Technicalbackground
TounderstandtheimportanceofDWDMandopticalnetworkinterconnection,itisnecessarytodiscussDWDMtechnologyresearchunderthepremiseofwhatproblemsthecommunicationsindustry,especiallyserviceproviders,arecurrentlyfacing.Thepowerfulfeaturesthatitbrings.Duringthenetworkdesignandconstructionperiod,theengineeringdesignermustmakeareasonableestimateofthenetwork'sfuturebandwidthrequirements.TheestimationofbandwidthrequirementsformostnetworksdeployedintheUnitedStatesandotherregionsisderivedfromclassicalengineeringformulaestimates,suchasPoisson(Poisson)probabilitydistributionmodels.Asaresult,estimatesoftheamountofbandwidthrequiredbythenetworkareusuallygivenbasedoncertainstatisticalassumptions.Forexample,itisgenerallybelievedthatundernormalcircumstances,individualswillonlyuse6minutesofnetworkbandwidthwithinanhour.However,thismathematicalmodeldoesnottakeintoaccountthatservicessuchasInternetaccess(theannualgrowthrateofdatatrafficforthisserviceis300%),faxes,multipletelephonelines,modems,teleconferences,dataandvideotransmission,etc.Datatraffic.Ifthesefactorsaretakenintoaccount,theuserusagemodelofthenetworkbandwidthisgreatlydifferentfromtheexistinginitialdesignestimates.Infact,inpeople'sdailylife,manypeopleusenetworkbandwidthfor180minutesorevenmorethananhouronaverage.
Obviously,operatorsurgentlyneedalargeamountofnetworkcapacitytomeettheever-increasingservicedemandsofcustomers.Itisestimatedthatin1997alone,thebandwidthcapacityoflong-distancetelephonetransmissionthroughapairofopticalcablesincreasedto1.2Gbps(millionbitspersecond).WhenthedatatransmissionspeediscalculatedinGbpsunits,theinformationof1,000bookscanbetransmittedthroughthenetworkpersecond.However,if1millionhouseholdswanttowatchthevideoprogramslaunchedonthewebsiteorusethenewlyemergingnetworkvideoapplications,then,inthisdemandsituation,thenetworktransmissionratemustreachtheterabitlevel(trillionbitspersecond).:Tbps).WhenthedatatransmissionspeediscalculatedinTbps,withinonesecond,thenetworkcantransmitthedatavolumeof20millionconcurrentduplextelephonesoralldailyreportspublishedinthepast300years.
Ofcourse,thedemandfornetworkbandwidthisgrowingveryfast.Forexample,somepeoplepredictthroughresearchthatfrom1994to1998,thenetworkcapacityoftheUnitedStatesLong-distanceExchangeNetworkOperator(IXC)willincrease7times,whilethenetworkcapacityoftheUnitedStatesLocalExchangeNetworkOperator(LEC)willincrease4times.Butinfact,itisestimatedthatitsnetworkcapacitywillincreaseby32timescomparedwithpreviousyears,andanothercompany'snewlyaddednetworkcapacityin1997reacheditsentirenetworkscalein1991.Somecompaniesclaimthatthesizeoftheirnetworkwilldoubleeverysixmonthsinthenextfouryears.
Inadditiontotheexplosiveincreaseinconsumerdemandforbandwidth,manyserviceprovidersarealsofacingthedilemmathattheavailablemarginoftheiropticalcablesisabouttorunout.Anindustryreportpointedout:In1995,theusedpartofburiedopticalcablesaccountedfor70%to80%ofthenetworkonaverage.Manytelecomoperators'opticalcableutilizationratehasalmostreachedtheupperlimitof100%effectiveutilization.Thereisalsoanembarrassingproblem:howcannetworkserviceoperatorsdeployandintegrateavarietyofcommunicationtechnologiesonaphysicalnetwork.Theneedsofconsumersandthepressureofcompetitionamongenterprisesforceoperatorstoprovideavarietyofservicesthataremoreeconomicalintermsofconstructionandoperatingcosts.Ontheotherhand,theyhavetobuildontheexistingnetworkfoundationasmuchaspossible.Deploytheseservicesontop.Fortunately,DWDMtechnologyhasemerged,anditisDWDMthatprovidestheseoperatorswithfeasiblesolutionstomeettheseneedsatthesametime.
UsingDWDMtechnologyallowsserviceproviderstoprovidee-mail,videoandmultimediaservicesbroughtaboutbytraditionalIPoverATMbearerdata,SONET/SDHbearervoiceandothertransmissionmethods.Atthesametime,withouttheneedConsideringthesedifferentdataformats-whethertheyareIP,ATMorSONET/SDH,DWDMcanequallyprovideunifiedbandwidthmanagementfunctionsforthesedifferenttransmissionmethods.AlltheabovethreecommunicationprotocolscanbeadoptedbyTheopticallayerofDWDMtechnologycanbetransmitted.Thisunifiedmanagementfunctionallowsserviceproviderstoflexiblymeetcustomers'bandwidthneedsthroughasinglenetwork.
Ifoperatorswanttosucceedincommercialoperations,oneofthekeypointsisthattheyneedaunifiedbearerplatformthatcancarryvariouscommunicationtechnologiesinaunifiedmannerandinterfacewiththesecommunicationtechnologies.Moreover,theTheplatformshouldalsogiveoperatorstheabilitytointegratecurrentandnext-generationtechnologies.Facetheabovethreeproblems:increasingservicedemand,exhaustionofopticalcablemargin,andunifiedhierarchicalbandwidthmanagement.Theserviceprovidermustfindaneconomicallyfeasiblesolution.Anobviousmeasuretoreducefiberopticcableconsumptionistolaymorefiberopticcables.Forthosenetworkswherethecostoflayingnewfiberopticcablescanbekepttoaminimum,thismeasurecanprovetobethemosteconomicalsolution.However,layingnewopticalcablesdoesnotencourageserviceproviderstoprovidenewservices,nordoesitallowoperatorstoobtainunifiedmanagementcapabilitiesforopticaltransportlayerbandwidth.ThesecondmeasureistousetimedivisionmultiplexingtechnologyTDMtoincreasethedatatransmissionrate.TDMdividesthetimeintosmallerintervalssothatmoredatacanbetransmittedatthesametime,resultinginanincreaseintheeffectivecapacityoftheopticalcable.Infact,thisisthesolutionalreadyadoptedintheindustry(DS-1,DS-2,DS-3,etc.).However,whenserviceprovidersonlyusethismeasure,eachoftheirnetworkexpansionhasasignificantjump,whichmeansthatthegrowthofnetworkcapacityisnotsmooth,anditisverylikelythattheywilleventuallygetmorethantheyoriginallydemanded.Bandwidth,inasense,thisisasituationthatmanyoperatorsareunwillingtosee,anditsmanagementcomplexityandinvestmentwillincreaseheadaches.TakingSONETtechnologyasanexample,thenextlevelofcapacityincreasedfrom10GbpsTDMto40Gbps(thishugeleapforwardthatmanypeopleareconvincedisunlikelyforTDMtechnologyinashortperiodoftime).TheNorthAmericantransmissionnetworkusingSONETandtheinternationaltransmissionnetworkusingSDHbothuseTDMtechnology.
