Background
ThebirthofSDHtechnologyhasitsinevitability.Withthedevelopmentofcommunication,theinformationrequiredtobetransmittedisnotonlyvoice,butalsotext,data,imagesandvideos.Coupledwiththedevelopmentofdigitalcommunicationandcomputertechnology,inthe1970sand1980s,T1(DS1)/E1carriersystems(1.544/2.048Mbps),X.25framerelay,ISDN(IntegratedServicesDigitalNetwork)andFDDI(Opticalfiberdistributeddatainterface)andothernetworktechnologies.Withtheadventoftheinformationsociety,peoplehopethatmoderninformationtransmissionnetworkscanprovidevariouscircuitsandservicesquickly,economically,andeffectively.However,duetothemonotonicityoftheirservices,thecomplexityofexpansion,andthelimitationofbandwidth,theabove-mentionednetworktechnologiesareonlyintheoriginalModificationsorimprovementswithintheframeworkarenolongerhelpful.SDHwasdevelopedunderthisbackground.Amongvariousbroadbandopticalfiberaccessnetworktechnologies,theaccessnetworksystemusingSDHtechnologyisthemostwidelyused.ThebirthofSDHsolvestheproblemofbeingunabletokeepupwiththedevelopmentofthebackbonenetworkanduserservicerequirementsduetothebandwidthlimitationoftheinboundmedia,whichhascausedtheaccess"bottleneck"betweentheuserandthecorenetwork,andatthesametime,ithasincreasedalargeamountofbandwidthonthetransmissionnetwork.Utilizationrate.SincetheintroductionofSDHtechnologyinthe1990s,ithasbeenamatureandstandardtechnology.Itiswidelyusedinbackbonenetworksandthepriceisgettinglowerandlower.TheapplicationofSDHtechnologyintheaccessnetworkcanreducethehugebandwidthinthecorenetwork.Advantagesandtechnicaladvantagesarebroughtintothefieldofaccessnetworks,makingfulluseofSDHsynchronousmultiplexing,standardizedopticalinterfaces,powerfulnetworkmanagementcapabilities,flexiblenetworktopologycapabilitiesandhighreliabilitytobringbenefits,andlong-termbenefitsintheconstructionanddevelopmentofaccessnetworks.
Основни принципи
Нивото на структурата на информацията, прието от SDH, се нарича синхронен транспортен модулSTM-N(SynchronousTransportMode,N=1,4,16,64),най-основниятмодулзаSTM-1,четириSTM-1синхронно мултиплексиранеформаSTM-4,16STM-1иличетириSTM-4синхроннимул типплексиранеформаSTM-16 и четириSTM-16синхронномултиплексиранеформаSTM- 64. Дори четири синхронни мултиплексирани STM-64 съставляват STM-256; SDHa приема структура на блокова рамка за пренасяне на информация, всеки кадър е съставен от 9 реда вертикални и 270 × N колони от байтове хоризонтално, всеки байт съдържа 8 бита и цялата рамка Структурата е разделена на три области: Раздел над глава (SOH)област,STM-NpayloadareaиManagementUnitPointer(AUPTR)area.Тази секция над главата сеизползва главно за работа в мрежата, управление,поддръжка и присвояване,за да се гарантира, че информацията може да се предава нормално и гъвкаво,и е разделена на RegeneratorSectionOverHead(RSOH)иMultiplexSectionOverHead(MSOH);payloadareai използвани за съхраняване на битове и малко количество битове, действително използвани за информационни услуги. ;указателят на единицата за управление се използва за указване на точната позиция на първия байт информация в областта на полезния товар в STM-Nрамките, така че полезният товар да може да бъде правилно разделен при получаване. Предаването на SDH кадър е подредено в поток от сериен код отляво надясно и отгоре надолу. Времето за предаване на всеки кадър е 125 μs и 1/125×100000 0 кадъра се предават за секунда. За STM-1 всеки брой кадри е 8bit×(9×270×1)=19440bit, тогава скоростта на предаване на STM-1 е 19440×8000=155,520Mbit/s; а скоростта на предаване на STM-4 е 4×155,520Mbit/s =622.080Mbit/s; Скоростта на предаване на STM-16 е 16×155.520(или 4×622.080)=2488.320Mbit/s.
