A Concise Guide to Mobile Backhaul Synchronization

A Concise Guide to Mobile Backhaul Synchronization

The Ver i f icat ion Experts

August 2012

WHITE PAPER

www.veexinc.comVeEX Inc. 2827 Lakeview Court, Fremont, CA 94538 USA Tel: +1.510.651.0500 Fax: +1.510.651.0505

www.veexinc.com

VeEX Inc. 2827 Lakeview Court, Fremont, CA 94538 USA Tel: +1.510.651.0500 Fax: +1.510.651.0505 www.veexinc.com 2

WHITE PAPER: A Concise Guide to Mobile Backhaul Synchronization

Table of Contents1. Introduction .............................................................................................................................. 3

2. Synchronizationvs.Syntonization ............................................................................................ 3

3. What is Mobile Backhaul and Why Does it Need to be Synchronized? .................................... 4

4. What is IEEE 1588v2 PTP Protocol? .......................................................................................... 6

5. PTP and Network Impairments ................................................................................................ 8

6. TheTelecomProfile .................................................................................................................. 9

7. 1588v2SynchronizationHelpers:BoundaryandTransparentClocks ..................................... 10

8. PTPTestingMethodologiesandStandards ............................................................................ 11

9. TX130M+:TheSwissArmyKnifeofMBHTesting ................................................................... 14

About VeEX® ................................................................................................................................. 16



1. Introduction

As the Mobile Backhaul increases bandwidth between radio equipment and core networks to keep up withdemand,legacyTDMlinksarebeingreplacedbymoreefficientEthernetlinks.However,withpacketswitchednetworks(PSN)beinginherentlyasynchronous,thequestionarisesastowhatcouldfillthevoidofprovidingthetightfrequencyandphasesynchronizationrequiredbythemobilebasestationsandotherreal-timeapplications.

SeveraltechnologieslikeIEEE1588v2Precisiontimingprotocol,ITU-TG.8261SynchronousEthernetandGPS-disciplinedclocksarecurrentlyavailabletofillthatgap.Sinceeachonehasitsstrengthsandweaknesses,andsolutionsmayincludeacombinationofthem,MobileBackhaulSynchronizationiscurrentlyahottopicbeingstudiedanddiscussedbystandardizationbodiesandserviceproviders’labsaround the world. Thiswhitepaperprovidesaconciseguidetothesenewsynchronizationtechniques,withanemphasisonthe IEEE 1588v2 Precision Timing Protocol (PTP).

2. Synchronization vs. Syntonization

TwoclocksarealignedinFrequency,alsocalledsyntonization,ifthedurationofasecondisthesameonboth,whichmeansthatthetimemeasuredbyeachclockadvancesatthesamerate.TwoclocksarealignedinTimeofDay/PhaseandFrequency,alsocalledsynchronization,iftheirtimemeasurementofan event is the same.

Thedifferencebetweensynchronizationandsyntonization(orfrequencysynchronization)isanimportantconceptformobilebackhaultechnologies,becauseasdescribedinthenextsection,somemobileapplicationscanfunctiononlywithsyntonizationwhileothersrequirefullsynchronization.

Figure 1. Frequency Syntonization and Frequency and Phase/ToD Synchronization



3. What is Mobile Backhaul and Why Does it Need to be Synchronized?

TheMobileBackhaul(MBH)isthetransportsegmentthatlinksthecellsitestotheirmobileswitchingandcontrolcenters.Themobilebackhaultechnologieshaveevolvedtosatisfytheskyrocketingdemandfordataservicesaswellasstreamlinethetransportofvoice,dataandcontroltrafficintoanallIPnetwork.

In2Gnetworks,TDMleasedlineswithT1/E1connectivitywereusedtocarryvoice/data.Theintroductionof3GsawthetransitiontowardsATMtransport,whereas4G/LTEnetworksnowmandateanallIPnetwork,hencelosingthephysicallayerclockreferencetotheintroductionofasynchronouspacketnetworks.

