrecisionTimeProtocolTelecomProfileforFrequencySynchronization,ITUTRecommendationG.8265.1,October2010
BackgroundReading:
SynchronizationLayerFunctions,ITUTRecommendationG.781,August2008
ArchitectureofTransportNetworksbasedontheSynchronousDigitalHierarchy(SDH),
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. ,
Definitionsandterminologyforsynchronizationinpacketnetworks,ITUT
RecommendationG.8260,
August
2010
TimingandsynchronizationAspectsinPacketNetworks,ITUTRecommendationG.8261,April2008
ArchitectureandRequirementsforPacketBasedFrequencyDelivery,ITUTRecommendation G.8265,October2010
ThankYou
SeniorTechnologist,
Symmetricom,Inc.
Email:[email protected]
26ConfidentialCopyright2012
Symmetricom,Inc.
2300OrchardParkway
SanJose,CA951311017
Tel:+14084287907
Fax:+14084286960
www.symmetricom.com