WhenSDHtransmitsservicesignals,theframesofvariousservicesignalstoenterSDHmustgothroughthethreestepsofmapping,positioningandmultiplexingasfollows:
(1)MappingistocombinevariousThesignaloftherateisfirstloadedintothecorrespondingstandardcontainer(C)afterthecoderateisadjusted,andthenthechanneloverhead(POH)isaddedtoformthevirtualcontainer(VC).Thedeviationoftheframephaseiscalledtheframeoffset.
(2) Позиционирането е процесът на получаване на информация за изместване на рамката в модула за принос (TU) или блока за управление (AU). Той преминава функцията на указателя на модула на притока (TUPTR) или указателя на модула за управление (AUPTR), за да се постигне.
(3)Theconceptofmultiplexingisrelativelysimple.Multiplexingisawayofadaptingsignalsfrommultiplelow-levelchannellayerstohigher-levelchannellayers,oradaptingmultiplehigh-levelchannellayersignalsTheprocessofenteringthemultiplexinglayer.MultiplexingistheprocessoforganizingTUintohigh-levelVCororganizingAUintoSTM-Nthroughbyteinterleaving.SincetheVCtributarysignalsprocessedbyTUandAUpointersarephase-synchronized,themultiplexingTheapplicationprocessisthattheprincipleofsynchronousmultiplexingissimilartotheserial-to-parallelconversionofdata.
Content
ThecontentofSDHincludestransmissionrate,interfaceparameters,multiplexingmodeandOAMofhigh-speedSDHtransportnetwork.ThemaincontentisbasedontheSONet(SynchronousOpticalNetwork)proposalsubmittedbyBellcore(nowTelcordiaTechnologies)toANSIin1985.ITU-Thasmadesomemodificationstoit,andmostofthemodificationsareinthelowermultiplexinglayer.Inordertomeetthecomplexityrequirementsoftheinterconnectionofvariouscountriesandregions.RelatedrecommendationsarecontainedinG707,G.708andG.709.SDHequipmentisonlypartiallycompatiblewithSONet,andthetwosystemscancarryeachother'sbusinessflows,butthealarmandperformancemanagementinformationbetweenthetwosystemscannotbeinteroperable.
За да поддържа предаването на различни услуги, SDH определи, че мултиплексирането с ниска скорост получава високи скорости, а мултиплексирането с висока скорост - по-високи скорости за получаване на различни комуникационни скорости. SDH използва основния модул за синхронно предаване STM1 като основна единица за предаване, а други по-високи скорости се получават чрез мултиплексиране на мулти pleSTM1s. Първото ниво на синхронния сигнал за предаване, определено от SDH, е STS-1 (SpaceTransportation System), скоростта на STS-1 е 51,840 Mbps и STS-1 е получено чрез мултиплексиране на множество нискоскоростни сигнали T1, T2 и T3 Сигнал с основна скорост STM1, където: T1 = 1,544 Mbps ,T2=6.312Mbps,T3=44.736Mbps.Триканален STS-1 е мултиплексиран и изпратен до основния модул за синхронно предаване STM1линия за предаване.STM1скорост е 3×51.840Mbps=155.520Mbps.
AdvantagesandDisadvantages
Advantages
(1)SDHtransmissionsystemhasaunifiedframestructuredigitaltransmissionstandardrateandstandardopticalinterfaceintheworld,sothatthenetworkmanagementThesystemisinteroperable,soithasgoodhorizontalcompatibility.ItcanbefullycompatiblewiththeexistingPDHandaccommodatevariousnewbusinesssignals,formingaglobalunifieddigitaltransmissionsystemstandard,andimprovingthereliabilityofthenetwork.