ThetelecommunicationsindustryhasadoptedtheSONETorSDHstandardtoprovideastandardsynchronousopticalfibernetwork,withitsflexibilitytomatchcurrentandfuturedigitalsignals.SONETorSDHachievestheabovegoalsbydefiningstandardtransmissionratesandopticalfiberinterfaces.Forexample,theterminalthatterminatestheSONETnetworkwillintroduceavarietyofelectronicandopticalsignals.ThesesignalswillbemultiplexedintheformofelectricalsignalsbeforebecomingthedataloadofSTS-1(anorganicpartoftheSONETnetworkframestructure).STS–1Theloadisthenmultiplexedandtransmittedinasinglefiberatasinglerate.Thesestandardratesare:OC–3,OC–12,OC–48,andeventuallyashighasOC–192.SDHhasaframestructuresimilartoSTM–n,anditssignalratecanreachtherangeofSTS–1toSTM–64.
SONETandSDHaretwocloselyrelatedstandards,anditisthesetwostandardsthathavelaidthefoundationforthetransmissionnetwork.Thesetwostandardsdeterminetheparametersofthetransmissioninterface,thetransmissionrate,theformatofthetransmissiondataandthesignalmultiplexingmode,andeventheoperation,management,maintenanceandprovision(OAM&P)characteristicsrequiredtoachievehigh-speedtransmission.Synchronoustransmissionmodemeansthatthelasersignalflowingthroughtheopticalcablesystemissynchronizedwiththeexternalclock.Theadvantageofthisisthatthedatastreamthattransmitsvoice,dataandimagesthroughtheopticalcablesystemcanflowinasmoothandregularmanner.Asaresult,eachlaserbeamcanbeeasilyidentifiedbytheoppositeend.
Technicalprinciple
DWDMfirstallocatestheincomingopticalsignaltoaspecifiedfrequency(wavelength,lambda)inaspecificfrequencyband,andthenmultiplexesthesignalintoanopticalfiber.Inthisway,thebandwidthofthelaidopticalcablecanbegreatlyincreased.Sincetheincomingsignalisnotterminatedattheopticallayer,therateandformatoftheinterfacecanbekeptindependent,whichallowsserviceproviderstointegrateDWDMtechnologywithexistingequipmentinthenetwork,whilegainingaccesstotheexistinglayingofopticalcables.Alargeamountofbandwidththatisnotavailable.
DWDMcancombinemultipleopticalsignalsfortransmission.Asaresult,theseopticalsignalscanbegroupedintothesamegroupandamplifiedatthesametimeandtransmittedthroughasingleopticalfiber.Thebandwidthofthenetworkisgreatlyincreased(seeFigure3)..Eachbearersignalcanbesettodifferenttransmissionrates(OC–3/12/24,etc.)anddifferentformats(SONET,ATM,data,etc.).Forexample,aDWDMnetworkcanmixOC–48(2.5Gbps)andOC–192(10Gbps)SONETsignalsonthebasisofDWDM.Inordertoobtainahugebandwidthofupto40Gbps.Whileachievingtheabovegoals,thesystemusingDWDMcanstillmaintainthesamelevelofsystemperformance,reliabilityandstabilityastheexistingtransmissionsystem-evenworse.LaterDWDMterminalscancarryatotalof80wavelengthsofOC-48toachieveatransmissionrateof200Gbpsorupto40wavelengthsofOC-192toachieveatransmissionrateof400Gbps.Thisbandwidthissufficientwithinonesecond.Transmissionof90,000volumesoftheencyclopedia.Thekeytechnologytorealizethishigh-speed,high-capacitytransmissioncapabilityistheopticalamplifier.Theopticalamplifierrunsonaspecificspectralfrequencybandandisoptimizedaccordingtotheexistingopticalfiber,whichmakesitpossiblefortheopticalamplifiertoamplifythelightwavesignal,therebyexpandingitstransmissionrangewithoutconvertingitintoanelectricalsignal.Ultra-widebandfiberamplifiershavebeenusedinpracticetoprovethatlightwavesignalscarrying100channels(orwavelengths)canbeeffectivelyamplified.Thenetworkthatusesthiskindofamplifiercanhandleterabit-levelinformationveryeasily.Atthisrate,suchanetworkmayeventransmitalltheTVchannelsintheworldatonceor500,000moviesatthesametime.
Taketheanalogyofahighway,anopticalfibercanalsoberegardedasamulti-lanehighway.TheTDMsystemintheusualsenseusesonelaneofthehighwaytoincreasethebandwidthbyacceleratingthedrivingspeedofthecaronthisonlylane.Intheopticalcablenetwork,theadoptionofDWDMislikeputtingthecarbehindonanunusedlaneonthehighway(increasingthenumberofwavelengthsforlayingopticalfibers)toobtainanincrediblyhugebandwidth.Thereisanotheradvantage:thisroaddoesnotcareaboutthetypeoftrafficrunningonit.Asaresult,the"cars"runningontheDWDMhighwaycanloadATMcells,SONETandIPpackets.
Systemcharacteristics
AnacceptableidealDWDMsystemshouldhavesomecommonkeycharacteristics.AnyDWDMsystemshouldhavethesecharacteristicssothatoperatorscanrealizethehugepotentialofthetechnology.ThefollowingquestionshelpdeterminewhetheraspecificDWDMsystemmeetstherequirements.
1.Whetherthesystemutilizesexistingequipmentandfacilities.The2.5GbpslevelDWDMsystemshouldbeabletofullyutilizetheexistingequipmentandopticalcablefacilities.
