opticalnetworkofsdhgraduatethesiss正文正文大学学位论文

本科生毕业设计（论文）PAGEIIIAbstractInrecentyears,withthedevelopmentoftheinformationsociety,theglobalInternetbusinessshowedanexplosivegrowthtrend.TheimplementationofthefiberopticalnetworkofSDHisonthebasisofthetransmissionplatformofZTEMP320,itcanprovideasimulationenvironmentfortheentireoptictransmissionnetwork.ThispaperstudiesthetechniqueofSDHnetwork,describesthebasicprincipleofSDHmappingmultiplexing,structureofframe,categoriesoftopology,andresearchesthemethodsofprotectionoftheopticnetwork.Accordingtotheinstructionrequirement,anoptictransmissionnetworkwith9nodesissetup,theservicesbetweentwonodesareconfiguredthroughthenetworkwiththecostsorminimumresourcesofthenetwork.ThesystemtimeclockisconfiguredandchoosenodeDasprimaryclock.OfficewireiscompletedandchoosenodeDascontrollerofthenetwork.Multiplexingsectionprotectionandpathprotectionisfinished.Aftercarefuldesign,realizedsimulations,performancetestsshowthatthesynchronousdigitalhierarchynetworkisreasonable,itcanbeusedforpracticalengineering.Keywords:SDH;networks;opticaltransmission;STM-N;networking;multiplexing;opticalinterfaceTableofContentsChapter1Introduction 11.1Background 11.2CharacteristicsofSDHnetwork 11.3Researchobjectivesandsignificance 2Chapter2SDHPrinciples 42.1SDHsignalframestructureandmultiplexingsteps 42.1.1SDHsignalframestructure 42.1.2SDHmultiplexingstep 62.2SDHequipmentlogicfunctionblocks 52.3Thebasicnetworktopology 62.4Self-healingnetwork 8Chapter3SDHNetworkArchitectures 103.1Apoint-to-pointSDHlink 103.2ASDHRingNetwork 113.3Multi-RingSDHTopology 123.4 SDHRingProtectionMechanisms 133.5 TheRingProtectionArchitectures 13Chapter4RealizationofOpticTransmissionNetwork 184.1Networktopology 184.2Networkingrealization 184.2.1Setupthenetworknodes 194.2.2Installationoftheboardsinnodes 204.3SystemClockconfigurations 234.4Officewireconfigurations 264.6Serviceconfiguration 274.6.1AtoI2mb/s*10serviceconfiguration 274.6.2Theresultsofservicesofnetwork 304.7Serviceprotectionconfiguration 324.7.1ServiceprotectionfromAtoB 324.7.1Serviceprotectionresults 354.8Multiplexingsectionprotection 35References 39Acknowledgements 40AppendixI 41PAGE46Chapter1IntroductionBackgroundTheSDHisaninternationalstandardthatishighlypopularandusedforitshighspeeddatatransferofthetelecommunicationanddigitalsignals.Thissynchronoussystemhasbeenspeciallydesignedinordertoprovideasimpleandflexiblenetworkinfrastructure.Thissystemhasbroughtaconsiderableamountofchangeinthetelecommunicationnetworksthatwerebasedontheopticalfibersasfarasperformanceandcostwereconcerned.CharacteristicsofSDHnetworkSDHhasbeenabletorapidlydevelopandthisisitsowncharacteristicsareinseparable,anditsspecificfeaturesareasfollows:(1)SDHtransmissionsystemsintheworldtohaveaunifiedframestructure,digitaltransmissionstandardrateandstandardopticalinterfaceisnetworkinterworkingsystem,andthereforehasgoodlateralcompatibility,itisfullycompatiblewiththeexistingPDH,andaccommodatesavarietyofnewtrafficsignals,theformationofaglobalunifiedstandarddigitaltransmissionsystemandimprovethereliabilityofthenetwork;(differentlevelsofthecodearrangement)(2)SDHaccesssystemflowstructureofthepayloadareaoftheframeisveryregular,andthepayloadissynchronizedwiththenetwork,whichusessoftware,adirecthigh-speedsignalcandropoutofthelow-speedtributarysignalstoachievethecharacteristicsofamultiplexovercomePDHplesiochronousmultiplexedmannerforallthehigh-speedsignalmultiplexinggradualdecompositionandregenerationprocessisgreatlysimplifiedbecausetheDXC,reducingtheback-interfacemultiplexequipment,improvedtheservicedeliverynetworktransparency;(3)Asaresultofthemoreadvancedadd-dropmultiplexer(ADM),digitalcross-connect(DXC),self-healingcapabilitiesandreassemblybranchnetworkbecomesstronger,withastrongsurvival.BecauseSDHframestructurearrangeda5percentoverheadbitsignal,itisparticularlypowerfulnetworkmanagementfunctions,andcanbeunifiedintoanetworkmanagementsystemforautomationnetworks,intelligent,channelutilizationandreducenetworkcostsandvascularviabilityplayedapositiverole;(4)DuetoavarietyofSDHnetworktopology,thenetworkthatitconsistsofveryflexible,itcanenhancenetworkmonitoring,operationmanagementandauto-configurationcapabilities,optimizenetworkperformance,butalsotonetworkoperationandflexible,safeandreliable,sothenetworkisverycompleteanddiverse;(5)SDHtransmissionandswitchingperformancehasfreecombinationoffunctionblockscomposedthroughitsfamilyofdevices,toachieveavarietyofdifferentlevelsandnetworktopologyisveryflexible;(6)SDHdoesnotbelongtoatransmissionmediumwhichcanbeusedfortwistedpair,coaxialcable,butforthetransmissionofSDH.Youneedtousehighdatarateopticalfiber.Thisfeatureindicates,SDHtrunkchannelssuitableforboth,butalsoforregionalchannels.Forexample,China'snationalandprovincialtrunkcablenetworkisusingSDH,anditisalsoeasytomixcompatiblewithfiber-opticcablenetwork(HFC).(7)SDHisstrictlysynchronizedtoensurethattheentirenetworkisstableandreliable,lesserror,andeasyadjustmentandmultiplexing.ResearchobjectivesandsignificanceOvertheyears,intelligentopticaltransmissiontechnologyhasbeenofconcernfortheindustry,ithopestobuildintelligentopticaltransmissionnetworktosolvetheproblemseveralaspectsoftheexistingtransmissionnetworkexist:first,thetraditionalfast-growingonlinenetworkisdifficulttoadaptthedataservicewithunpredictability,dynamicallocationofnetworkbandwidth.Second,thetraditionalopticaltransmissionnetworkmainlyrelyonmanualconfigurationofnetworkconnections,time-consuminganddifficulttoadapttothemodernnetworkexpansionneedsnewbusiness.Third,inordertoimprovethereliabilityofbearerservices,thetraditionalopticaltransportnetworksneedtosetasidealargeamountofsparecapacity,reducingtheefficiencyofthetransportnetworkbandwidthusage.Inaddition,thetraditionalwaytoprotectbusinessnetworksisrelativelysimple,thelackofadvancedprotection,restorationandroutingfunctions.SynchronousDigitalHierarchy(SDH)iswidelyusedworldwidemetrobackbonenetwork,telecommunicationnetworkandWAN.SDHtransmissionsystemisacombinationofhigh-speedlarge-capacityopticaltransmissiontechnologyandintelligentnetworktechnology.Itprovidesconvenientandeffectivedatamultiplexinganddemultiplexingfunctions,andcanreliablytransmitdata.Chapter2SDHPrinciples2.1SDHsignalframestructureandmultiplexingsteps2.1.1SDHsignalframestructureTheSTM-N(SynchronousTransportModulelevel-1)isthefiberopticnetworktransmissionstandard.Ithasabitrateof155.52Mbit/s.Higherlevelsgoupbyafactorof4atatime:theothercurrentlysupportedlevelsareSTM-4,STM-16,STM-64andSTM-256.Beyondthiswehavewavelength-divisionmultiplexing(WDM)commonlyusedinsubmarinecabling.FramestructureTheSTM-1frameisthebasictransmissionformatforSDH(SynchronousDigitalHierarchy).ASTM-1framehasabyte-orientedstructurewith9rowsand270columnsofbytes,foratotalof2,430bytes(9rows*270columns=2430bytes).Eachbytecorrespondstoa64kbit/schannel.TOH:TransportOverhead(RSOH+AU4P+MSOH)MSOH:MultiplexSectionOverheadRSOH:RegenerationSectionOverheadAU4P:AU-4PointersVC4:VirtualContainer-4payload(POH+VC-4Data)POH:PathOverheadFramecharacteristicsTheSTM-1baseframeisstructuredwiththefollowingcharacteristics:Length:270column×9row=2430bytesByte:1-byte=64kbit/sspeechchannelDuration(Framerepetitiontime