Wdm the Transmode Way

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WDMCWDM

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WDMtheTransmodewayIntroductionandContent

1.Opticsandcommunications

2.WDMnetworkingtechnologies

3.Creatingthetopology

4.Addingtraffic

5.Operatingthenetwork

Summary

Index

3.Creatingthetopology3.1Executivesummaryofchapter3Usingthetransmissionandmultiplexingtechnologiesdescribedinchapter2,endtoendlightpathspassingmultipleintermediatenodescanbecreated.Thelightpathsactasopticalcircuitsthatareroutedthroughthenetworkpermanentlyorsetupbydemand.Inchapter3thefocusisontheselightpaths:

UsingWDMwavelengthstocreateacompleteopticaltransportnetwork.Routingofwavelengthsbetweennetworknodesusingadddropmultiplexers(OADMandROADM)andwhytherearemultipletypesofROADMs.WDMintheaccessnetwork.HowWDMlightpathscanbeconfiguredtoprotectthetransportnetworkandmakeitmoreresilient.

3.2TransportnetworksintelecomTheequipmentandfibershandlingthephysicaltransportofsignalsinthetelecommunicationsnetworkareoftenreferredtoasthetransportnetwork.Howshouldsuchanetworkbedesignedandoperated?Therearedifferentwaystosolvethistask.

Whetheranationwideormetropolitannetworkisbeingconstructed,thenetworkdesignermustconsidertwodifferentaspectsofnetworklifeitsplannedexpansionovertimeanditsdaytodayoperations.

PLANNEDEXPANSIONTHELONGERTERMPERSPECTIVEPlanningofthenetworkisalongertermactivitytypicalperformedofflineinanofficeenvironment.Whenplanningthetransportnetwork,factorssuchastheseareimportant:

Locationofnodesbasedonsiteavailabilityandcost.Availabilityandcostoffiberssinglefiber,fiberpairs,owncables,leaseddarkfiber.Customer/userlocations,typesoftraffic,capacityneedsnowandinthefuture.Futureexpansionofthenetworktopologyandcapacity.Needofredundantlinksandequipmentforprotectionagainstfaults.

OPERATIONSTHESHORTERTERMPERSPECTIVETheoperationalperspectivecoversthedaytodayoperationsofthenetwork.Thesetasksareperformedindirectcontactwiththeopticalnetwork,typicallyfromanetworkmanagementcenter(NMC).Examplesofoperationaltasksthatinfluencethenetworkdesignandputrequirementsontheflexibilityofthenetworkare:

Proceduresforconnectinguserstothenetwork.Proceduresforexpandingthenetworkandaddingmorelinks.Manualorautomaticproceduresforprotectionswitching,i.e.thereroutingoftrafficonalternativelinksincaseofanodefailureorlinkoutage.

Boththelongerandshortertermactivitiesrequireasetofflexibleandmanageablenetworkelementsthatcanformthetransportnetwork:TheseelementsaretheTMSeries.

3.3WDMasthetransportnetworkTheTMSeriesisafourthgenerationopticalnetworkingsystemthatcombinesthemostadvancedopticaltransmissiontechnologieswiththeswitchingoflightpathsandthepacketizationofinformationintoamultifunctionalpacketopticaltransportnetwork.Itsprincipalelementsarethetranspondersandmuxpondersthatallowtraffictoenterandleavetheopticalnetworkandtheopticalfilters,multiplexers/demultiplexersandreconfigurableopticaladddropmultiplexersthatmultiplexandsendwavelengthsoflightindifferentdirectionsasdirectedbythecontrollingmanagementsystem.(Figure28)

Figure28.AtypicalWDMopticalnetworkcoveringaccesstolonghaul.

Anopticalnetworkprovidescircuitswitchedendtoendopticalchannelsorlightpathsbetweennetworknodesandtheirusers,theclients.Alightpathismadeupofawavelengthbetweentwonetworknodesthatcanberoutedthroughmultipleintermediatenodes.Theintermediatenodesdirectthewavelengths.Theopticalnetworkmaythusbethoughtofasawavelengthroutingnetwork.Lightpathsaresetupandtakendownasrequiredbytheusersofthenetwork.

ItisimportanttorememberthatthelightpathsinaWDMnetworkareendtoendconnections,andshouldbeconsideredastheequivalentsofuninterruptedwires,stretchingfromonepointinthenetworktoanotherwhilepassingoneorseveralnodes.ThisisasignificantdifferencefromtheprinciplesofclassicalTDMopticaltransportnetworks,suchasSDHandSONET,wherethesignalsareregeneratedateachnodetheequivalentuninterruptedwirestretchesonlybetweentwonodes.Hence,aWDMnetworkrequirescarefulwavelengthplanning,todefinewhereeachwavelength(wire)startsandends,whileanSDH/SONETnetworkmakesallsignalsavailableineverynodepassed.Theendtoendaspectalsoaffectshowthepowerbudget(i.e.signalattenuation)iscalculated:InaWDMnetwork,theopticaltransmissioncharacteristicsforawavelengthhastobecalculatedforthecompletedistancethelightpathtraverses;forSDH/SONETanewpowerbudgetiscalculatedforeachhopbetweentwoadjacentnodes.(Figure29)



Figure29.ComparisonbetweenanSDH/SONETandaWDMnode.IntheSDH/SONETnode(left)alltrafficsignalsareregeneratedandswitched,makingthemavailableforaddanddrop.IntheWDMnode(right)onlyselectedsignals(wavelengths)areavailableforaddanddrop,therestareglassedthrough.