2.Thesystemusesfluorideorsilicon-basedfiberamplifiers.Inthe1530-to1565-nmspectrum,theperformanceofsilicon-basedopticalamplifiersandfluorideopticalamplifiersequippedwithfiltersaregood.However,fluorideopticalamplifiersaremoreexpensivetoimplement.Thelong-termreliabilityoffluoridefiberhasnotbeentested.
3.Whethermanualinterventionisrequiredwhenchangingthechannel.Whenincreasingordecreasingthenumberofopticalchannelstoachievethebestsystemperformance,theopticalamplifierwillautomaticallyadjust.Thisisveryimportant,becauseifthereisonlyonechannelinahigh-energysystem,theself-phasemodulationphenomenonwillleadtoadecreaseinsystemperformance.Ontheotherhand,ifthepoweristoolow,theamplifiercannotobtainsufficientgain.
4.Whetherthesystemisstableandreliable.TheDWDMsystemthathasbeenwell-engineeredandconstructedprovidesitsownreliability,systemavailabilityandsystemredundancy.Althoughfiltersareoftenaffectedbyhumidenvironments,thisisnolongeraproblem.
5.Whetherthelaserpumphasaconnector.Theopticalamplifierhastwokeycomponents:erbium-dopedopticalfilterandamplifier.Whenthelaserpumpactivatestheerbiumelementwithalaserofaspecificwavelength,erbiumwillactasagainmediumtoamplifytheincominglasersignal.Ifaconnectorisusedinsteadofdirectbonding,slightdirtonthesurfacemaydamagetheconnector.
6.Whetherthenumberofwavelengthsandtransmissionrateofthesystemcanbeupgraded.AlthoughtheanswerisyesforvariousDWDMsystems,itisalsoimportanttoplanfortheupgrade.Iftheserviceprovideradoptsacertainmethodtoassembletheirnetworkasawholeandthenupgrade,thenthefollowingsituationmayhappen:thenetworkneedsmorepoweroradditionalsignal-to-noiseratioincrease.Forexample,everytimethesupplierdoublesthenumberofchannelsorbitrate,anadditional3decibelincreaseinsignal-to-noiseratioisrequired.
7.Doesthesystemprovideastandardmaintenanceinterface?STL1interfacecanbewidelyusedinDWDMsystem.Theinterfaceshouldbeadaptedtotheserviceprovider’susualmaintenanceplan.
Keytechnologies
Keycomponents
Theuseofopticalnetworkstobuildfuturehigh-speed,large-capacityinformationnetworksystemsneedstofocusonhigh-speedopticaltransmission,multiplexingandde-multiplexingUsetechnology.Optical-basedadd/dropmultiplexing(OADM)technology,opticalcrossinterconnection(OXC)technologybetweennetworks,integratednarrow-band,high-speed,wavelength-tunablelow-noisedetectortechnology,andopticalfibernetworktrunktransmission,TheintegratedlightsourceoftheDFBlaser/opticalmodulatorwitharateofupto4OGbit/s,wavelengthtunable,andhighlystablegaincoupling.
1)Opticalfibertransmissionisgenerallyconsideredthatsingle-modefiberSMFhasalargedispersion,whichisgoodforreducinginterferencecausedbyfour-wavemixing(FWM),butitrequiresalotofcompensationfibers.TheactualexperimentshowsthatwhenSMF(G.652)andDSF(G.653)areusedinWDMsystem,theharmofSPMandXPMisless,notasseriousasimagined.TheoriesandexperimentsinthepastshowthattheFWMinterferenceofDSFfiberisseriousanditisnotsuitableforWDMsystem.However,afterRamanamplificationisused,theamplificationeffectisdistributedalongthefiberinsteadofbeingconcentrated,sothetransmittedopticalpowercanbereduced,andtheFWMinterferencecanbereduced,soWDMcanstillachievebetterresultsinDSFfibertransmission.Polarizedmodedispersion(PMD)andchromaticdispersioncompensationareinevitablyproblemsencounteredinlong-distanceandlarge-capacityWDMsystems.Ifyouwanttogetawideandflatband.Thentherearecertainrequirementsforthedispersionanddispersionslopeofthedispersioncompensationdeviceatthesametime.
2)DWDMlightsourceWDMopticalnetworkrequireshighspeed(largecapacity),lowchirp(toincreasethetransmissiondistance),andstableworkingwavelengthforthelightsource.Forthisreason,itisnecessarytoresearchanddevelophighspeed,lowchirp,Ahighlystablelightsourcewithadjustableworkingwavelength.Fromtheperspectiveofworldwidedevelopmenttrends,integratedlightsourcesarethepreferredsolution.Theintegrationoflasersandmodulatorscombinesthefunctionsoflaserwavelengthstabilityandtunabilitywiththehigh-speedandlow-chirpfunctionsofthemodulator.Therearemanytypesofintegratedlightsources:OneisthemonolithicintegrationofDFBsemiconductorlasersandelectro-absorptionmodulators.ThesecondisthemonolithicintegrationofDFBsemiconductorlasersandM-Zmodulators:therearealsomonolithicintegrationofdistributedBraggreflector(DBR)lasersandmodulators,andhybridintegratedDBRlaserscomposedofsemiconductorandfibergrids.
3)DWDMdetectorwavelengthtunablenarrow-bandopticaldetectorisahigh-efficiency,high-signal-to-noiseratioopticalreceivingtechnologyfordownloadingvoicechannelsinWDMopticalnetworks.Inordertogreatlyreducethesizeofthesystem,considerintegratingthepreamplifiercircuitandthedetectortogether.Eachdetectorofthistypeofdevicemustcorrespondtoadifferentchannel,sothedetectormustbenarrow-band,andtheresponsepeakwavelengthmustbealignedwiththecenterwavelengthofthechannel,sotheresponsebandwidthmustbetunablewithinacertainrange.Inaddition,thecrosstalkbetweenthedetectorsisrequiredtobesmall.TheResonantCavityEnhanced(RCE)photodetectorintegratesanarrow-bandtunablefilterandadetector,andisthefirstchoiceforthistypeofdetector.