Synchronizationofthebasestationsinfrequencyandphase/timeisanimportantrequirementformobiletechnologies. This allows for smooth call handover between neighboring cells (to avoid dropped calls) and is vitalforRFencoding.Thestandardshavestricttimeandphasesynchronizationrequirements,forexampletheGSMstandardcallsforafrequencysynchronization(syntonization)of±50ppb(partsperbillion)forbothRFfrequencygenerationpurposeandclockingofthetimebase.WhileLTE-TDD(timedivisionduplex)technologycallsforphasesynchronizationofneighboringcellswithin±3µsaswellasfrequencysyntonization.Thetablebelowsummarizesthefrequencyandphasestandardrequirementsfordifferentmobile technologies.

Figure 2. Mobile Backhaul Evolution

Table 1. Frequency and Phase accuracy requirements

eNodeB

NodeB

BTS

2G

3G

LTE

BSC

RNC

TDM

ATM

IP/Ethernet

S-‐GW MME

Circuit Switched Core Network

Voice

Voice

Packet Switched Core Network

Data

Evolved Packet Core Network

Iub IuCS

IuPS

Abis A

GPRS Network Data Gb

Packet Switched Core Network

S1 Voice Data

Radio Interface Technology

Frequency Accuracy

Phase Accuracy

CDMA ± 50 ppb ± 3 µs

CDMA2000 ± 50 ppb ± 3 µs

GSM ± 50 ppb N/A

UMTS-‐FDD ± 50 ppb N/A

UMTS-‐TDD ± 50 ppb ± 2.5 µs

LTE-‐FDD ± 50 ppb N/A

LTE-‐TDD ± 50 ppb ± 3 µs



Inthe2G/3Gnetworks,basestationshavemettheserequirementsinfrequencyaccuracybyrecoveringtheirclocksfromtheT1/E1TDMbackhaulconnections.ButwiththemigrationtoanEthernetbackhaul,thiscapabilityhasbeenlost.Alternatively,satelliteGPSreceiverscanbeusedateachcellsitetorecoverfrequency as well as phase with a high accuracy, but as GPS receivers require outdoor placement and have inherent cost and security concerns, other technologies like IEEE 1588v2 Precision Timing Protocol andITU-TG.8261SyncEhaveemergedasviablealternativesformobilebackhaulsynchronizationpurposes. The table below presents a summary of the pros and cons of each technology.

Carriers choice for one technology or another will depend on many factors including if there is a need forfrequencysyntonizationaswellasphasesynchronization.Inaddition,amixoftechnologiescanbedeployedtoachieveendtoendsynchronization;forexamplehavingaGPSbasedPRC(PrimaryReferenceClock)distributingtimingvia1588v2tothecelltowers;orusingSyncEforfrequencydistributionand1588forToDdistribution.

Afterthisoverviewofvariousavailablesynchronizationtechnologies,itistimetocoverthedetailsoftheIEEE 1588v2 PTP protocol.

TDM GPS ITU-‐T G.8261 IEEE 1588v2

Method Physical signal carries synchroniza3on in the line code

Uses GPS satellite receiver

Physical signal carries synchroniza3on in the line code

Packet based network sync based on 3ming packets exchanged between master and slave clocks

Frequency sync Yes Yes Yes Yes

Phase sync No Yes No Yes

ToD distribu3on

No Yes No Yes

Pros Legacy reliable technology

Highly accurate freq and phase distribu3on

Not subject to packet network impairments (delay/jiKer)

Can be deployed over legacy Ethernet networks and elements

Cons Carriers are migra3ng to packet networks

Need line of sight from antenna to sky Subject to jamming Possible degrada3on of civilian satellite signal for military reasons

Requires all network elements to be upgraded for SyncE physical layer support

Clock recovery impacted by packet network impairments (delay/jiKer)

Table 2. Frequency and Phase distribution technologies



4. What is IEEE 1588v2 PTP Protocol?

ThePTPprotocolreliesonadistributionmethodwheretiminginformationisincludedinpacketsexchangedbetweenmasterandslave(s)clocksusingexistingEthernetnetworks.TheappealofthePTPprotocol is that the PTP messages are transported on the same network as the regular data and voice trafficanddoesnotrequireallnetworkelementstobeupgradedwithdedicatedhardwaretosupportthePTPprotocol.Theclockgrandmasteristheultimatesourceforclocksynchronizationwiththedownstreamslaveclocksrecoveringtheirlocaltimefromthemessagesreceived.