Thelightpathsoftheopticalnetworkhaveseveralimportantcharacteristics:

Theyaretransparent,i.e.theycancarrydataatvariousrates,withdifferentprotocolsetc.Thisenablestheopticallayertosupportavarietyofhigherlayerprotocolsconcurrently.Wavelengthanddatarateusedaresetbytheterminatingnodes.Henceanindividuallightpathmaybeupgradedtohighercapacitybysimplychangingtrafficunitsinthestartandendnodes,withoutaffectinganyequipmentinintermediatenodes.ThisisafundamentaldifferencetoSDH/SONETaswellasnetworksofinterconnectedEthernetswitches.Lightpathscanbesetupandtakendownondemand,equivalenttotheestablishmentofcircuitsinacircuitswitchednetwork.Alternativelightpathscanbeconfiguredandkeptinstandbymodesothatintheeventofafailure,trafficmaybereroutedandtheservicemaintained.Wavelengthscanbereused.Ifalightpathusingaparticularwavelengthendsinonenode,thesamewavelengthcanbereusedinanotherlightpathheadinginanotherdirection.ThewholeconceptofWDMandlightpathsisbasedonanalogopticaltransmissiontechniques,makingparameterssuchasdispersion,signalattenuation,opticalsignaltonoiseratioandinterferenceoverthewholelengthofthepathimportanttocontrol.

3.4NodesandnetworkelementsThelightpathsoftheopticalnetworkpassnodesofdifferenttypes,eachcomprisingoneormoremanagednetworkelements.Theprincipalnodesoftheopticalnetworkfromatopologyperspectivearetheterminalmultiplexer,theopticaladd/dropmultiplexer(OADM)andthereconfigurableopticaladd/dropmultiplexer(ROADM).Thesenodesallow

lightpathstoentertheopticalnetworkandtoberoutedtoanydesiredpointofexit.9

3.4.1TheterminalmultiplexerClientsoftheWDMopticalnetworkareinterfacedtothenetworkviatranspondersandmuxponders.Theprincipaldifferenceisthatatransponderisasignal/wavelengthconverter(onesignalinandonesignalout),whilethemuxponderhascircuitrythatcombinesseveralclientsignalsintoonelinesignalandviceversa.Thetransponder/muxponderandanassociatedmultiplexer/demultiplexerareoftenreferredtoasaterminalmultiplexer

(terminalmux)orterminalnode.10

9Theopticalcrossconnect(OXC)issometimesalsoreferredtoasaprincipalnodeofanopticalnetwork.However,inmostcasesthesamefunctionalitycanbeachievedwithcombinationsofROADMunits;hencetheOXCisnotdescribedhere.10Theterminalmultiplexerissometimescalledanopticallineterminal(OLT),especiallyinresidentialbroadband

accessnetworkapplications.

Thetransponder/muxponderisanopticalelectricalopticalunitthatadaptstheincomingsignaltoaformatforuseinsidetheopticalnetwork.Theincomingwavelengthmayneedtobeconverted.Overheadfornetworkmanagement,forwarderrorcorrectionandotherpurposesmustbeadded.Biterrorscountedandstatisticsforwardedtothemanagementsystem.Andinthecaseofthemuxponder,severalbitstreamsaretimedivisionmultiplexedintoahigherratebitstream.Thenextchapterdescribesthefunctionsofthetranspondersandmuxpondersinmoredetail.(Figure30and31)



Figure30.Blockdiagramofaterminalmultiplexerwithatransponderandamuxponder.

Figure31.ATMSeriesterminalmultiplexercombining2.5Gbit/sand10Gbit/sdataratesonCWDMandcomprisingtwomuxponders,atransponderandaCWDMmultiplexer/demultiplexer.Allunitsarehousedinonesinglechassis.

3.4.2Theopticaladd/dropmultiplexer(OADM,ROADM)ThelightpaththathasenteredtheopticalWDMnetworkviaaterminalmultiplexermustberoutedtoitsdestinationviaintermediatenodesthatcandirectthewavelengthtowardsthedesiredpointofexit.Thetaskofroutingthelightpathsisperformedbytheopticaladd/dropmultiplexer(OADM)andthereconfigurableopticaladd/dropmultiplexer(ROADM).(Figure32)

Figure32.AnationalopticaltransportnetworkcomprisingmultipleOADMandROADMnodesthatroutelightpathsbetweenvariousclientsofthenetwork.

Consider,forexample,thesituationdepictedinthefollowingdiagram.Afiberringspansametroarea,withtrafficoriginatingandleavingtheringat10locations,wherelocationsHUBAandHUBBareactingascentralhubsforthetraffic.(Figure33)



Figure33.Anexampleofametropolitanfiberringwithprotection(trafficissentintwodirections)andadd/dropofDWDMwavelengths.Thisisalsoanexampleofhowthesamewavelengthmaybereusedondifferentsegmentsofthering.

Sucharingtopologyrequiresanopticalelementthatcanremoveandaddwavelengthsfromtheringatdemandandforwardthemtowardstheclientfacingequipmentitrequiresanadd/dropmultiplexer.Severalapproachescanbeusedwhenimplementinganadd/dropmultiplexer,andtheTMSeriescomprisesopticalfilters,bandsplitterunits,mux/demuxesandcompleteROADMsforthispurpose.

3.4.2.1TheopticalfilterasOADMAfullypassiveopticalfiltercanbeusedinCWDMandDWDMnetworkstoadd/droponeormorewavelengths.Themainadvantageoftheopticalfilterapproachisitssimplicityanddirectscalabilityonlywhenmorechannelsaretobedropped,morefiltersneedtobeinstalled.Themaindisadvantageistheattenuationthatisintroducedateachfilterpointandtheplanningrequiredinassigningwavelengthstothedesiredlightpaths.(Figure3435)

Figure34.Principleofapassive,filterbased,opticaladd/dropmultiplexer.Onewavelength(1)ispassedthroughtheOADM,andanotherwavelength(2)isdropped.Newsignalsarethenaddedwhenthewavelengthiscontinued.