4)Thewavelengthconversionall-opticalwavelengthconversionmoduleisusedattheaccessendtoconverttheopticalsignalfromtherouterorotherequipment,andconverttheopticalsignalonthenon-matchedwavelengthtothestandardspecifiedbytheITUThewavelengthistheninsertedintotheopticalcoupler;andwhenitisusedinthewavelengthswitchingnode,itexchangestheopticalpathandperformsthewavelengthreusefunction,soithasaveryhugeroleinthewavelengthroutingall-opticalnetwork.Broadbandtransparencyandfastresponsearethebasicrequirementsofwavelengthconverters.Amongthevarioustechnologiesofall-opticalwavelengthswitching(includingcross-gainmodulation,cross-phasemodulation,four-wavemixing,andnonlinearopticalloopmirror)technologies,themostpromisingall-opticaltransponderisbasedonsemiconductoropticalamplifiers(SOAs)Thecross-phasemodulationprincipleisintegratedintotheMach-Zehnderinterferometer(MZI)orMichelsoninterferometer(MI)toformaband-wavelengthconverter.Itisrecognizedasanidealsolutionforwavelengthconversioninhigh-speed,high-capacityopticalnetworks.
WhenusingWDMnetworkingonalargescale,especiallyduringchannelscheduling,itmaybenecessarytoconvertacertainwavelengthtoanotherwavelength,ortheentirebandmustbeconverted.TheopticalbandconverterdevelopedbyLucentusesthesecond-ordernonlinearcoefficientx(2):x(2)ofLiNbO3toconverttheopticalwavelength.TheopticalwaveguideisaperiodicpoledLiNbO3opticalwaveguide(Periodicallypoledwaveguide).
5)Opticalamplifiersneedamplifiersinordertoovercometheattenuationintheopticalfiber.Erbium-dopedfiberamplifierEDFAhasbeenwidelyusedinlong-distancecommunicationsystems.Itcanprovideaflatgainbandwidthofabout30nminthe1550nmwindow.
ForwidebandEDFAamplifiers,gainflatnessovertheentireWDMbandwidthisparticularlyrequired.Recently,therehasbeenadual-bandfiberamplifier(DBFA)basedonerbium-dopedfiber,anditsbandwidthcancovertherangeof1528～1610nm.ItiscomposedofconventionalEDFAandEBFA(Extendedbandfiberamplifer).SimilarproductsincludeBellLab'sUWOA(Ultra-WidebandOpticalAmplifier),whichhasanavailablebandwidthof80nmandcanamplifyupto100wavelengthchannelsinasingleopticalfiber.ItcoverstheCband(1530～1656nm)andLband(1565～1620nm).
UKImperialCollegehasdevelopedabroadbandRamanamplifier.StimulatedRamanAmplify(StimulatedRamanAmplify)istodirectlyaddtheopticalpumppowertotheconventionalopticalfiber,andusethenonlinearityoftheopticalfibertoamplifytheopticalsignal.ThesingleopticalpumpRamanamplifierhasanarrowgainbandwidth,andtheRamanamplifierwithtwoopticalpumpswithwavelengthsof1420nmand1450nmcanobtainaverywidebandwidth(1480~1620nm).ThegainofRamanamplificationcanreach30dB,andthenoisefigureislessthan6dB.Theopticalpumppoweris860mW.
6)Opticaladd/dropmultiplexer(OADM)andopticalcross-connector(OXC)opticaladd/dropmultiplexerOADMs(OpticalAddDropMuxs)achieveselectiveground/drop(drop)inWDMfiberoradd)Therequiredopticalwavelengthchannelofanyspecificrate,formatandprotocoltype.Itistheinterfacebetweenthehigh-speedandlarge-capacityWDMopticalfibernetworkandtheuserinterface.OADMisgenerallyasingle-chipintegrationorhybridintegrationofmultiplexers,demultiplexers,andopticalswitcharrays.OADMdeviceswithtunablewavelengthareunderdevelopment,andbreakthroughshavebeenmade.Inaddition,thecrossinterconnectionbetweenWDMopticalnetworkswillgraduallytransitiontoacompletelyopticalform.TherehavebeenreportsontheworkofmonolithicintegratedOXClaboratoriesintheworld,butmoreworkisfocusedonthekeycomponents,mainlywavelengthconversiondevicesforsolvingnetworkcongestionandrationalutilizationofnetworkresources.AWG(ArrayWaveguideGrating)isanewkeydevicemostsuitableforDWDMmultiplexinganddemultiplexingandasacoredevicetoformOADMandOXC.BecausetheAWGcanbeefficientlycoupledwiththequartzfiber,theinsertionlossisverylow,andlow-costintegrationcanberealized.Inaddition,theAWGalleviatestherequirementfortheintegrationofthelightsourcearray,andtheDWDMgoalcanbeachievedbycouplingmultiplesingle-wavelengthlaserstoit.Thekeytotheresearchistomasterthepreparationtechnologyofthick-layerwaveguidesandtrytoavoidtheintroductionofpolarizationdispersionduetostress,andevenleadtodevicerupture.
7)Opticalswitch,opticalwaveguideswitchintegratedareaarrayisalsoakeycomponentofOXCandOADM.MostpracticalopticalswitcharraysarerealizedbyLiNbO3opticalwaveguideswitches.Thiskindofopticalswitchmatrixisdifficulttoachievelarge-scalemonolithicintegration,especiallydifficulttoachieveOEICintegrationwiththeoperatingcircuit.Therearealsothermo-opticalswitchesthatuseSiO2/Si,buttheresponsespeedisslowerontheorderofmilliseconds,whichisonlysuitableforchannelsHandover,fortheexchangeofcells/packets,theresponsespeedcannotmeettherequirements.Inordertorealizetheexchangeofcells/packets,theresponsetimemustreachatleasttheorderofmicroseconds.Thequasi-real-timeexchange(suchasexchangeinacomputernetwork)needstoreachtheorderofnanoseconds.TheutilizationrateofinformationresourcesinthenetworkisdeterminedbytheintegrationscaleofOXCandtheflexibilityofoperation,sothefinalOXCshouldbemonolithicintegrated.Thekeytechnologyisthedevelopmentofhigh-speedresponseSi-basedlightguideopticalswitches,andtheSOI-typeSiO2/Siwaveguideopticalswitchcomposedofelectricalinjectionbendingeffectcanrealizeopticalswitchoperationoflessthanmicroseconds,andisexpectedtorealizelarge-scalemonolithicintegration.
HertzLaboratorieshasdevelopedaveryhigh-speedopticalswitch,whichcansampleat160Gbit/sopticaldatastream.Itsworkingprincipleis:thefour-wavemixinggeneratedinthesemiconductoropticalamplifierusingtwoopticalpulseswithwavelengthsof1302nmand1312nmrespectivelycanbeusedtocheckthe1550nmopticalsignalpulsesampling.Thiskindofhigh-speedswitchissuitableforextractingtheroutingaddressdirectlyfromtheopticalIPsignalinthefutureinordertorealizetheopticalIP(IPoverOptical).