Figure 3. 1588v2 Time distribution



In order to recover the clock, the slave device needs to be able to compensate for the network propagationdelayandtheclockoffsetbetweenthemasterandslave.ThisisdonebyutilizingthetimestampscontainedinSyncandoptionalFollowupmessagesandaDelayRequest/DelayResponsemessage exchange.

1. The master starts the exchange by sending a Sync message to the slave. The message contains thetimet1atwhichthepacketwassent.Theslavenotesthetimet2atwhichthismessagewasreceived.Dependingonthemasterhardwarecapabilities,thesendingtimemaynotbeaccurateenoughintheoriginalSyncmessage;thereforeitcanbeoptionallyfollowedbyaFollowupmessagecontainingtheaccuratesendtimet1oftheSyncmessage.AclocksupportingSyncandFollowupmessagesisreferredtoasatwo-stepclock,whereasaclocksupportingonlySyncmessages is referred to as a one-step clock.

2. TheSlavesendsaDelayRequestmessagetothemasterandnotesthesendtimet3.UponreceptionoftheDelayRequestthemasternotesthetimet4andembedsthisinformationintheDelay Response message.

3. Theslaveisnowarmedwith4timestamps.Thenetworkdelaybetweenmasterandslaveisestimatedbyaveragingthetimebetweenmasterandslave(t2-t1)andthetimebetweenslaveandmaster(t4-t3).Itisimportanttonotethatthiscalculationassumesthatthenetworkdelaysaresymmetrical.

4. Thetimingoffsetbetweenslaveandmasteristhendeterminedbysubtractingthedelayfromthedifferencebetweent1andt2.

5. WiththeDelayandOffsetinformation,theslaveisabletoalignitsclocktothemaster.6. Sync,optionalFollowup,Delay_ReqandDelay_Respareexchangedrepeatedlythroughoutthe

durationofthe1588v2sessiontocompensateforclockdrift.

Iffrequencysynchronization(withoutphasesynchronization)isneeded,thenonly2timestampscanbeused,andtherateofchangeintheslaveclockisalignedtothemasterbycomparingthetimestampsoftheSyncmessages.Iffullfrequencyandphasesynchronizationaredesired,thenall4timestampsmustbe used.

Figure 4. PTP Messages



5. PTP and Network Impairments

Asyoucanunderstandfromthistimerecoverymechanism,networkasymmetry(delaydifferencebetweenthesendpathandreceivepath)andPacketDelayVariation(PDV)arecriticalelementsfortheaccuracy of the recovered clock.

ThePTPalgorithmusesasitsmainassumptionthatthedelaybetweenthemasterandtheslaveisequivalent to half of the round trip delay, but if network asymmetry is introduced in the physical layer whenthePTPmessagesdonottravelviathesamesendandreceivepaths,theaccuracyoftimerecoverywillbeimpacted.Thenetworkshouldbeengineeredtominimizesuchdifferences,oriftheasymmetryvalue is known the PTP algorithm can correct for it.

A chain of network elements between the master and the slave with variable network load and varying queuingandprocessingdelayscancausedelayvariationsinthePTPmessages.BecausethePTPalgorithmassumesaconstantnetworkdelay,changesinpacketarrivaltimeareproblematicfortheslave.Itcannotidentifythedifferencebetweenvariationinpacketdelayandatimingdriftinthemaster.MostoftenspecificalgorithmsareimplementedintheslavetofilteroutasmuchaspossibleofthePDV.Toalesserextentpacketloss,packeterror,orduplicatedpacketsarealsocommontrafficimpairmentsthatcanaffectPTP.

Howmuchnetworkimpairmentsaretolerableandhowdotheyaffectclockstability?ThesearetopicscurrentlyunderstudyintheITU,andreinforcetheneedtotestPTPinrealisticnetworkconditionspriorto live deployment.



6. The Telecom Profile

ThetelecomprofiledefinedinITU-TG.8265.1collectsthebestpracticesPTParchitectureandparametersinordertosecurefrequencysynchronizationinacarrier-gradeenvironment.ITU-TG.8275.1iscurrentlybeingworkedonbytheITUinordertodefineaPTPprofileforTime/Phasedistribution,althoughitisnotyetratifiedatthetimeofthispaper.

