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Measuring 802.1AS Slave Clock AccuracyAlon Regev

Introduction to IEEE 1588 and IEEE 802.1AS

• IEEE 1588 & IEEE 802.1AS standards define how to synchronize time accurately between nodes on a network

• IEEE 1588 standardized the use of physical layer timestamps to compute network delays and define synchronization events

• This achieves much higher timing accuracy than legacy protocols (such as NTP*) where timestamping is typically done in SW

• IEEE 802.1AS is a 1588 “profile” with fewer options, and extended physical layer options • Faster clock locking• Allows for easier / lower cost implementation• Every device in the path (endpoints and relays) must support 802.1AS

* NTP refers to the “Network Time Protocol” defined by IETF RFC 5905.

IEEE 802.1AS: How it works

• Best Master Clock Algorithm (BMCA) is used to select a Grand Master• Grand Master periodically sends the clock using sync messages• Each Relay corrects timing information based on the delay through the cable as well as the delay through the

relay itself• The Relay acts as a slave in the port in which it receives a clock and as a master on other ports

• Slave endpoints receive timing information and correct for the delay through the cable.

PicturecourtesyofMichaelJohasTeener,postedtoWikipedia.Seehttps://en.wikipedia.org/wiki/File:Clock-Master.pdf

Time Correction in a Bridge

• Bridge Delays are now relatively constant, since they are just cable delays, without queues or buffers– 1588 calls this a “transparent clock”, required in 802.1AS

• A “correction field” in the FollowUp is incremented by the upstream delay and the residence time (t3-t2)– The correction field plus the precise origin timestamp plus the upstream delay is the correct time

master clock 802.1AS bridge

Synch tx: t1

rx: t2

queue synch

every 100ms

Synch

"downstream" device

FollowUp (t1)

FollowUp (t1*)

tx: t3

d

know: upstream path delay (d),

bridge delay (t3-t2),

calculate: t1*= t1 + d + (t3-t2)

t3-t2

note:

t3 and t2 are bridge-local times,

they do not have to be synch'd

to the master since only their

difference is significant

SlidecontentcourtesyofMichaelJohasTeener,presentedatDeterministicEthernetTutorial.Seehttp://www.ieee802.org/802_tutorials/2012-11/8021-tutorial-final-v4.pdf

Path Delay Processing

• Done infrequently since delays are stableSlidecontentcourtesyofMichaelJohasTeener,presentedatDeterministicEthernetTutorial.Seehttp://www.ieee802.org/802_tutorials/2012-11/8021-tutorial-final-v4.pdf

Why measuring slave clock accuracy is important

• Whilethe802.1ASalgorithmworkswell,imperfectimplementationsand/orhardwareissuescancausetheslaveclocknottotrackthemasterclock

• Itisimportanttoverify&certifyimplementationstomakesuretheyarecorrect.• Avnuhasdetailedtestplanstovalidate802.1AS• Typically,PTPorgPTP implementationsthathavenotbeenvalidatedwillhaveconformanceissues

• Tovalidatetheclockaccuracyonaslavedevice,welookattheTimeError(TE),whichisthedifferencebetweenthetestreference(802.1ASmaster)andtheDUT(802.1ASslave).

• ThisneedstobesampledoveralongperiodoftimetofindouttheMAXTE.

Different methods for measuring the slave Clock accuracy

1. Compare1PulsePerSecond(1PPS)orsimilaroutputbetweenthemasterandslave

2. “Ingressmethod”whereaslaveclockreportsthetimeerroritisseeingeachtimeitreceivesasyncpath

3. “Egressmethod”wheretheslaveprovidesthePTPtimeofmessagesthatitsent,whichcanbecomparedtothetimethesemessagesarereceivedbythemaster

4. “ReverseSyncmethod”wheretheslavesendssyncmessagesbacktothemastertovalidatethetiming• ThisisessentiallyavariationontheEgressmethodwherethemessageusedisaSyncmessageandthereis

noneedforadditionalmessagingtoidentifythePTPtimewhenthemessagewassent.

Thedetails,pros,andconsofeachmethodaredetailedinthenextslides.