Themicro-mirrorarrayopticalswitchtechnologybasedonMEMS(Micro-elecromechan-icalsystems)technologyisalsoahotspotintechnologydevelopment.Comparedwithtraditionalelectronicdevices,theuseofMEMStechnologyinopticalnetworkshastheadvantagesoflowcost,fastspeed,smallsize,largecommunicationcapacity,smallsize,flexibleandvariable,transparenttobitrateandprotocol,andcross-electroniclimitationstoincreasenetworkspeed..Buttheswitchingspeedisnotuptotherequirement.Micromechanicaltechnologycanalsobeusedasavariableopticalattenuator.Itsworkingprincipleistouseelectrostaticattractiontochangethepositionoftheshieldingsheetinthemicromachinetocoverthelightguideareaoftheopticalfiber,therebychangingthelightattenuation.Thedevicecanbecontrolledbyopticalsignalsandcanbeusedtomake:opticalattenuators,opticalpowerstabilizers,opticalpowerequalizersandopticalbandswitches.
Anothertypeofopticalswitchisapolymerdigitalswitchingdevice.OpticalwaveguidedevicesmadeofPolymermaterialsarebecomingmature.Thepolymermaterialiseasytoprocessandlowincost.Theapplicationofvoltageontheelectrodecancontroltheopticalsignaltopassthroughornottopassthroughtheopticalwaveguide.Theproblemisthatitiseasytofallofffromthesubstratesiliconwafer,easytoabsorbwaterandaging.
Networkstructureclassification
Opticalnetworkcanbedividedintoringnetwork,meshnetwork,starnetworkandbusstructureaccordingtophysicalconnection.Comparedwiththemeshtopology,theringtopologyhasmanyadvantages,forexample:thecostoflinksharingislow,thelinkcanbeshared,andwhenthereisalargeburstofdataflow,theworkingfiberandtheprotectionfibercanbeusedatthesametimetoreducetheloadoftherouter,Therebyavoidingtheneedforcachingontherouterside.
Multi-wavelengthnetworkscanbedividedintosingle-hopnetworksandmulti-hopnetworks.Inasingle-channelnetwork,thedataflowfromthesourcetothedestinationpassesthroughthenetworklikeanopticalflow,andthereisnoneedforelectricalconversionatanynodesinthemiddle.Therearetwotypicalsingle-hopnetworksfromthewayofopticalnetworkrouting:BroadcastandselectnetworkandWavelengthroutednetwork.
Thebroadcastandselectionnetworkareconnectedbypassivestar-shapedcouplingdevicestoconnectmultiplenodesaccordingtothestar-shapedtopology.Thebasicprincipleistosendintheformofbroadcast,andthereceivingendcanselectivelyfilterandreceive.Thiskindofnetworkismainlyusedforhigh-speedlocalareanetworksorwideareanetworks.Therearetwoworkingmodes:fixedwavelengthopticaltransmissionandtunableopticalreceptionorfixedwavelengthreceptionandtunabletransmissionwavelength.Therearetwoshortcomingsofbroadcastandselectionnetworks:Oneisthewasteofopticalpower.Thetransmittedopticalpowerissharedtoallreceivers,regardlessofwhetherthereceiverisacommunicationpartner.Inthisway,fortherealizationofthecommunicationnode,thelosscausedbytheopticalsplittingisincreased.Thesecondispoorscalability.NnodesneedtouseatleastNwavelengths.Addinganoderequiresaddingawavelength,andthetunablerangeofeachreceivermustbecorrespondinglyincreasedbyawavelength,andwavelengthreusecannotbeperformed.
Incontrast,thekeyelementofthewavelengthselectionnetworkisthewavelengthselectionswitch,whichisalsodividedintotwotypes:thewavelengthremoteswitchingmodeandthewavelengthconversionswitchingmode.TheformeristodynamicallyexchangedatasignalsbetweencommunicationsbychangingWDMrouting.Thelatterswitchesthedatatoanotherwavelengthchannelthroughwavelengthconversion.
Ifopticalswitches,wavelengthconverters,tunablefilters,arrayedwaveguideroutersandotherphotonicdevicesareusedinthenodes,aflexible,scalable,andreconfigurableopticalnetworkstructurecanbeformed.
Wavelengthrouting
Theopticalnetworkisformedbyconnectingwavelengthroutersandendnodestoeachotherthroughopticalpaths.Obviously,eachlinkcansupportmanysignalformats,buttheyarealllimitedtowavelengthgranularity.Wavelengthswitches(orwavelengthrouters)havethefollowingtypesofcomposition:
Non-reconstructedswitches:Thecorrespondingrelationshipbetweeneachinputportandoutputportisfixedandthewavelengthisthesame.Itcannotbechangedonceitisbuilt.
Reconfigurableswitchesrelatedtowavelengths:Thecorrespondingrelationshipbetweeninputportsandoutputportscanbedynamicallyreconstructed,butthisrelationshiphasnothingtodowithwavelength.Thatis,everyinputsignalhassomefixedoutputports.
Wavelengthselectivereconfigurableswitch:Ithasboththedynamicreconfigurationoftheportandtheroutingfunctionaccordingtotheinputwavelength.
Giventhephysicaltopologyofanetworkandasetofend-to-endopticalchannelsthatneedtobeestablishedonthenetwork,decidingroutingandassigningwavelengthsforeachbandwidthrequestisawavelengthrouteroutingproblem.Therearethreemaintypesofwavelengthroutingproblemsinopticalnetworks:
First,whentheall-opticalwavelengthconversionmoduleisnotused,theproblemofadaptivenetworkwavelengthandroutingdynamicallocation(RWA)isrealized.ThesolutionistodetermineandoptimizeCriteria,wavelengthandroutingallocationalgorithm.Italsoincludesadynamicself-healingrecoveryalgorithmforfaultrecoveryroutingwhentherequiredsystemcostisminimal.
Second,whenthereisanall-opticalwavelengthconversionmodule,howtousethewavelengthconversionmoduletoreducewavelengthcongestionalgorithmresearch,includingtheuseofwavelengthconversionmodulesystemperformanceincreaseandwavelengthroutingopticalnetworktopology,networksizerelation.