Figure35.TheopticalfilterusedasanOADM

3.4.2.2Themux/demuxasOADMAnalternativeapproachforaddinganddroppingchannelsatanintermediatesiteistodemultiplexallthelinewavelengthsandextract/addthedesiredchannels,whilelettingtherestofthewavelengthspassthrough.(Figure36)

Figure36.Add/dropofawavelengthsusingapairofmux/demux.

Thisapproachismoreefficientthanfiltersifmanychannelsaretobedroppedatonelocation.Sinceallwavelengthsarecateredforfromthebeginningandthemux/demuxhasafixedattenuation,thisapproachalsobecomesmoreflexibleandrequireslessadvanceplanningthanwithopticalfiltersinseries.However,theamountofequipmentneededandthusthecostishigherthanforafiltersolution.Also,theamountofpatchcordsforinterconnectingthewavelengthstobepassedthroughaddstothecomplexityandcancreatehandlingproblems.

FortheTMSeriesadd/dropwithopticalmux/demuxcanbeusedonbothsinglefiberandfiberconfigurationswithCWDMandonfiberpairconfigurationswithDWDM.



CWDMandonfiberpairconfigurationswithDWDM.

Whenlargernumbersofchannelsaretobedroppedtheaboveprinciplecanbeextendedbyuseofabandsplitterunit,whichextracts/insertsawholebandofwavelengthsfordemultiplexingbyamux/demuxasshowninthefollowingdiagram.(Figure37)

Figure37.OADMina40channelDWDMsystem.8channelsareadded/droppedasabandbythebandsplitterunitandmadeindividuallyavailableviatwomux/demuxes.Theremaining32channelsarepassedthroughthebandsplitterunittransparently.

3.4.3TheROADMInasmallandstaticopticalnetwork,OADMnodesoftheabovetypesmaybethebestsolution.However,inlargernetworks,thefrequentestablishmentandreassignmentoflightpathsmakeremotereconfigurabilityaverydesirableattributeinanOADM.Reconfigurabilityreferstotheabilitytoselectthedesiredwavelengthstobedroppedandaddedonthefly,asopposedtohavingtoplanaheadanddeployappropriateequipment.Reconfigurabilityallowslightpathstobesetupandtakendowndynamicallyasneededbetweennetworknodesandisthetaskofthereconfigurableopticaladd/dropmultiplexer,theROADM.(Figure38)

Figure38.Theaddingofnewlightpathswithredandbluewavelengthsrequireschangesinnodeconfigurations.

ROADMsareusedinbusandringnetworkstoenableflexibleadd/dropofwavelengthsandhitlessexpansionwherewavelengthscanbeaddedwithoutinterruptionsoftrafficonadjacentchannels.Whenusedinameshedopticalnetwork,ROADMscanprovidetotalflexibilityintheroutingoflightpaths.TheflexibilityofROADMsthusbenefitstheoperatorwantingtoadapttochangingsubscriberrequirements,aswellasincreasingnetworkavailabilitybysimplifyingprotectionswitchingandrestorationoflightpaths.Itcanevenbeusedforsettinguplightpathsdynamicallyondemandinspecialapplications,forexampleifthereisamajormediaeventatasiterequiringbandwidthjustforafewhours.(Figure39)



Figure39.Exampleofameshedopticalnetwork.WithmultidegreeROADMnodesinthenodesthelightpathscanbedirectedtoanydestination,providingmaximumflexibilityforthenetworkoperatorandsuperiorresiliencetolinkoutages.

AROADMbasednetworkdecreasestheoperatorstimetorevenuesinceservicescanbeprovidedrapidlywhenlightpathsaresetupremotelywithouttheneedofdispatchingtechnicianstonetworknodes.CommissioningandoperationoftheentirenetworkbecomessimplifiedandthecentralizedmanagementofROADMnodesenablesmoreautomation,reducingtheriskformanualerrors.

ROADMnodesalsohavesignificantadvantagesfromanetworkplanningperspective.FreewavelengthallocationwithROADMnodessimplifiesnetworkplanningandreducestheeffectsofinaccuratetrafficforecasting.ROADMnodessimplifytrafficengineeringandoptimizationofnetworkuse.Theyallowforbetterwavelengthutilizationsincewavelengthsaremanagedseparatelyratherthanincompletebands.TheTMSeriesROADMunitsalsoincludeintegratedvariableopticalattenuators(VOAs)foreachwavelength,whichgreatlysimplifiespowerbalancingofthelightpaths.

3.4.3.1ROADMprinciplesROADMunitscanbedesignedaroundmux/demuxesandopticalswitches,butthemostcommonarchitecturetodaymakesuseofa1xNwavelengthselectiveswitch(WSS)thatindividuallycanswitchthewavelengthsonitsinputstoitsoutput.(Ndenotesthenumberofinputstotheswitch.)TheTMSeriesROADMunitshaveaWSSontheaddside,i.ewhereaddedsignalsarecombinedwiththelinesignal.Thisarrangementgivesfullcontrolofallsignallevelsonaddedandpassedchannels,aprerequisiteforsecurenetworkoperationswithoutanytransmissionlevelproblems.

RecenttechnologydevelopmentshavemadeWSSbasedROADMunitsaffordable,notonlyinlonghaulnetworks,butalsointhemetronetworks.HavingthecapabilitytodeployROADMnodesinmetroapplicationsisofsignificantvaluesinceconfigurationchangesarenormallyquitefrequentinmetroandmetroaccess.HencetheTMSeriesROADMnodesareanoptimalchoicewhenimplementingaregional,metroormetroaccessopticalnetwork.(Figure40)

Figure40.ThemainelementsoftheTMSeries1x2ROADMpluginunit.

Asshowninthefigureabove,theincomingwavelengthsfromwestareallsplitviaanopticalcouplerandmadeindividuallyavailablelocallyviaademuxwhenusingtheTMSeries1x2ROADMpluginunit.Localwavelengthstobeaddedaremultiplexedandaddedtotheincomingsignalfromeastinthe2x1WSS.EachoftheincomingWSSportsissettoacceptoneorseveralwavelengths,theonlylimitationbeingthatnotwowavelengthsoverlap,i.e.arethesame.Thus,theWSScanforeachwavelengthdecideifitshouldbetakenfromlineeastorbelocallyadded.