Itisrecommendedtoimplementtestmodescontainingatleastoneoftheabovemethodsineachslave• Implementingmultiplemethodsallowscross-validationofthemeasurementmethods

SeeAvnurecommendationsathttp://avnu.org/wp-content/uploads/2014/05/Avnu-Testability-802.1AS-Recovered-Clock-Quality-Measurement-1.0_Approved-for-Public-Release.pdf

Using 1PPS outputs to validate slave time synchronization

• BothmasterandslaveoutputapulsethatstartsoneachsecondboundaryofthePTPclock• Anoscilloscopeisusedtocomparethetimeofthetwooutputs

• eachmeasurementshowsaphasedifferenceintheclock• Usingmultiplemeasurements,maximumtimeerrorandjittercanbeascertained.

Advantagesof1PPSmethod:• Knownindustrystandardwhichalreadyexistsonmanydevices.• Onlyanoscilloscopeisneededtomakethistest.

Shortcomingsof1PPSmethod:• Implementationofthe1PPSitselfissometimesimperfect,causingafixedphaseoffsetorjitter.• Somedevicescan’teasilysupporthavinga1PPSoutput(i.e.deviceswithmechanicalconstraintsthatpreventthis).• Thedevicesundertesthavetoberelativelyclosetogether• Notpracticalforlarge-scaletesting(suchasvalidatingtimesynchronizationinanentiresportsarenaorfactory).

Ingress method

IngressmethodreliesonSlavetodeclareit’serroratthemomentofreceivingaSyncmessage(Example:“WhenyousentmethelastSyncmessage,myrecoveredclockwhenreceivingitwasX”)Twoflavorsdefinedin1588,onethatreportserror,andonethatreportsT1andT2– sameprinciple!

DUT(Slave) TESTER(Master)

T1SyncT2Sync1.SlavesavesT2in

recoveredclocktimeofreceivingSyncmessage

2.SlavereceivesandsavesT1andsyncs

3.SlavereportsT1andT2orthecomputedoffsetFromMaster

4.MastercomputesrecordstimeerrorfromslaveprovideddataoffsetFromMaster=T2-T1-knownpathdelay

T1SyncT2Sync1.SlavesavesT2in

recoveredclocktimeofreceivingSyncmessage

2.SlavereceivesandsavesT1andsyncs

3.SlavereportsT1andT2orthecomputedoffsetFromMaster

4.MastercomputesrecordstimeerrorfromslaveprovideddataoffsetFromMaster=T2-T1-knownpathdelay

Ingress method pros/consAdvantagesofIngressmethod:• Reportingcanbedonein-bandwith1588proposedTLVsattachedonsignalingmessageorsimplylocallylogged/storedtobe

accessedremotelythroughYANG(idealforlivedeployments)• DoesnotrequireanyextracapabilityontheSlaveexceptsendingTLVs*/storingthedata

ShortcomingsofIngressmethod:• SamplingoftimeerrorisdependentonthetimingofincomingSyncmessages

ItmightnotreflecttheactualmaximumtimeerrorbetweentwoincomingSyncmessages• TheSlavecanreportavaluethatisnottheactualone

Thiscouldbeduetoafixedphaseoffset,codingerror,orduetointentionallyreturningvaluesthatmakethedeviceseembetter(veryhardtodetect,exampleinbackupslides)

=>theIngressmethodisnotadequateforcertificationtesting

*ATLV(orTypeLengthValue)isamessageformatdefinedinIEEEStd.1588.

Egress method

Egressmethod- Slavereportsit’srecoveredtimeatmomentwhenaneventmessageissent(Example:“Slave:thelastPdelayReqIsentwasatglobalgPTPtimeXaspermyrecoveredclock”)

AdvantagesofEgressmethod:• Testerevaluatestheerror,itdoesnotrelyontheSlavetoreportit• Reportingcanbedonein-bandwith1588proposedTLVsattachedonsignalingmessage• TimeofreportingisdecoupledfromthereceivetimeoftheincomingSyncmessages

ShortcomingsofEgressmethod:• Complicatedtouseindeployednetworks– needssupportfromdirectlyconnectedMasterofDUT

DUT(Slave) TESTER(Master)

1.SlavesendsPdelayReqandsavesT1- timeofsending

3.SlavesendssignalingmessagesindicatingT1

4.Testerreceivessignalingmessages,andcomputesTE=T2-T1-PD

2.TesterreceivesPdelayReqatT2andsavesvalue

Problems in using Egress method for gPTPForgPTPcommonsenseisthatreportingwouldbebasedonthesentPdelayReqeventmessages.IfPdelayReqmessagesaretimedimmediatelyorclosetoreceivingaSyncmessage(whencorrectionoftheclockismade),theymightnotrevealtheactualtimeerrorthedeviceisexperiencing.