Inaddition,torealizetrueadaptiveroutingandwavelengthallocation,theroutingproblemundertherestrictionofbusinessflowmustalsobeconsidered.ThemostidealsituationisthattheDWDMopticalnetworknodemonitorstheserviceflowontheopticalchannel,andincreases/decreasesthenumberofopticalchannelsandincreases/decreasesthedatarateoftheopticalchannelaccordingtotheusageconditionsaccordingtothecorrespondingalgorithm.
Theuniqueattributeoftheopticalnetworkisthatitcanrealizewavelengthrouting.Thesignalpaththroughthenetworkisdeterminedbythewavelength,sourcesignal,statusinformationofthenetworkexchange,andwavelengthchangeinformationintherouting.Figure2showstheestablishmentofanopticalpathinawavelengthroutingnetworkbasedonawaveguidegratingrouter(WGR).WGRnodesallocatewavelengthsthroughwavelengthroutingalgorithms,andtheapplicationofwavelengthconverterscanincreasetheflexibilityofthenetwork.
Wavelengthadd/dropmultiplexing(WADM)canbedirectlyconnectedtoarouter,makingitpossibletoestablishanopticalpathbetweenthetwo.BecauseInternetdatahasahighasymmetryinthedevelopedandreceivingchannels,theexistingcommunicationsystemdesignedonthebasisofsymmetricvoiceservicescannotadapttothisasymmetricservice.Oneadvantageofdirectlyconnectingtheroutertothediscretewavelengthisthattheopticalsystemcanperformthecorrespondingflowgroomingfunctiononthewavelength-basedopticaldomaindirectlyaccordingtotheflowofInternetdata.
Networksynchronizationandsecurity
BecausetheDWDMsystemprovidesmultiplexingofdifferentwavelengthswithnotimerelationshipwitheachother,itdoesnotneedtobesimilartotheclocksysteminSONET.However,toensuretransmissionquality,synchronizationtechnologymaystillbeneededintheWDMsystem.
Fiberopticscaneasilyachievesecureconnections.Quantumcryptographytechnologyusesthemostbasicprincipleofquantumcomplementarity(quantumcomplementarity:basedonthefactthatparticlesandwavesaremutuallyexclusiveintheirbehaviors,andatthesametimearetwoinseparableelementsthatcompletelydescribeaphenomenon)principleisoneofthem.Allowtwousersfaraparttouseasharedrandombitsequenceasthekeyforcryptographiccommunication.Verycomplextraditionalencryptionmeasuresareimplementedthroughcomplexandstrongmathematicaloperations.ComparedwiththeQuantumKeyDistribution(QKD)technology,itprovidesanewtypeofencryptionbasedonitsname.Basicphysicalprinciplesareaneffectivewaytoprotectandencryptusefulinformation.
PowerEqualizationTechnology
Comparedwithpoint-to-pointWDMsystems,animportantfeatureofWDMopticalnetworksisthatthepowerofeachchannelatthesamereferencepointinthenetworkisdifferent.Inanend-to-endWDMsystem,thepowerofeachwavelengthatthesignalsendingendisequal.Inanopticalnetwork,theopticalsignalfromthelocalnodeismultiplexedandtransmittedwithothersignalsthathavedifferentdistancesandthushavedifferentopticalpowers.Eveniftheopticalsignalsaremultiplexedtogetherandtransmittedforacertaindistance,theirpowermayalsobedifferentbecauseoftheslightlydifferentresponsesofEDFAs,opticalfilters,andopticalswitchestoeachwavelength.AfterwavelengthsignalsofdifferentpowerspassthroughthecascadedEDFAsystem,thepowerofcertainwavelengthsmaybefurtherreduced,deterioratingthechannelperformance.Inaddition,duetoreasonssuchastheaddanddropoftheopticalnetwork,reconfigurationornetworkrestoration.Theopticalpowerofeachwavelengthchannelenteringthenodeisalsodifferent.Sincetheopticalsignalhastogothroughmultiplenodesandlinks,theopticalpowerdifferencebetweeneachwavelengthchannelisaccumulated,resultinginthesamesignal-to-noiseratioofeachopticalchannel.Thesystemservicequalityisaffected,andevensomechannelsaredegradedtoanunacceptablelevel.Therefore,intheopticalnetwork,itisnecessarytobalancetheopticalpowerofeachwavelengthatthenodetoensurecommunicationquality.
Theimbalanceofthechannelintheopticalnetworkcanseriouslydeterioratethenetworkperformance,sotheequalizationofthechannelisanimportantbasisfortheperformanceoftheopticalnetwork.Manyequalizationschemeshavebeenproposed,suchasAOTFfilterandMZfilter.Tuner,FPfiltertuningprogram,andattenuatortuningprogram,etc.,theseprogramsarebasedontheuseofopticalcomponentssuchastunableattenuatorsandactivedevicessuchasSOA-basedchannel-levelequalization.OnemethodistodetecttheinterruptionoftheinputmultipleopticalsignalsontheOMUXdiskontheterminal.Afterthismessageisprocessedbythemonitoringsystem,itwillbenotifiedtoallstationsonthewholelinethroughthemonitoringchanneltocontroltheoutputdynamicsoftheopticalamplifiersofeachstation..Anothermethodistodesigninputandoutputopticalsignalmonitoringpointsonvariousopticalamplifierdisks.Throughthemonitoringsubrack,centralizedmonitoringandanalysisofeachwavelengthchannelinthelinesignalisrealized,thatis,theopticalmonitoringfromtheopticalamplifierdiskIntroduceopticalsignalsatonepointandperformonlineanalysistoobtaintheworkingstatusofanywavelengthchannel,suchasopticalpower,opticalwavelength,andsignal-to-noiseratiooftheopticalpath.WhenthepowermonitoringpointislocatedbeforethepoweramplifierEDFAinthe0XC/OADM,whenthesignalpowerorthetotalpowerofthesignalandnoiseineachchannelismonitoredandadjusted,thissolutionhasagoodequalizingeffectontheunevennessofeachchannel.However,iftheopticalpoweroftheentiremultiplexsectionfluctuates,allaffectedpasseswillbeadjustedaccordingly,whichnotonlyincreasestheadjustmenttime,butalsocomplicatestheadjustmentprocess.Thesituationisparticularlyprominentwhenthenumberofwavelengthssupportedbythelinkincreases.Inaddition,undercertaincircumstances(iftheequalizationcapabilityhasreachedthelimit),powerequalizationcannotbeachievedonlybychannel-levelequalization.Therefore,inordertoadapttotheinfluenceofnetworkconfigurationandnetworkreconstructiononeachopticalchannel,opticalpowerbalanceinWDMopticalnetworkisanimportantresearchcontentofWDMopticalnetwork.