(2)DifferentlevelsofcodestreamsoftheSDHaccesssystemarearrangedregularlyinthepayloadareaoftheframestructure,andthepayloadissynchronizedwiththenetwork.Itusessoftwaretotransferhigh-speedsignalsonceDirectlyaddandinsertlow-speedtributarysignals,whichrealizesthecharacteristicsofone-timemultiplexing,andovercomestheprocessofPDHquasi-synchronousmultiplexingtodecomposeallhigh-speedsignalsstepbystepandthenregenerateandmultiplexthem.SinceDXCisgreatlysimplified,theback-to-backisreduced.Interfacemultiplexingequipmentimprovesthetransparencyofnetworkservicetransmission.
(3)Duetotheuseofmoreadvancedadd/dropmultiplexers(ADM),digitalcross-connect(DXC),networkself-healingfunctionandreorganizationfunction,itisverypowerfulandhasastrongsurvivalRate.Because5%oftheoverheadbitsofthesignalarearrangedintheSDHframestructure,itsnetworkmanagementfunctionisparticularlypowerful,anditcanformaunifiednetworkmanagementsystem,whichcanhelpthenetworkautomation,intelligence,channelutilization,andreducenetworkmaintenanceandmanagementfees.Survivabilityplayedapositiverole.
(4)DuetothemultiplenetworktopologiesofSDH,thenetworkitconstitutesisveryflexible.Itcanenhancenetworkmonitoring,operationmanagementandautomaticconfigurationfunctions,optimizenetworkperformance,andalsomakenetworkoperationflexible,Safeandreliable,makingthenetworkfunctionverycompleteanddiversified.
(5)SDHhastheperformanceoftransmissionandswitching.Thecompositionofitsseriesofequipmentcanbefreelycombinedthroughfunctionalblockstorealizenetworksofdifferentlevelsandvarioustopologies,whichisveryflexible.
(6)SDHisnotexclusivetoacertaintransmissionmedium,itcanbeusedfortwistedpair,coaxialcable,butSDHisusedtotransmithighdataratesandrequiresopticalfiber.ThisfeatureshowsthatSDHissuitableforuseasatrunkchannelaswellasabranchchannel.Forexample,mycountry'snationalandprovincialcabletelevisiontrunknetworksuseSDH,anditisalsocompatiblewithfiberopticcablehybridnetworks(HFC).
(7)FromthepointofviewoftheOSImodel,SDHbelongstothephysicallayerofthebottomlayer,anddoesnothavestrictrestrictionsonitsupperlayers.ItisconvenienttoadoptvariousnetworktechnologiesonSDHandsupportATMorIPtransmission.
(8)SDH е строго синхронизиран, което гарантира, че цялата мрежа е стабилна и надеждна, с малко грешки и лесна за повторна употреба и настройка.
(9)Thestandardopenopticalinterfacecanachievehorizontalcompatibilityonthebasicopticalcablesection,reducingnetworkingcosts.
Disadvantages
(1)Effectivenessandreliabilityareacontradiction.Increasingeffectivenesswilldefinitelyreducereliability,andincreasingreliabilitywillalsoreduceeffectivenessaccordingly.AgreatadvantageofSDHisthatthereliabilityofthesystemisgreatlyenhanced(thedegreeofautomationofoperationandmaintenanceishigh),whichisduetotheadditionofalargenumberofoverheadwordsforOAMfunctionsintheSDHsignal-STM-NframeSection,thiswillinevitablyleadtothetransmissionofthesameamountofeffectiveinformation,onlywhenthePDHsignalis140Mbit/smultiplexedintotheSTM-1signalframe,theSTM-1signalcanaccommodate63×2Mbit/sTheamountofinformation,butatthistimeitssignalrateis155Mbit/s,whichishigherthantheE4signal(140Mbit/s)withthesameinformationcapacityofPDH,whichmeansthatthetransmissionfrequencybandoccupiedbySTM-1isgreaterthanthetransmissionofPDHE4signalfrequencyband.