Thetelecomprofilewasdefinedfortelecomnetworkswherenosynchronizationsupportisavailablefrom the network outside of the messages exchanged between the master and the slave and it is applicableformobilebackhaulsynchronization,thekeyfactsonthetelecomprofileareoutlinedbelow:

• PTP Transport:thePTPmessagesaredefinedtobetransportedoverIPv4andUDP,oftenVLANwillbeusedfortrafficprioritizationandsegregationpurposes.

• Unicast vs. Multicast:althoughbothunicastandmulticastPTPmechanismsaresupported,thetelecomprofileusesunicasttransmission.Theslaveneedstobeprovisionedwiththeunicastaddressof the master and request service.

• Domains:adomainnumberisdefinedbyG.8265.1intherangefrom4to23,thedomainnumberincludedinthemaster/slavemessagesisusedtoisolatethedifferentPTPmaster-slavecommunicationpairssharingthenetwork.

• One-way vs. Two-way: two-way PTP message exchange from slave to master and master to slave is definedinthe1588v2standard,howeverthetelecomprofilesallowsforone-waycommunicationwhereonlySync(andoptionalFollow_up)messagesfrommastertoslaveareincluded,sincefrequencydeliverydoesnotrequiretheuseofDelay_Req/Delay_Respmechanism.

• One-step vs. Two-step:Atwo-stepmasterclockusesaSyncmessagewithtentativetimestamp,followedbyaFollow_upmessagewithaprecisetimestamp,whereasaone-stepmasterclockusesonlyaSyncmessagewithprecisetimestamp.Sinceone-steportwo-stepoperationismostlyafunctionofthemasterclockhardwarecapabilities,bothareallowedinthetelecomprofile,andtheslave clock must be able to accept both.

• Sync/Follow-up rates:Theprofiledefinesaminimumrateof1Sync/Follow-upper16secondstoamaximum rate of 128 per second.

• Delay_Req/Delay_Resp rates:Theprofiledefinesaminimumrateof1Delay_Req/Delay_Respper16seconds to a maximum rate of 128 per second.

• Announce rate:Theprofiledefinesaminimumrateof1Announcemessageper16secondstoamaximum rate of 8 per second, with a default of 1 per 2 seconds. The announce message is sent by the master to indicate its quality level to the slave.

• Signaling message rate:Notdefined,willbebasedontheexpirationoftheservicerequest.Thesignaling messages are used to request service from the master.

TherangeofSyncandDelay_reqmessagerateallowedinthetelecomprofileisverylarge,itisuptotheslavetorequestthemostefficientratebasedonitslocaloscillatorquality,theclockperformanceobjectivesaswellastheseverityofthenetworkimpairments(PDV,congestion,delay)encountered.



7. 1588v2 Synchronization Helpers: Boundary and Transparent Clocks

Eventhough1588v2iscurrentlydeployedworldwideforfrequencydistribution,itiscommonlyacceptedthatthetechnologyisnotsuitableforphase/ToDdistributioninthepresenceofnetworkimpairmentslike PDV or asymmetry. It is the subject of ongoing work in standard ITU-T G.8275.1. While no decisions havebeenmadeatthetimeofthispaper,thedistributionofPhaseandToDwilllikelyrequiretheuseofintermediate specialized elements like Transparent clocks or Boundary clocks.

Boundary ClockTheBoundaryClock(BC)isamultiportdevice.Ononesideitactsasanordinaryslaveandrecoverstheclock from the master, on the other side it serves as master to ordinary slave clocks. It acts as a PTP regeneratorinthemiddleofthenetworkandisdesignedtoeliminateupstreamPTPpacketjitterinorder to provide stable clock to the downstream slaves.

Transparent ClockTheTransparentClock(TC)isamultiportswitchthatisnotamasteroraslaveclock,butanelementthatforwardsthePTPpacketswhilemodifyingthetimestampsintheSyncandDelaymessagestoaccountforthedelaysincurredbetweenreceptionandtransmissionofthemessageswithintheswitch(residencetime).Theslaveclockusesthecorrectionfieldaddedbythetransparentclockinitsalgorithmtocompensateforthedelayvariationthatwasaddedwithintheswitch.