):125μsi.e.8000frame/sRate(Framecapacity):2430×8×8000=155.520Mbit/sPayload=2349bytes×8bits×8000frames/sec=150.336Mbit/sFig.2.1StructureofSDHframeSDHframestructureusedtocarryablockofinformation,eachframeiscomposedoflongitudinalandtransverserows9columns270×Nbytes,eachbytecontaining8bit,theframestructureisdividedintoasectionoverhead(SectionOverhead,SOH)area,STM-Npayloadareaandsnap-pointer(AUPTR)threeregionaldistrict.TheabovemethodcanbeobtainedwhenNisanarbitraryvaluewhentherate14,16,64,256.AscanbeseenfromFigure1,theentireframestructureisdividedintothreeregions:thesectionoverhead(SOH)totheinformationpayloadareaandsnappointer.Sectionoverhead(SOH)istheSDHframestructureinordertoensurethenormaltransmissionofinformationandadditionalbytesfornetworkoperation,managementandmaintenanceofuse,itslocationintheSTM-Nframestructureisthefirst1~9×Ncolumnsthefirst.ThefiguredescribesbasicSDHframeconsistingof9rowsand90columns.SDHframeiscomposedof810octets(bytes).Transmissioniscarriedoutrowwisefromlefttorightandfromtoptobottom.Bitsaretransmittedserially.TheSTS-1frameofSDHiscomposedofsectionoverhead,transportoverhead,payloadoverheadanddatapart.TheframestartswithfixedA1/A2bitpatternof0xf628usedforbit/octetsynchronization.SDHisreferredasoctetsynchronous.ThefirstthreecolumnsofSDHframeisreferredastransportoverhead.Thenext87columnsoftheframearereferredasSynchronouspayloadenvelope(SPE).PayloadoverheadispartofSPE.Fig.2.2SectionoverheadSTS-1datarateisabout51.84Mbps.Letusexaminehowthishasbeenachieved.EverySDHframerepeatsonceevery125micro-sec.90columnsin9rowsand8000timespersecondand8bitsperoctetgiveusdatarateof51.84Mbps.STSistheabbreviationofSynchronousTransportSignal.STS-1isreferredasOC-1(OpticalCarrier)afterscramblingisdoneonSTS-SDHmultiplexingstepThemultiplexingprinciplesofSDHfollow:Mapping-AprocessusedwhentributariesareadaptedintoVirtualContainers(VCs)byaddingjustificationbitsandPathOverhead(POH)information.Aligning-ThisprocesstakesplacewhenapointerisincludedinaTributaryUnit(TU)oranAdministrativeUnit(AU),toallowthefirstbyteoftheVirtualContainertobelocated.Multiplexing-Thisprocessisusedwhenmultiplelower-orderpathlayersignalsareadaptedintoahigher-orderpathsignal,orwhenthehigher-orderpathsignalsareadaptedintoaMultiplexSection.Stuffing-SDHhastheabilitytohandlevariousinputtributaryratesfromPDH.Asthetributarysignalsaremultiplexedandaligned,somesparecapacityhasbeendesignedintotheSDHframetoprovideenoughspaceforallthesevarioustributaryrates.Therefore,atcertainpointsinthemultiplexinghierarchy,thisspacecapacityisfilledwith"fixedstuffing"bitsthatcarrynoinformation,butarerequiredtofilluptheparticularframe.Functionstoachieveinclude;reusetheisalow-costchannellayermultiplesignalsthroughthecodetomakeitintothehigh-speedadjustmentchannelormultiplehigh-speedchannellayercodesignalsthroughtheprocessofadjustmenttomakeitintothemultiplexlayer.Amongthem,reuseisreuseroadmap,roadmapITU-Thasavarietyofprovisions,butanationalandregionaluseonlyone.China'suseofSDHsignalsaremultiplexedwiththeline10showninFigure2.3.Fig.2.3.SDHmappingmultiplexingprincipleThefigureonthepreviouspageillustratestheITU-TSDHmultiplexingstructure.Thenotationsintheboxes,suchasC-1,VC-3,andAU-4,areexplainedinthetableafterthefigure.Atthelowestlevel,containers(C)areinputtovirtualcontainers(VC).ThepurposeofthisfunctionistocreateauniformVCpayloadbyusingbit-stuffingtobringallinputstoacommonbit-ratereadyforsynchronousmultiplexing.Variouscontainers(rangingfromVC-11at1.728Mbit/stoVC-4at139.264Mbit/s)arecoveredbytheSDHhierarchy.Next,VCsarealignedintotributaryunits(TUs),wherepointerprocessingoperationsareimplemented.TheseinitialfunctionsallowthepayloadtobemultiplexedintoTUgroups(TUGs).Asthefigureillustrates,thexNlabelindicatesthemultiplexingintegerusedtomultiplextheTUstotheTUGs.ThenextstepisthemultiplexingoftheTUGstohigherlevelVCs,andTUG-2andTUG-3aremultiplexedintoVC-3(ANSImappings)andVC-4.TheseVCsaremultiplexedwithfixedbyte-stuffingtoformadministrationunits(AUs)whicharefinallymultiplexedintotheAUgroup(AUG).ThispayloadthenismultiplexedintotheSynchronousTransportModule(STM).TheinformationstructurelevelSDHmultiplexingstepiscalledSynchronousTransportModuleusingSTM-N(SynchronousTransport,N=14,16,64),infourfoldmodule,thebasicmoduleSTM-1.FourSTM-1synchronousmultiplexingconstituteSTM-4,16个fourSTM-1orSTM-4synchronousmultiplexingconstituteSTM-16,thestructureshowninFigure2.4.Fig.2.4StructureofSDHsynchronousmultiplexingSDHtransmissionnetworkiscomposedofdifferenttypesofnetworkelementsconnectedviafiber-opticcablescomposedofdifferentnetworkelementSDHnetworktocompletethetransferfunction:up/downoperations,cross-connectservices,suchasself-healingnetworkfailure.TM-TerminalMultiplexerTerminal:terminalmultiplexerusedonthesiteofthenetwork,suchasthetwoendpointsofachainwhichisadual-portdevice,showninFig.2.5.Fig.2.5TerminalMultiplexerADM-Add/DropMultiplexer:add/dropmultiplexerforSDHtransmissionnetworkadapteratthesite,suchasthechainofintermediatenodes,orSDHnodeontheringisthemostusedonline,themostimportantonenetworkelement,whichisathree-portdevice,asshowninFigure2.6.Fig.2.6Add/dropmultiplexerREG-regenerativerepeater:opticaltransmissionnetwork,therearetworegenerativerepeater,andapurelyopticalregenerativerepeater,themainpoweramplifiertoextendtheopticaldistanceoftheopticaltransmission;otherisforpulseregenerationshapingtheelectricregenerativerepeater,mainlythroughtheoptical/electricalconversion,samplingtheelectricalsignal,reproductionjudgmentplastic,electrical/opticalconversion,linenoisedoesnotaccumulateinordertoachieveandensuretransferofthesignalwaveformonlineintegrity.Aftertalkinghereisaregenerativerepeater,REGisatwo-portdevice,onlytwolineports-W,E,showninFigure2.7.Fig.2.7RegenerativerepeaterDXC-digitalcross-connectequipment:digitalcross-connectequipmenttocompletethemaincross-connectfunctionSTM-Nsignal,itisamulti-portdevice,itisactuallyequivalenttoacross-matrix,tocompletethecross-connectbetweenthevarioussignals,suchasFigure2.8.Fig.2.8Digitalcross-connectequipment2.2SDHequipmentlogicfunctionblocksSDHequipmentlogicfunctionblocksisshownasfigure2.9.SPI:SDHphysicalinterfaceTTF:transferterminalfunctionsRST:regeneratorsectionterminationHOI:high-levelinterfacesMST:multiplexsectionterminationLOI:low-levelinterfacesMSP:MSPHOA:high-levelassemblerMSA:multiplexsectionadaptationHPC:high-endchannelconnectionPPI:PDHPhysicalInterfaceOHA:overheadaccessfunctionLPA:low-levelchanneladapterSEMF:synchronousequipmentmanagementfunctionLPT:lower-orderchannelterminalMCF:messagingfeatureLPC:low-levelchannelconnectionSETS:SynchronousEquipmentClockSourceHPA:high-endchanneladapterSETPI:SynchronousEquipmentTimingPhysicalInterfaceHPT:higher-orderchannelterminal