TheROADMunitinthediagramabovehasfiberlinksintwodirections,asforexampleinaringtopology.Thenumberoffiberlinkdirectionsto/fromaROADM(oranyotheropticalnetworknode)isoftenreferredtoasthedegreeoftheROADM.Inmeshnetworksandinterconnectedringtopologiestherearenodesthathaveahigherdegree,forexample3or4,referredtoasmultidegreeROADMnodes.

Ifwewanttocreateacomplete2degreeROADMnodewhereanywavelengthcanbeaddedordroppedinboththewestandeastdirections,twoofthejustdescribed1x2ROADMpluginunitsarecombinedbacktoback.(Figure



westandeastdirections,twoofthejustdescribed1x2ROADMpluginunitsarecombinedbacktoback.(Figure41)

Figure41.Acomplete2degreeROADMnode.

3.4.3.2ColorlessROADMFurtherflexibilitycanbeaddedtotheROADMbymakingthemux/demuxunitswavelengthindependent,i.e.makingitpossibletoaddordropanyatanyoftheirports,creatingacolorlessROADMnode.(Figure42)

Figure42.Acolorless2degreeROADMnode.

Whencombinedwithtunabletransceiversintheattachedtransponders/muxponders,theoperatorcannowchangethewavelengthforaservicewithoutmovingthetransponder/muxpondertoanewportonthemux/demux.FortheTMSeriesROADMnodes,suchwavelengthreconfigurationscanbemadecompletelyremotelyfromtheTransmodeNetworkManager(TNM)systemwithouttheneedtovisitthesite.(Figure43)



Figure43.ManagementofwavelengthswiththeTransmodeNetworkManager(TNM).

AfurtheradvantageoftheTMseriesROADMunitsisthattheyareallimplementedaspluginunitswhichmaybelocatedinanyoftheavailablechassis.Thismeansthatanynode,largeorsmall,easilycanbeupgradedwithROADMfunctionalitywhenthenetworkgrows.

3.4.3.3DirectionlessROADMIntheROADMnodeconfigurationsinthepreviouschapters,aparticularaddedisphysicallydeterminedtogoeitherintheeastorwestdirection,dependingonwhichofthetwomuxesthetransponder/muxponderisconnectedto.Thiscanbeadisadvantage,forexampleinprotectiveswitchingandmaymeanawasteofavailablewavelengths.Byaddingonemore1x2ROADM,itispossibletocreateadirectionlessROADMnodewheretrafficfromanyaddedportcanbesentineithereastorwestdirections.(Figure44)

Figure44.Adirectionless2degreeROADMnode.

3.4.3.4HigherdegreeROADMnodesUsing4x1and8x1WSSunitsitispossibletodesignROADMnodesformeshednetworks,withnodesofhigherdegreethantwoandwithmorethantwoincomingandoutgoingfiberdirections.TheTMSeriescomprisesone1x4ROADMunitfor40channelDWDMsystemsandtwo1x8ROADMunitsfor40or80channelDWDMsystems,allsuitablefortheseapplications.

The4or8addportsuseawavelengthselectiveswitch(WSS)todynamicallyselectwhichoftheDWDMchannelsontheITUTCbandgridtobeaddedtothelinesignalforeachaddport.AnOpticalCouplerisusedtodistributetheincominglinesignaltothedropports.ADWDMadddropfilterorMux/Demuxunitisalwaysusedforthelocallyterminatingtraffic.

Similarlytothe1x2ROADMpluginunit,theTMSeries1x4and1x8ROADMsalsoincludevariableopticalattenuator(VOA)functionalityonallwavelengthsaddedtothelinesignalbytheWSS.Thisfacilitateschannelpowerbalancinginamplifiednetworks.

Groupingofportsondifferentunitscanbemadeinthenodemanagementsoftwaretoenablethesettingofidenticalchannelselection.Alsorestrictionsonchannelsselectioncanbemadeonindividualorgroupedportstosimplifycommissioningandminimizeriskforfaultyhandling.

Boththe1x4andthe1x8ROADMunitsconsumeslessthan6W.Lowpowerconsumptionincombinationwithasmallfootprintreducessitecostsandenablesmorecapacitytobehandledatsiteswithrestrictionsonpowerconsumption,coolingandspace.

3.4.3.5ContentionlessROADMUsingacombinationof1x4and1x2ROADMsafullycontentionlessROADMnodefor2degreesmaybedesigned.Asshowninthefollowingdiagram,wavelengthscannotbeassignedarbitrarilyinthedirectionlessROADMdescribedearlier:Ifonewavelength1issentine.g.thewestdirection,thesamewavelength1cannotbereusedintheeastdirection.Byaddinganextrasetof1x2ROADMunitsfullfreedomofwavelengthallocationispossible;theROADMnodebecomesbothdirectionlessandcontentionless.However,especiallyforhigherdegreenodes,theamountofequipmentneededforacontentionlessROADMmaymakeitscostprohibitive,althoughexpectedtodecreaseasnewcomponentsbecomeavailable.(Figure45)

Figure45.Adirectionlessandcontentionless2degreeROADMnode.Themux/demuxunitscanbemadecolorlessandcombinedwithtrafficunitshavingtunabletransceiversforadditionalflexibility.

Thefourindividualadddropportsofthe1x4ROADMenablehitlessredirectionoftrafficinmultidegreenodes.Bygroupingfourunitsandinterconnectingtheadddropports,a4degreenodeiscreated,wheretrafficfromanylinecanbedirectedtoanyotherlineorbelocallydropped.(Figure46)



Figure46.Afourdegreenodeimplementedbyfour1x4ROADMs.