AddingBCorTCinanetworkwilldefinitelyhelpintheaccuracyoftheclockrecovery,butaccumulationofthesedevicesinthechain,orintroductionofnonPTPspecializedelementswithinthenetworkwillstillbeanissue.Regardlessoftheimplementationchoices,testingthenetworkbeforeandduringPTPoperationisarequirement.Thefollowingsectionwilldescribetheclockperformanceobjectiveandmeasurement methods.

Figure 5. Boundary clock

Figure 6. Transparent clock



8. PTP Testing Methodologies and Standards

ThecharacterizationofpacketnetworkmetricsandcorrespondinglimitsisstillanongoingresearchtopicintheITUandMEF.Belowisalistofstandardsofinterestinthisfield:

• ITU-TG.8260providesdefinitionsandterminologyforsynchronizationinpacketnetworks• ITU-TG.8261Timingandsynchronizationaspectsinpacketnetworks• ITU-TG.8261.1Packetdelayvariationnetworklimitsapplicabletopacketbasedmethods

(Frequencysynchronization)• ITU-T G.8265 Architecture and requirements for packet-based frequency delivery• ITU-TG.8265.1Precisiontimeprotocoltelecomprofileforfrequencysynchronization• ITU-TG.8271.1Timeandphasesynchronizationaspectsofpacketnetworks• MEF22.1MobileBackhaulPhase2ImplementationAgreementRequirementsforimplementing

Carrier Ethernet for Mobile Backhaul

Asdescribedinprevioussections,1588v2PTPclocksynchronizationaccuracyisaffectedbynetworkimpairmentsofPacketDelayVariation(PDV)andnetworkasymmetry.PDVandasymmetryarecreatedby many factors including: the number of network elements on the master to slave and slave to masterpaths,thetrafficpatternandnetworkload,aswellastrafficprioritizationandtrafficshapingmechanisms enforced in the network. Therefore it is very hard to model and know beforehand the severityanddistributionofthenetworkimpairments.Fieldtestingoftheseparametersisacriticalpieceneededtoestablishconfidenceinthedeployment.

PTPtestingcanbebrokendownintotwophases:functionaltestingtoestablishprotocolandprovisioninginteroperabilityofthePTPelements,andnetworkperformancetestingtoestablishrecovered clock accuracy.

Protocol TestingTheITU-TG.8265.1standardTelecomprofilenarrowsdownthefieldofacceptablevaluesandparametersforPTPmasterandslavesinteroperability,howevervalidationofinterworkinginthelivenetworkisdesired.Thefollowingparametersaretobeprovisionedand/orproperlynegotiated:

• Unicastormulticastmessagedelivery• SyncandDelay_RequestMessageratesnegotiationandservicegrant• Bestmasterclockselection• ClockIDverification• Domain number assignment• VLAN CoS provisioning



Frequency Performance TestingTheperformanceoftheslaveclock’sfrequencysynchronizationistestedagainstthejitterandwanderlimitsspecifiedinITU-TG.823forE1/E3andG.824forDS1/DS3.Masksdefinethetolerablejitterandwandervalues;theEquipmentClock(EC)maskisusuallyusedtodefinethepass/failperformanceofthe1588v2 slave recovered clock.

TheevaluationofwanderisdonebymeasuringTIE(timeintervalerror)andcalculatingtheMTIEandTDEV. MTIE (in nanoseconds) is the Maximum Time Interval Error. It measures the maximum delay variationoftheslaverecoveredclockcomparedtothereferenceclock.TDEVistheTimeDeviation(innanoseconds)oftheslaveclockasafunctionoftheintegrationtime.Itprovidesinformationaboutthespectral content of the phase noise of the signal.

Phase Performance TestingTheperformanceoftheslave’sphasesynchronizationaccuracyismeasuredbyanalyzingthe1pps(pulsepersecond)signalfromtherecoveredclockandmeasuringitsdeviationfromthereferencesignal.Theaccuracylimitsaredefinedbythemobiletechnologytested(refertotable1).ForexampleLTE-TDDrequiresanaccuracyof±3µs.