Fig.2.9SDHequipmentlogicfunctionblocks2.3ThebasicnetworktopologySDHnetworkelementSDHnetworkisinterconnectedviaopticalcablefromthedevice,thenetworknode(NE)andthegeometricarrangementofthetransmissionlineontheconfigurationofthenetworktopology.Theeffectivenessofthenetwork(channelutilization)reliabilityandeconomyislargelyrelatedtoitstopology.ThebasicFig.2.10basicnetworktopologyLinearstructure:Tomakethepointbetweentwoadjacentnon-connectionfunctioncanberealized,whichrequiresthatallpointsshouldhavetheconnection.Thisisaneconomicaltopology.Intermediatepointsshouldbeused(ADM)dropmultiplexer,bothendsofthefirstandlastuse(TM)terminalmultiplexers.Chain-likestructure:thisnetworktopologytoallnodesinthenetworkisoneoftheseries,andbothendsopen.ThistopologyischaracterizedbyamoreeconomicalintheearlySDHnetworkusedmore,mainlyforprivatenetworks(suchastherailwaynetwork).Starstructure:thisisthenetworktopologynetworkelementasaspecialnodewithothernodesconnectedtoeachnetworkelement,theothernodesineachnetworkelementdisconnected,NEnodesofbusinesstogothroughthisparticularnodeadapter.Thisnetworktopologymaybecharacterizedbytheunifiedmanagementofaparticularnodetoothernetworknodes,bandwidthallocationinfavor;cost,butthereisaspecialsecuritynodeprocessingcapabilitiesandpotentialbottlenecks.Theroleofspecialnodessimilartandemswitchingnetwork,thistopologyusedforthelocalnetwork(accessnetworkandusernetwork).Treestructure:thiscanbeseenasacombinationofnetworktopologychaintopologyandstartopology,therearealsopotentialbottlenecksspecialnodesecurityandprocessingcapabilities.Ring:Ringtopologyisactuallyconnectedtothechaintopologyreferstoboth,sothatthenetworktopologyintheformofanetworkelementinanyonlinenodeisnotopeningup.Thisiscurrentlythemostwidelyusednetworktopologies,mainlybecauseithasastrongsurvival,i.e.strongself-healingfunction.Ringnetworkcommonlyusedinthelocalnetwork(accessnetworkandusernetwork),interofficetrunknetwork.Mesh-likestructure:allthenodesinthenetworkelementtwenty-twoconnectedtoformamesh-shapednetworktopology.Thisnetworktopologybetweentwonodesinthenetworkelementprovidesmultipletransmissionroutes,makingthenetworkmorereliable,thereisnobottleneckproblemsandfailures.However,duetohighredundancysystemwillmakethesystemlesseffective,highcostandcomplexstructures.Shapedmeshnetworkismainlyusedforlong-distancenetworktoprovidehighnetworkreliability.2.4Self-healingnetworkTheconceptofself-healingwhenthenetworkfails,withouthumanintervention,caninaveryshortlapseoftimefromtheautomaticfailurerecovery,allowinguserstofeelthenetworkfails.Thebasicprincipleistomakethenetworkhavetheabilitytofindfaultandtofindalternatedeliveryroute,re-establishcommunicationwithinashortertimeframeandapplicationoftheprincipleofprotectionswitchingline.Lineprotectionswitchingisthesimplestformofself-healingnetworks,thebasicprincipleisthatwhenafaultoccurs,theworkingchannelswitchedtotheprotectionchannel,sothatthebusinesscancontinuetosend.(1)1+11+1protectionmethodusingconcurrentexcellentincome,namelytheprotectionofclientsegmentsandworktogetheratthesendingend(bridge)permanently,butpreferredtoacceptthereceivingendsignalbasedonthegoodperformanceoffaultconditions.(2)1:Nprotectionofaprotectedsegment,Nworkingsection,theprotectionsegmentconsistsofNsegmentscommunalwork,workwheneitheronesegmentfails,canswitchedtotheprotectionsegment(usingAPSprotocol).1:1modewhichis1:NmodeexceptionIII.Applicationoftheprincipleofself-healingandself-healingreferstotheuseofadd-dropmultiplexer(ADM)toformaringnetworktoachieveaself-healingprotection.Self-healingbystructuralclassification:Channelswitchingringandringswitchingmultiplexsection;classifiedbythenumberoffiber:two-fiberringandfour-fiberring;presstransferdirectiontoreceiveandsendsignalsCategory:unidirectionalandbidirectionalring.1.two-fiberunidirectionalpathprotectionringusingtwoopticalfibers,inwhichasignalfortransmissionservice,calledW1fiber,theotherforprotection,calledP1fiber.Thebasicprincipleofusingthe1+1protectionmode(thefirstendofthebridge,theendoftheswitchover),namelytheuseofP1W1fiberandopticalfibersimultaneouslycarryingtrafficsignalsaretransmittedinbothdirections,andthereceiveronlyselectonepreferredway.2.two-fiberbidirectionalpathprotectionringtwo-fiberbidirectionalpathprotectionringstillusingtwoopticalfiber,1+1and1:1canbedividedintotwoways,thewayinwhichtheone-waychannel1+1protectionringisbasicallythesame(concurrentexcellentincome),butthereturnsignalintheoppositedirection(bidirectional)only.ItsmainadvantageistheuseoftheprotectiondeviceassociatedtothesameringorlinearADMoccasion’sequipmentusedwiththefunctionofchannelreuse,therebyincreasingthetotalvolumeofinsertoperations.3.four-fiberbidirectionalmultiplexsectionsharedprotectionringfour-fiberbidirectionalmultiplexsectionsharedprotectionringineachsection(betweennodes)usingtwoworkingfiber(pitch-catch,W1andW2)andtwotoprotectthefiber(pitch-catch,P1andP2),whereW1andW2,respectively,clockwiseandcounterclockwisebidirectionalsignaltransmissionbusiness,P1andP2,respectively,whiletheformationofthetwodirectionsW1andW2protectionringsoneachnodecorrespondingswitchingswitchforprotectionswitchingpurposes.ThemaximumcapacityofthestructureofthebusinessuptothecapacityofasingleADMKtimes(Kisthenumberofnodes),namelyK×STM-N.4.two-fiberbidirectionalmultiplexsectionsharedprotectionringtwo-fiberbidirectionalmultiplexsectionprotectionringusingatimeslotinterchange(TSI)technology.InanopticalfiberatthesametimecontaintheworkingpathandtheprotectionpathW1P2,whiletheotherfibercontainedintheworkingpathandtheprotectionpathW2P1.Halfofthepredeterminedchanneloneachfibercarryingtheworkingpath(W),theotherhalfofthechannelcarryingtheprotectionpath(P),theworkingpathofanopticalfiber(W1),theoppositedirectionoftheprotectiveringontheotherfiberpath(P1)toprotect;viceversa.Chapter3SDHNetworkArchitectures3.1Apoint-to-pointSDHlinkSDHnetworkscanbeconfiguredaspoint-to-pointlinks,linearconfigurations,meshtopologiesorringsandmulti-ringtopologies.Thereexistonlytwoterminalmultiplexernetworkelementsatbothendoflinkinpointto-pointlinksasshowninFigure3.1.Thisnetworkstructurecaneitherbeprotectedorunprotected.Inprotectedlinks,twoextrafibersareneededtobereservedforprotectionincaseofafailure.Fig.3.1apoint-to-pointSDHlinkInlinearnetworks,SDHADMnodesareconnectedinalinearfashionwheretwoterminalmultiplexersexistatbothendsasshowninFigure3.2Thistopologyprovidesdropandinsertcapabilitytoallnetworkelements.Theremaybeunprotectedlinearnetworks,establishingtwofiberconnectionsbetweentwoADMsorprotectedwithfourfiberconnectionswheretwoofthemareworkingandothertwoservingasabackuporprotectionpairFig.3.2alineardropandinsertSDHnetworkInlinearnetworks,eveniftwofibersareusedforprotectionbetweentwoADMs,itispossibleforallfourfiberstobecutatthesametime.Thus,ringsarethemostcommonlyusedtopologybecausetheyprovideanalternatepathtocommunicatebetweenanytwonodes,asshownintheFigure3.3.Fig.3.3A3.2ASDHRingNetworkAtwo-fiberringcanbeoperatedeitherasaunidirectionalring,orasabi-directionalring.Inringarchitecture,eithertwofibersorfourfiberprotectioncanbeselected.Inunidirectionalrings,trafficislimitedtoonefiberandflowsthesamewayaroundthering.Thesecondfiberworksastheprotectionfiberandisusedtoproviderestoration.Inabi-directionalring,trafficissentonbothfibers,sobothareworkingfibers.Inordertoprovidetherestorationincaseofafailure,halfofthecapacityonbothfibersisreservedforbackup,ortwomorefibersaredeployedexclusivelyforprotection.RingprotectiontypeswillbediscussedinSection2.3.2.Twoormoreringscanbeconnectedconfiguringamulti-ringtopologyasseeninFigure2.11.Inthistopology,theexpensivedigitalcrossconnectequipmentisrequiredathub-nodes.Eachringhastheirringprotectionmechanisms.Itisbettertoha