The8individualadddropportsofthe1x8ROADMenablehitlessredirectionoftrafficinevenhigherdegreenodes.Bygroupingupto8unitsandinterconnectingtheadddropports,upto8degreenodescanbecreated,wheretrafficfromanylinecanbedirectedtoanyotherlineorbelocallydropped.A50GHzcompatibleMux/Demuxisusedtoseparatetheterminatedchannels.(Figure47)

Figure47.An8degreenode.Allconnectionsarejustshownforline1.

Itispossibletocreatedirectionlesshigherdegreenodesbyusinganextra1x8ROADMunittodirectthelocaltraffictothepreferredlinefiber.Eachwavelengthcanbedirectedasrequiredonanindividualbasis.Itispossibletohavebothfixedanddirectionlessadd/dropsinthesamenode.(Figure48)

Figure48.Afourdegreenodewithbothfixedanddirectionlesstraffic.

3.5WavelengthmanagementTrafficunitswithpluginandtunabletransceivers,multidegreeROADMsandcolorlessmux/demuxunitsenableatremendousflexibilityinopticalnetworkdesignandoperations,butalsoputstringentrequirementsonwavelengthmanagement.TheTransmodeEnlighten11softwaresuiteforplanning,design,commissioningandmanagementofanintegratedpacketopticalnetworkincludesthenecessarytoolsforthistask.CentralizedwavelengthmanagementisperformedfromtheTransmodeNetworkManager(TNM),acomprehensive,carrierclassElement,NetworkandServiceManagerforTransmodesintegratedlayer1andlayer2networkingsolutions.

TNMincludesseveralfeaturesofhighvalueforefficientwavelengthmanagement,forexample:

Theextensiveinventorymodule.TheROADMprovisioningapplication.TheOpticalControlPlanewithitsapplications.Integratedhandlingofalienwavelengths.



NotonlydoestheinventorymoduleofTNMkeeptrackofallactiveequipmentinaTMSeriesopticalnetwork.Theinventorymayalsobeusedtoregistereveryfilter,mux/demuxandotherpassiveunitinthenetworkandthenkeeptrackofhowthevariouswavelengthsareallocated.Havingalsothepassiveelementsavailableintheinventorygreatlysimplifiesplanningandallocationofwavelengthsthroughouttheopticalnetwork.

TheROADMprovisioningapplicationinTNMautomaticallyreadsROADMparametersettingsfromthenodesandenablestheoperatortoremotelyaddchannelstoanadd/dropport.IftheROADMprovisioningapplicationisactivated,theTNMautomaticallyidentifiespotentialchannelsthroughtheunconfiguredROADMsofthenetwork.TNMchecksthatnowavelengthconflictsoccurthroughouttheopticalpathandthenautomaticallyconfigurestheROADMstocreatetheopticalpath,includingstartingnecessarycontrolloops.Thishighlyautomatedprocessgreatlyreducestheriskformisconfigurationswhilereducingtheconfigurationtimebyupto90%.

3.5.1TheOpticalControlPlaneinTNMTheOpticalControlPlane(OCP)inTNMprovidesadvancedfunctionalitytosimplifycentralizedcommissioning,tuningandplanningoftheopticalnetwork.Currently,theTNMOpticalControlPlanecomprisestwomodules:TransmissionControlandChannelControl.

3.5.1.1TransmissionControlTransmissionControlisaTNMapplicationthatsupportscommissioningandtuningofamplifiedopticalnetworks,therebyreducingtheoperationalcostsassociatedwithsettingupandmaintainingsuchnetworks.TransmissionControlworksinconcertwiththeOpticalChannelMonitoring(OCM)units,theVariableOpticalAttenuators(VOAs)andthetransceiversinthenetworktomeasureandpresentpowerlevelsatvariouspointsofanopticallightpath.TransmissionControlallowstheoperatortoselectoneormorelightpathsinthenetwork,showthepowerlevelineachpointthathasmeasurementcapabilityandthenmaketheoptimalpowersettingsinattenuatorsandamplifiers.

3.5.1.2ChannelControlChannelControlisaTNMapplicationthatallowstheoperatortoselectanumberofopticallinksandshowtheroutingofusedandavailablechannels()andsubchannels(sub)throughouttheselectedlightpath.

ChannelControlprovidesroutinginformationforchannelsandsubchannelsandidentifieschannelsas:

ActiveChannelschannelsthatarecarryingtraffic.Activechannelhavetranspondersconnectedandcliententriesdefined.PossibleChannelspreprovisionedchannelsthatareavailabletobetakeninservice.Possiblechannelsmayormaynothavetranspondersconnectedbutnocliententriesaredefined.ReservedChannelspreprovisionedchannelsthathavebeendedicatedforparticularfuturepurpose.Reservedchannelsmayormaynothavetranspondersconnectedbutnocliententriesdefined.

3.5.1.3HandlingofalienwavelengthsAnalienwavelengthisastandardCWDMorDWDMwavelengththattransportstrafficthatdoesnotoriginateandterminateinTransmodeequipment,e.g.IPpacketssentbetweentworoutershavingopticalinterfacesconnecteddirectlytotheportsoftwomux/demuxunits.SuchtrafficdoesnotpassanyTMSeriestrafficunitsandisonlytransportedthroughtheopticalnetworkbetweentwopassiveports.AlthoughnotbeingofTMSeriesorigin,thefollowingTNMmanagementfeaturesarestillavailableforalienwavelengths:

TransmissionControl,i.e.thealienwavelengthispowerbalancedinthesamewayandtogetherwiththeTMSeriesnativewavelengths.Opticalprovisioning.Aseparatepassiveadministrativestate.Servicetopology.Servicealarmsifthewavelengthpassesanopticalchannelmonitoring(OCM)unit.

(Figure49)

Figure49.Analarmcanbetriggeredalsoforanalienwavelengthifitpassesanodewithanopticalchannelmonitoring(OCM)unit.

3.6WDMintheaccessnetworkModernizationoftheaccessnetworkisapressingissueformanyoperators,duetotherapidlyincreasingtrafficvolumesgeneratedbynewapplicationsdemandedbytheirsubscribersandusers.Mobileoperatorsaredeployingfourthgeneration(HSDPA,LTE)mobilenetworkstosupportsmartphones,tabletsandmobilecomputing,thusrequiringlinkswithGbit/sdataratestothecellsites.Enterprisesareincreasinglydependentoncentralizedcloudcomputing,whiletheresourceswithinthecloudneedtobeinterconnectedbyhighcapacitylinks;bothtrendsdrivinguptheneedformoredatatransportcapacityintheaccessnetwork.Inparallel,consumerssubscribetovideoondemandandotherInternetbasedmediaservices,making100Mbit/saccessandmorearequirementforeachhousehold.

WDMasatechnologyanswersmanyoftheoperatorsdemandsandhasanimportantroleinthemodernizationoftheaccessnetwork.

3.6.1WDMaggregationringsAnimportantapplicationofWDMandtheTMSeriesisforefficientcapacityupgradesoftheaggregationnetworks,carryingtrafficfrommultipleremoteaccesssitesto/fromacentralhubsite.Forexamples,theremotesitesmaybeamobilecellsite,aDSLAMservingresidentialInternetusersoranEthernetdemarcationunitinanindustrycampus.WithWDM,eachsuchremotenodecanbeallocatedadedicatedwavelengthoverthering,carryingitstrafficto/fromthecentralhub.(Figure50)



thecentralhub.(Figure50)

Figure50.ComparisonofanaggregationnetworkbuiltwithEthernetswitchesandaWDMaggregationnetwork.

InaWDMring,alltheremotenodeshavetheirownpredeterminedcapacityonthelinktowardsthehub,resultinginadeterministictrafficpatternandeasybandwidthmanagement.Ethernetrings,ontheotherhand,sharetheavailablelinkcapacitybetweenalltheremotenodes.Anincreaseincapacityononenodestealscapacityfromtherest.

Ethernetringsalsorequirethesameuplinkinterfaceonallswitches/nodeswhichleadstoexpensiveforkliftupgrades,shouldcapacityonanyofthenodesneedtoberaised.InaWDMnetworkeachnodeuplinkisindependentfromalltheothersandeachnodecanbeupgradedindividuallywhenneeded.

InaWDMringeachnodeequipmentisindependentoftheothers(nosharedhardware)andtheprotectioncanbehandleddirectlyonlayer1withlessthan50msdelay.Ethernetringsaresensitivetosingleunitfailureandneedcomplexlayer2orlayer3redundancyschemestoovercomethis.

WithWDMaggregationitiseasytocombineEthernetandTDMtrafficbackhaulingonthesamefiber,whileEthernetringsneedmorecomplexremappingsolutions(pseudowireetc.)orseparatefiberpairstocopewithlegacyTDMtraffic.ThisisofspecificvaluetomobileoperatorswhichoftenrequiretransportofbothlegacyTDMtrafficandnewEthernettrafficfromthecellsites.

TheTMSeriesisideallysuitedforbuildingWDMaggregationrings.LowcostCWDMringsolutionsusingonlypassiveopticalfiltersandcoloredinterfacesonalreadyexistingroutersandswitchesareeasilyimplemented.Byincludingactivetranspondersandmuxponders,thetrafficaggregation,protectionandmonitoringcanbemademoreefficient.Andforlongerdistancesandmorechannels,completeDWDMsystems,ifnecessarywithamplifiers,canbedeployed.Inaddition,thepacketopticalfunctionalityoftheTMSeriestranspondersandmuxpondersenablesintegrationoflayer2functionspreviouslyresidingintheEthernetswitchesdirectlyintotheopticalaggregationring.

3.6.2PointtopointandPassiveOpticalNetworks(PON)inaccessExpandingthetelecommunicationsnetworktoeachandeveryhouseholdandeachandeveryenterpriseistherealchallengeofanynetworkdesigner.Thenumberofendpointsterminatingthenetworkbecomesmassive,andsodoestheassociatedcostofcivilworksandequipmentneeded.Whilemostlargerandmediumsizedenterpriseshavehaddedicatedconnections(leasedlines)fordatatraffictotheirpremisessincethe1980s,residentialInternetusershaveprimarilybeenrestrictedtouseaccessnetworksthattakeafreerideonthealreadyinstalledinfrastructureof

telephonylinesorCATVHFC12networks.

12CableTV(CATV)usinghybridfiberandcoax(HFC)distributionnetworks.

Thecostofdeployingnewfiberisobviouslyaprohibitingfactorforamorerapidexpansionofhighspeedaccess,andvariousschemesareusedtoreducethenumberoffibermilesintheaccessnetworks.Thetwomaincompetingcategoriesarepointtopointfiberaccessnetworksandpassiveopticalnetwork(PON)accessnetworks.Aswewillsee,WDMhasanimportantroleinextendingthecapacity,securityandflexibilityofthesenetworks.

Apointtopointfiberaccessnetworkisafairlystraightforwardstructurethatmimicstheclassicaltelephonyaccessnetworkinthatithasacentralnodewithindividualfibercablesreachingouttoeachsubscriber.Thenetworkisstarshaped;sometimesendinginthebasementofamultidwellingbuilding,whereanopticalelectricalconversionisdoneandatraditionalstarshaped,electricalEthernetLANisinstalledreachingouttoeachandeveryapartment.(Figure51)



Figure51.Topologyofapointtopointbroadbandaccessnetwork.