Figure 7. MTIE and TDEV results



MEF 22.1 Service Calls Compliance TestingTheMEF22.1standarddefinesanew“VeryHighPriority”classofserviceforMBHsynchronizationtraffic.

CompliancetotheMEFparametersshouldbetestedinthenetwork,you’llfindbelowasummaryoftheperformanceobjectives.

Notethatcomplianceoftheotherclassesofserviceisalsoimportanttoestablish,sinceMBHsynchronizationanddatatrafficsharethesamepath.ToolsfollowingtheITU-TY.1564standardcanbeused for this purpose.

Table 3. One way CPOs across PT for Point-to-Point Mobile Backhaul service (Source: MEF22.1 standard)



9. TX130M+: The Swiss Army Knife of MBH Testing

Insomeinstances,TDMisstillusedforlegacyvoicetrafficwhileEthernetisusedfordatatraffic.OthertypesofhybriddeploymentsmixlegacyTDMbasestationsandTDMnetworkcontrollers,butlinkedbyapackedbasedMBHnetworkcarryingbothsynchronizationanddata.And4Gnetworksuseunifiedpacketbasedcoreandaccessnetworks,butcanuseamixofSyncEandPTPforsynchronizationpurposes.

Withalltheseoptions,oftencoexistingwithinthesamecarriernetwork,onemaythinkthatMBHtestingrequiresamyriadofdifferenttestequipmentandspecializedskills,butthankfullytheVeEXteamhasdevelopedtheperfectall-in-oneMBHtester.TheTX130M+handheldtesterprovidesalltheinterfacesandfeaturesforcompleteMBHtestinginarobustchassiswithaneasytousegraphicaluserinterface.

Synchronization TestingTheTX130M+supportsboth1588v2PTPandSyncE,masterandslaveclockstatefulemulations.ItprovidesPTPprotocolmonitoring,decoding,andpacketsstatistics.PTPandSyncErecoveredclockwander can be directly measured on the unit and the recovered slave clock can be translated to output E1/DS1signal.CriticalPDVmeasurementsforSyncandDelay_reqmessagesarecontinuouslymeasuredandreportedinagraphicalformat.PTPpacketsstatisticswithmessagescaptureandprotocoldecodearealso available on the unit.

Itcanalsocomparetwo1PPSreferenceorrecoveredtimingsignaltoverifyaccuracyandstability.

Figure 8. TX130M+ Synchronization testing screenshots: 1588 messages, Sync announce, PDV messages, and Results



Legacy TDM Mobile Backhaul Network TestingTheTX130M+supportstraditionalPDH/DSnBERtestingwithdefectandanomalybasedperformancemonitoring. E1, E3, T1 and T3 pulse shape analysis with standard pass/fail masks and signal level measurementsaswellasFrequency(datarate)anddeviationmeasurementsandOutputjittermeasurementforPDH/DSn(E1,E3,T1,andT3).

WandermeasurementsareavailableforPDH/DSn(E1,T1)outputs,recoveredSyncEand1588v2,andbetweentraditionalreferenceclocksignals(1.5MHz,2MHz,10MHz,1PPS,amongothers);real-timeTIE data export for MTIE and TDEV post-analysis.

Ethernet TestingCapableofgeneratingEthernettrafficat100%bandwidthoncopperorfiberGigEinterfaces,theTX130M+supportstheRFC2544andY.1564standardsnecessarytotestcompliancetoMEF22.1.Mostimportantly,trafficgenerationandanalysiscanbedonewhileslaveormastermodeemulationsareactive;thereforePDVstatisticsandrecoveredclockstabilitycanbebenchmarkedunderrealisticnetworkconditionseliminatingtheneedofmultipletesters.

Hybrid TestingAuniqueSYNCmodeallowstheclocksonbothPDH/DSnandEthernetinterfacestobesynchronized,sothat simultaneous BERT measurements can be performed. This integrated approach eliminates complex setupstypicalofusingmultipletestersanddrasticallyreducestestingtimeofhybridSynchronizedPacketand legacy TDM services.



The Ver i f icat ion Experts© 2012 VeEX Inc. All rights reserved. D08-00-009 A00 2012/08

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TheVeEXteambringssimplicitytotomorrow’snetworks.

Notes

Documents

A Concise Guide to Mobile Backhaul Synchronization