Overheadprocessing
Inorderforanopticalnetworknodetosupportopticalnetworking,itmusthaveOAM(operation,management,andmaintenance)informationfortheopticalpath,soitmusthaveoverheadprocessingcapabilities.Therearetwowaystocarryoverheads:follow-upandshared-route,eachwithitsownadvantagesanddisadvantages.Therearethreewaystoprovideoverhead:subcarriermodulation(SCM),suchaspilottone(PilotTones);opticalmonitoringchannel(OSC);digital"wrapper"(Digital"Wrapper").
HowtocombineWDMsystemwithIPnetworktotransmitIPinformation(commonlyreferredtoasIP0verWDM)isanextremelyimportantissue,becauseIPdataserviceswilloccupyadominantpositioninthenearfuture.WhenrealizingdirectIP0verWDMwithoutusingSONET/SDHequipment,youneedtoconsiderhowcertainfunctions(suchasprocessingvariousoverheadbytes)performedintheoriginalSONET/SDHcanbeimplementedinthenewsystem.Onesolutionis:thetransmissionoverheadoflighthastwoparts,onepartisintheopticalcontainerframestructure,whichcorrespondstothesegmentoverheadofSONET/SDH,andtheotherpartisnotintheframe,butistransmittedbymodulatedpilottone.TheopticallayeronlyhasthemultiplexingfunctionofWDM.
Opticalnetworkingtechnologyprovidestransmissionnetworkingtechnologyontheopticallayer,suchasfastroutingandswitchingofOChontheopticalpath(OCh)layer;(Rate)Slotcapacity(justlikemanagingtimeslotsinexistingnetworks),whereafrequencyslotisanopticalpath.
Samefrequencycrosstalk
Inthetraditionalpoint-to-pointwavelengthdivisionmultiplexing(WDM)system,duetowavelengthselectiondevices(suchaswavelengthdivisionmultiplexer/demultiplexerandTheperformanceofthetunableopticalfilterisimperfect,andcrosstalkwilloccurbetweenadjacentwavelengthchannels.Thiscrosstalkiscalledinter-frequencycrosstalk.Itisakindofadditivecrosstalk,whichmanifestsasacertainpowerofnoisesuperimposedonthesignal,whichdeterioratestheextinctionratioofthesignal.Whentheopticalnetworkisformed,theinfluenceofthiskindofcrosstalkcontinuestoaccumulate,anditcanbefilteredoutbyaddinganopticalfilterinfrontofthereceiver,sotheimpactonthesystemissmall.
InaWDMopticalnetworkbasedonwavelengthdivisionmultiplexingtransmissionandwavelengthcross-connection(OXC),whensignalsofthesamewavelength(frequency)indifferentinputlinksaresenttothesameopticalswitch,Aftercompletingtheopticalcross-connectionasrequired,itissenttothecorrespondingwavelengthdivisionmultiplexer.Duetotheimperfectperformanceofthedevice,thesignalofonechannelwillcontainthecrosstalkofotherchannelsafterpassingthroughthecrossdevice.Whenmultiplechannelsarere-coupledtogether,inter-frequencycrosstalkwillbeconvertedintosame-frequencycrosstalk,thatis,crosstalkwiththesamefrequencyasthesignallight.Itcanbethecrosstalkbetweenthesamewavelengthindifferentlinksorthecrosstalkbetweenthesamesignalanditself.WhentheopticalchannelpassesthroughmultipleOXCs,duetotheroleofthewavelengthselectiondeviceineachOXC,inter-frequencycrosstalkwillnotaccumulateasthenumberofnodesincreases.However,thesame-frequencycrosstalkandthesignalareinthesamewavelengthchannel,andwillnotbeaffectedbythewavelengthselectiondevice,andwillbeinterruptedandaccumulatedasthenumberofnodesincreases.Therefore,thesamefrequencycrosstalkneedstobestudiedemphatically.
ThesamefrequencycrosstalkintroducedbyOXCcanbedividedintophasecrosstalk(thephaseofcrosstalklightisrelatedtothemainsignal)andincoherentcrosstalk(thephaseofcrosstalklightisnotrelatedtothemainsignal).WhenpartoftheenergyofthemainsignalbecomescrosstalkthroughtheOXC,thecrosstalkopticalsignalmaybecoherentwiththemainsignal.Thisismainlydeterminedbythetransmissiondelaydifferencebetweenthecrosstalkopticalsignalandthemainsignalandthecoherencetimeofthelaser.Whenthetransmissiondelaydifferenceislessthanthelasercoherencetime,thissame-frequencycrosstalkbecomescoherentcrosstalk.Inordertoreducetheimpactofcrosstalkonthesystem,thedelaydifferenceofdifferentopticalpathsshouldbegreaterthanthecoherencetimeofthelaserwhendesigningOXC.
Developmenthistory
TheresearchanddevelopmentofDWDMopticaltransmissionsystemisprogressingrapidly.ThetransmissioncapacityofDWDMinthelaboratoryhasreached6.4Tb/s.In2000,thecapacityofcommercialsystemswillreach1.6Tb/s.TheDWDMsystemthatmultiplexes30-40wavelengthshasbeenusedonalargescale,andthe100-160wavelengthsystemisabouttobecommercialized.Themultiplexingwavelengthinthelaboratoryhasexceeded1000wavelengths.
In1996,BellLabsintheUnitedStatesfirstconductedaDWDMtransmissionexperimentwithatotalcapacityof1Tb/s.Thiswasthelatestrecordatthetime,butthisrecordwasrefreshedinlessthanayear.AtOFC'97,NECannouncedtherealizationofa2.6Tb/sDWDMtransmissionexperiment,whichisknownastheworld'slatestrecord.However,thisrecordwasonlykeptfortwoyearsandwasrefreshed.AtOFC'99,NTTannouncedthecompletionofthe3Tb/sOTDM+DWDMtransmissionexperiment,andSiemensalsoannouncedthatitachievedatransmissionof80×40Gb/swithatotalcapacityof3.2Tb/s.TheexperimentbrokeNEC'srecord.Inthesameyear,NortelannouncedtwoworldrecordsonTelecom'99,namelythehighestrecordofsinglechannel80Gb/sandtotalcapacity6.4Tb/s.Butthesetworecordshavejustbeenannounced.AtthenewinventionexhibitioninNovember,Lucentannouncedtherealizationofsingle-channel160Gb/sandDWDM16Tb/stransmissionexperimentalrecords,leavingNortelfarbehind.Whowillthenextworldrecordbelongto?