Themainadvantagesofapointtopointaccessarchitectureareitstopologicalsimplicityandthestrictseparationoftrafficto/fromeachsubscriber.Significantdisadvantagesarethedeploymentcostofalltheindividualfibers,themassiveamountoffibersthatmustbeterminatedatthecentralnodeandthatpointtopointaccessnormallyrequiresactiveequipment(typicallyanEthernetswitch)ateverysubscriberlocation.AlthoughpopularinsomeNordiccitynetworksforresidentialaccessandoftenusedforenterpriseaccess,pureEthernetbasedpointtopointbroadbandaccessisnotlikelytodominatethefuturebroadbandaccessnetworks.

Apassiveopticalnetwork(PON)isapointtomultipoint,accessarchitectureinwhichpassiveopticalsplittersareusedtoenableasingleopticalfibertoservemultiplepremises,typically16128.APONconsistsofanopticallineterminal(OLT)attheserviceproviderscentralofficeandanumberofopticalnetworkterminals(ONTs)nearendusers.PONreducestheamountoffiberandcentralofficeequipmentrequiredcomparedwithpointtopointarchitectures.(Figure52)

Figure52.Topologyofapassiveopticalbroadbandaccessnetwork(PON).

TwomaintypesofPONsystemshavebeenstandardized:TDMPONandWDMPON.TDMPONs(i.e.BPON,EPON,GPON)useseparatewavelengthsforthedownstreamandupstreamdirectionsonthesinglefiberandtimedivisionmultiplexingbetweentheindividualsubscribersconnectedtothefiber.DownstreamsignalsarebroadcasttoallpremisessharingmultiplefibersandonlyreceivedbytheappropriateONT.Upstreamsignalsarecombinedusingamultipleaccessprotocol,usuallytimedivisionmultipleaccess(TDMA).TheOLTsallowsatimeslotassignmentsforupstreamcommunicationfromeachoftheOLTsinturn.BecauseaTDMPONreliesupontimesharing,ithasaninherentcapacitylimitation,aswellasasecurityproblem,sinceallinformationistheoreticallyavailableateveryendpoint.

AWDMPONcombinesthededicatedbandwidthofapointtopointnetworkwiththefibersharingarchitectureinherentinthePONtopology.AWDMPONusesafiltertoseparatethewavelengthsofaWDMstreamfordeliverytoeachindividualsubscriberONT.Thus,onlyonefiberisrequiredatthecentralsitewhilethewavelength(channel)topologyispointtopoint.

AkeyadvantageofWDMPONistheuseofacompletelyseparatedownstreamwavelengthforeachofthesubscribers.Thisseparatewavelengthprovidesmorebandwidthtoeachsubscriber,bettersecurity,andenhancedoperationalcontrolsincethereisnopotentialinterferencebetweenwavelengths.Similarly,adedicatedupstreamwavelengthprovidesalmostunlimitedcapacitytoeachsubscriber,coveringallpotentialfuturegrowth.(Figure53)

Figure53.PrincipleofaWDMPONaccessnetwork.

3.6.3TheTransmodeiWDMPONsolutionWhileWDMPONsatisfiesallfuturecapacityandsecurityrequirementsinbroadbandaccess,itisstillatechnologyinitsinfancy,withvariousimplementationalternativesunderstandardization.Severalofthesuggestedstandardsrelyuponspecialcomponentshavinghighcostsduetosmallseriesandlimiteddeployment,e.g.whenWDMPONisimplementedwithCandLbandtransmissionandnonITUstandardizedwavelengths.

TransmodehasthereforeselectedanalternativeapproachtoWDMPON:UsingthesamebasicelementsasformetroandlonghaulWDM,TransmodesiWDMPONsolutionoperatesintheCband.Byusingthesamewavelengthsfor



andlonghaulWDM,TransmodesiWDMPONsolutionoperatesintheCband.Byusingthesamewavelengthsforaccessasthoseusedinthemetroandlonghaulnetworks,itispossibletodevelopacommonWDMaccessstructurethatcombineslegacyenterprisetraffic,mobilebackhaul,residentialInternetaccessandhighcapacityWDMPONusersinthesameopticalnetwork.(Figure54)

Figure54.ATransmodeiWDMPONaccessnetwork.Usingonewavelengthfromaccesstocoreprovidesopticaltransparencyandeliminatesopticalelectricconversions.

TheadvantagesofusingITUstandardWDMintheaccessnetworkarevaried:

EasytocombineFTTB,mobilebackhaul,Enterpriseleasedlinesandmoreinthesameinfrastructure.Providesonesolutionforalltypesofdataratesandapplications.Eachservice(SDH/SONET,GbE,FC,FICON)canbeallocateditsownwavelength.OpenandstandardizedDWDMgridanduseofthestandardITUCbandmakesitpossibletocapitalizeonstandardcomponentdevelopmentcurveandpricereductions.Opticaltransparencyfromaccesstocoreeliminatestheneedforopticalelectricconversions.

TransmodesiWDMPONsolutionleveragesthepassiveandactivecomponentsalreadydescribed,butalsoincludesspecialelementstofacilitatewavelengthallocationtosubscribers,accordingtonormalWDMPONprinciples.Akeyelementistheuseofpluggable,colorlessDWDMSFPswithinjectionlockedFPlasersintheONTatthecustomersite.ThecolorlessSFPisautomaticallytunedtotheincomingseedingwavelength,thuseliminatingmanualtuningateachsubscriber.(Figure55)

Figure55.WavelengthallocationiniWDMPON:TheseedingboardinstalledintheTMSeriescentralofficeOLTbroadcastsabroadbandlightsource.AfilterforwardsasinglewavelengthtotheSFP,whichlocksonthecorrectfrequency.