1. ConsideraSlaveclockdrifting400nsbetweeneach2Syncmessages(125msinterval)2. ANDnotproperlyimplementingsyntonization(algorithmorrateratiocalculus)3. Thetimeerrorfunctionlookssimilartoasaw(resetsto~0ateachSyncreceived)4. IfeventPdelayReqissentcloseafterSyncisreceivedmeasurederrorislowerthanone5. BecausePdelayReqintervalisamultipleoftheSyncinterval,thiswouldhappenateverytime

Notjusttheory,thiswasexperiencedwithrealdevicesinlabtesting!

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TE=50ns TE=50ns

1s

TE=200nsTE=200ns

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TE=350nsTE=350ns

1s

Reverse Sync Method

DUT(Slave) TESTER(Master)

T1Sync

T2Sync

Norm

algPTP

Ope

ratio

n

T1SyncReverse

T2SyncReverseRe

verse

Messages

1. SlavecalculatespathdelaytoMaster

2. SlavesynchronizestoMaster

3.MastercalculatespathdelaytoSlave

5.MastercalculatestimeerrorofSlave

TheReverseSyncMethod1. Theslavesynchronizesit’s

timetothemaster2. TheReverseSyncisenabled

ontheslave(testmode)3. TheMaster(tester)calculates

thepathdelaysothatitcanaccountforthiswhenvalidatingthesyncmessages

4. TheslavesendsSyncmessagesinthereversedirection(usingadifferentdomain)

5. Observations:theTESTER(Master)willcalculatethetimeerrorintherecoveredclockoftheDUT(Slave)

Adjust Reverse Sync rate for better samplingDUT(Slave) TESTER(Master)

1Sync

Interval

1Sync

Interval

1Sync

Interval

1Sync

Interval

90%Sync

Interval

90%Sync

Interval

90%Sync

Interval

90%Sync

Interval

90%Sync

Interval

90%Sync

Interval

ReverseSync

90%Sync

Interval

ReverseSync

Advantages of Reverse Sync for Integrated Devices• OnintegrateddevicesthathaveanintegratedSlaveandBridge,thereisnoobservabilityoftheSlavePdelay

mechanism• UsingSyncmessagesasEgress,theBridgecancorrectforthe“residencetime”oftheReverseSyncthesamewayas

theywouldnormallydoforthestandardgPTPinstance⁻ ThebridgejustforwardstheReverseSync(withtimingcorrections)toitsMasterporttotheTester⁻ Thisiseffectivelyusingfunctionalitydefinedin802.1AS-rev,usingonedomainforSynchronizationandadifferent

domaintodotherecoveredclockmeasurement• IfpropertimeissetinReverseSyncpreciseOriginTimestamp thereisnoneedforadditionalegressTLV• Implementationisstraightforward– canbedonefromexisting“SendSync”code

Slave TESTER(Master)Bridge

DUT:IntegratedECUwithSlave+Bridge

ResidenceTime

Summary

It is important to test 802.1AS slave clock accuracy

Multiple methods are now defined allowing slave clock accuracy to be measured including- 1PPS- Ingress Method- Egress Method- Reverse Sync Method

It is recommended to implement multiple methods to cross-check results

Summary: Advantages & Disadvantages of Each Method

Method Advantages Disadvantages

1PPSoutputs • Knownindustrystandard• Onlyanoscilloscopeisneededtomakethis

test

• Hardtovalidateimplementation• Notavailableonalldevices• DUTsmustbeclosetogether• Notpracticalforlarge-scaletesting

IngressMethod • Reportingcanbedonein-band• Noneed forextracapabilitiesontheSlave

• SamplingoftimeerrorisdependentonthetimingofincomingSyncmessages

• TheSlavecanreportavaluethatisnottheactualone

EgressMethod • Testerevaluatestheerror,itdoesnotrelyontheSlavetoreportit

• Reportingcanbedonein-band• Timeofreportingisdecoupledfromthe

receivetimeoftheincomingSyncmessages

• Complicatedtouseindeployednetworks–needssupportfromdirectlyconnectedMasterofDUT

ReverseSync • Alladvantages ofegressmethod,plus• Canbeusedonintegrateddevices• NoneedforadditionalegressTLV• Implementationisstraightforward

• Requires SyncgenerationonSlave

Questions?