China’sdomesticDWDMmarketisthesame.SeveralmajorcommunicationequipmentsuppliershaveenteredtheDWDMmarkettocompete.Foratime,theChinesedomesticmarketwasalsohotlyspeculated.
Networkdesign
WhendesigningaDWDMopticalnetwork,thecostshouldbeminimizedandtheselectednetworkstructurecanprovidetheexpectedroutingcapabilitiesandprotectionandrecoverycapabilitiesasplanninganddesignThemaingoal.Themostchallengingprobleminnetworkdesignisoptimization.Thegoalofoverallnetworkoptimizationistominimizethecostofnetworking.
Accordingtothecomplexityofthenetworkandthedifferentoptimizationgoals,networkdesignandplanningcanbecarriedoutinsteps,orcentralizedandunified.Step-by-stepdesigncancompletethedesignprocessofthenetworkundertheconditionofanacceptableamountofcalculation,andtheunifieddesignschemecanconsidertheoverallsituationofthenetwork,sothedesignedstructureisrelativelysuperior.Inthelaterstageofnetworkusabilityanalysis,thedesignresultsareevaluatedthroughmoreextensivetests,suchasperformanceanalysisandusabilityevaluationofthedesignednetworkunderunexpectedanddynamicbusinessconditions,andtheevaluationresultscanbefedbacktotheoriginaldesignprocessingalgorithmIn,theimplementationofthenextdesignisfurtheroptimizedthroughslightparameteradjustments.
Whenoptimizingthedesignofthemeshopticalnetwork,thedesignermustfirstdeterminetheamountofnetworkresourcesrequiredtocarryacertainamountofbusiness.TherequirednetworkresourcesincludethescaleofOXCslocatedatdifferentnodes.,Thenumberoffibersandthewavelengthrequirementsbetweennodes.
Thedesignofameshopticalnetworkisdividedintothreesteps:topologydesign(thatis,determiningtheuseoflinks);serviceroutingandcapacityallocation(thatis,determiningthestatusofservicetransmissionbetweennodes);andcapacityIdleallocation(thatis,determiningtheinvulnerabilitycountermeasuresandresourceoccupancystatusofthenetworkwhenthenetworkfails).
Thedesignandplanningprocessoftheringopticalnetworkcanbedividedintotwomajorsteps:
First,thedefinitionofthebasicsituationoftheringnetwork.Itismainlytodeterminewhethertheringnetworktobedesignedisanewdesignfromscratchoraringopticalnetworkbasedontheexistingnetwork.Inaddition,itisalsonecessarytodeterminewhetherthenetworktobedesignedisahierarchicalmulti-layernetworkorasingle-layernetwork;
Second,segmentationandpositioningoftheringnetworkstructureandfunctionsbasedontheroutingandwavelengthdistributionwithinthering.Comparedwiththemeshnetworkdesign,theringnetworkinterconnectiondoesnotneedtoperformtheallocationandplanningprocessofidlecapacitybecausetheidlecapacityitselfhasbeenembeddedintheringnetwork.
Networkoptimization
DWDMnetworkoptimizationreferstotheuseofscientificalgorithmtoolstooptimizethecalculationandoptimizationofDWDMlinksaccordingtovariousparametersoftheactuallineopticalcable,suchasattenuationanddispersion.Configureandperformspecificoptimizationadjustmentsduringprojectexecutiontoeliminateorsuppressdistortionanddegradationinthesignaltransmissionprocessasmuchaspossible,sothattheDWDMnetworkisinarelativelyoptimalworkingstate,andtheentireprocessofhigh-qualitytransmissionisensured.DuringthepropagationofservicesignalsthroughtheDWDMsystem,duetothecharacteristicsofthesystemandthetransmissionmedium,signaldistortionsofvaryingdegreeswilloccur.Signalsatratesbelow2.5Gb/sarenotaffectedbydistortionduetolowrates;high-ratesignalsabove10Gb/sareverysensitivetothesesignaldistortionsandaregreatlyaffected.Therefore,systemoptimizationisimportanttoensurehigh-speedandlarge-capacityDWDMThehighperformanceofthenetworkisessential.Thefactorsthatcausesignaldistortionmainlyfallintothefollowingtwocategories:oneislineardistortion,andtheotherisnonlineardistortion.
GeneralizedDWDMnetworkoptimizationisaprocessinvolvingawiderangeofaspects,includingthedeterminationofthebasicconfigurationduringprojectbidding,themeasurementofactualparametersduringprojectexecution,andtheadjustmentofDCMmodulesandpumpsbasedonthemeasurementresults.Theactualparametersetting,signalpre-emphasisadjustmentandotherprocessesofthecardandeachspecificsegment.DWDMnetworkoptimizationisascientificandrigorousprocess.Itmustbestrictlyinaccordancewithrelevantoperatinginstructionsandcalculationresults,anditisnotallowedtoimagineordoitatwill;inaddition,inviewofthecleanlinessofopticalfiberconnectors,itisnecessaryfor2.5Gb/sor10Gb/srateDWDMnetworks.Itisimportanttoensurethateachopticalconnectoroftheentirelinkisintactandclean,otherwiseitwillaffecttheoptimizationresultsandcausesystemperformancedegradation.TheredundancyofthemainparametersoftheoptimizedDWDMsystemisgenerally:asingle-segmentattenuationof3dB,andafull-rangedispersionofabout400ps/nm.Ifthefollowingconditionsoccur,generallyneedtobere-optimized;thelineattenuationchangestoolargeandcannotberecovered,itneedstoberecalculated,andifnecessary,theopticalpowerparametersneedtobechanged;thelinefibertyperemainsthesamebutthelengthchangessignificantly,anditneedstoberecalculated.NeedtoadjusttheDCMmodulesequenceorreplacetheDCMmodule;ifthelinefibertypechanges,itwilldefinitelyneedtorecalculate,adjusttheDCMmodule,andre-optimize.
Inshort,forthenewly-builtDWDMlink,onlyscientificrigorous,preciseandeffectivesystemoptimizationcancompletelygetridofthetraditionalDWDMlink.Toachievethelimitationofobtaining,trulyrealizehigh-speed,large-capacity,long-distance,andhigh-performancetransmission.