3.7NetworktopologiesOpticalnetworkscanbeclassifiedaccordingtotheirtopologies:Lineorpointtopoint,star,ringandmeshshaped.Thelineandstartopologieshavesinglepointsoffailure,andshouldbeavoidedwhenresilienceagainstoutagesisimportant.Therearemanyargumentsaboutwhicharethebest:Ringormesh.Veryoften,thedebatemixesupthetopologyofthephysicalfibersinstalledandthelogicalconnectivitythatcanbeachievedbyswitchingandroutingoverthefibersathigherlayers.Thephysicalconnectivitymattersmostwhenresilience,distanceanddelayarebeingconsidered.

Manymetropolitannetworksstartasringsandevolvesgraduallytomeshthecrossoverdependsonfibercosts,equipmentcosts,trafficloading,degreeofmeshrequiredforresilienceetc.Ascanbeseenfromthediagrambelow,thecostofameshednetworkishighatlowertrafficvolumes,butbecomesincreasinglyattractiveastrafficgrows.Thediagramisbasedonsimulations.

WiththeadventoftodayscostefficientROADMnodes,itisnowpossibletoimplementresilientandflexiblemeshednetworksalsointhemetroareaandmetroaccessareas.(Figure56)



Figure56.Simulationofnetworkcostasafunctionoftrafficfordifferenttopologies.

3.8Resilienceandprotection

3.8.1CalculatingtheavailabilityBeingafundamentalpartofthetelecommunicationsinfrastructure,theopticalnetworkmustalwaysbeoperational.Threevariablesdefinethequalityofanetworkfromaresilienceperspective:

Availabilityisthefractionoftotaltimethatafunction,forexampleanetworkconnection,isavailablei.e.whattheusersexperienceasaworkingsystem.Availabilityismeasuredin%oftimeandtypicallyliesintherangeof99.999%(fivenines)ormore.

Thereliabilityofanetworkelementisthefractionoftotaltimethatanobjectisworking.Reliabilityisoftenmeasuredasmeantimebetweenfailures(MTBF)atimethatshouldbeaslongaspossible.Therepairtimedefineshowquicklyanobjectcanberepairedandputbackinservice.Repairtimeismeasuredasmeantimetorepair(MTTR)andshouldpreferablybeasshortaspossible.

LetuslookataconcreteexampleonhowprotectionswitchingandringtopologiescanhelpimprovetheavailabilityonanopticallightpathfromGlasgowtoLondon.ThecalculationisbasedonSDHtechnologyandpurelyforillustrativepurposes,i.e.thevaluesusedforactivecomponentsarenotdirectlyapplicablefortheTMSeries.

Lookingfirstatanunprotectedlightpath,thephysicalnetworkcanbedrawninamoreeasilyunderstoodform.Thetotalavailabilityoftheunprotectedpathissimplyfoundbymultiplyingtheavailabilitiesofeachoftheelementsinthechain.(Figure57)

Figure57.Availabilityforanexamplenetwork.

Usingprotectionswitchinginthenodes,thelightpathmayfindalternativeroutesbetweenitsendpoints,forexample,shouldafiberbreakoccur.(Figure58)



Figure58.Availabilityforaprotectedandarestoredexamplenetwork.

Calculationoftheavailabilityinaprotectednetworkisabitmorecomplicatedthaninanunprotectednetwork:

1. Firstmultiplytheavailabilitiesoftheunprotectedpartstogether(inthiscasethetributarycardateitherendofthepath).

2. Calculatetheavailabilitiesofthetwodiversepathsbymultiplyingtheavailabilitiesofeachoftheelementsinbothchains.

3. Combinethetwochainsbymultiplyingtheirunavailabilitystogether(unavailability=1availability).4. Finallymultiplytheavailabilityfromstep1withtheavailabilityfromstep3.

Therestoredexamplebreaksthesingle,long,diverserouteintoseveral,shorter,diversesections.Theresultofthisisthatitcansurvivemultiplesimultaneousfailureswhilsttheprotecteddesigncanonlysurviveasinglefailure.

3.8.2TMSeriesresiliencefeaturesTheTMSeriesincludesnumerousfeaturesthatcanbeusedtominimizetheimpactofbothfiberbreaksandfailuresinindividualcomponentsontheoverallnetworkavailability.

Oneliveandoneredundantclientsystemmay,forexample,beconnectedtotwoseparateandindependenttranspondersormuxponderssharingthesameopticalfiberviaapassiveopticalcouplerunit.Incaseoffailureoftheliveclientsystemorthecorrespondingtransponder/muxpondertraffic,theredundantpathisautomaticallymadeactiveinlessthan50ms.Thetransponders/muxpondersmayevenbelocatedinseparateTMSerieschassistofurtherminimizetheriskofsinglepointoffailure.(Figure59)

Figure59.ClientandequipmentprotectionwiththeTMSeries.

Transponderscanbeequippedwithmultipleoutputsforalternativefiberroutesandsignalsfromincomingfiberscanbesplitintotwoormorepaths.Switchingbetweentheworkingpathandthealternativepathisfullyautomaticandtakeslessthan50ms.Therefore,multipathlineprotectioncaneasilybeachievedinTMSeriesopticalnetworks

withouttheneedforGMPLS/ASON13software.(Figure60)

Figure60.Threeexamplesoflineprotectionwithtransponders.

13GeneralizedMultiProtocolLabelSwitching(GMPLS)isaprotocolsuiteextendingMPLS(seechapter4)tomanage

furtherclassesofinterfacesandswitchingsuchastimedivisionmultiplexers,layer2switchesandwavelength



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furtherclassesofinterfacesandswitchingsuchastimedivisionmultiplexers,layer2switchesandwavelengthswitches.ASON(AutomaticallySwitchedOpticalNetwork)isaconceptfortheevolutionoftransportnetworkswhichallowsfordynamicpolicydrivencontrolofanopticalorSDHnetworkbasedonsignalingbetweenauserandcomponentsofthenetwork.

Ifseveralfibersareavailableonagivenlink,theTMSeriesalsoallowsforfiberprotection,byusingamechanicalfiberprotectionunit.(Figure61)

Figure61.Fiberprotection.

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