(2)Механизмът за регулиране на стрелката е сложен
TheSDHsystemcandirectlydownloadlow-speedsignalsfromhigh-speedsignals,eliminatingtheneedformulti-stagemultiplexing/demultiplexing.Therealizationofthisfunctionisaccomplishedthroughthepointermechanism.Theroleofthepointeristoindicatethepositionofthelow-speedsignalatalltimes,sothattherequiredlow-speedsignalcanbecorrectlysplitduring"unpacking",whichensuresthattheSDHisseparatedfromthehigh-speedsignal.Realizationofthefunctionofdirectlydownloadinglow-speedsignalsinthemiddle.ItcanbesaidthatpointersareamajorfeatureofSDH.Buttherealizationofthepointerfunctionincreasesthecomplexityofthesystem.ThemostimportantthingistomakethesystemproduceauniquejitterofSDH-combinedjittercausedbypointeradjustment.Thiskindofjittermostlyoccursattheboundaryofthenetwork,anditslowfrequencyandlargeamplitudewillcausetheperformanceoflow-speedsignalstodeteriorateafterbeingremoved,anditwillbequitedifficulttofilterthiskindofjitter.
(3) Въздействието на широкото използване на софтуер върху сигурността на системата
OneofthemajorcharacteristicsofSDHisthehighdegreeofautomationofOAM,whichalsomeansthatthesoftwareoccupiesaconsiderableamountoftimeinthesystem.Theproportion,whichmakesthesystemvulnerabletocomputerviruses,especiallyintoday'subiquitouscomputerviruses.Inaddition,humanerroroperationsandsoftwarefailuresatthenetworklayerhaveafatalimpactonthesystem.Inthisway,thesecurityofthesystemhasbecomeaveryimportantaspect.Therefore,themaintenancepersonneloftheequipmentmustbefamiliarwiththesoftwareandchooseareliablenetworktopology.
Application
DuetothemanycharacteristicsofSDHmentionedabove,ithasmadetremendousdevelopmentinthefieldofwideareanetworkandprivatenetwork.TelecomoperatorssuchasChinaMobile,ChinaTelecom,ChinaUnicom,andRadioandTelevisionhaveallbuiltSDH-basedbackboneopticaltransmissionnetworksonalargescale.Utilizelarge-capacitySDHlooptocarryIPbusiness,ATMbusinessorleasethecircuitdirectlytoenterprisesandinstitutions.Andsomelarge-scaleprivatenetworkshavealsoadoptedSDHtechnologytosetupSDHopticalloopswithinthesystemtocarryvariousservices.Forexample,thepowersystemusesSDHloopstocarryinternaldata,remotecontrol,video,voiceandotherservices.
FortheunitsthataremoreurgentfornetworkingandhavenopossibilitytosetupdedicatedSDHloops,manyofthemhaveadoptedthemethodofleasedcircuitsfromtelecomoperators.BecauseSDHisbasedonthecharacteristicsofthephysicallayer,unitscancarryvariousservicesonleasedcircuitswithoutbeingrestrictedbytransmission.Therearemanybearermodes,whichcanbetheuseofintegratedmultiplexingequipmentbasedonTDMtechnologytoachievemultipleservicemultiplexing,ortheuseofIP-basedequipmenttoimplementmulti-servicepacketswitching.SDHtechnologycantrulyguaranteethebandwidthofleasedcircuits,andisbetterthanVPNsintermsofsecurity.SDHleasedlineshavebeenwidelyusedingovernmentagenciesandcompaniesthatattachgreatimportancetosafety.Generallyspeaking,SDHcanprovideE1,E3,STM-1orSTM-N(N=4,16,64)andotherinterfaces,whichcanfullymeetvariousbandwidthrequirements.Atthesametime,intermsofprice,ithasalsobeenacceptedbymostunits.
Developmenttrend
Asanewgenerationofidealtransmissionsystem,SDHhastheabilityofautomaticroutingselection,convenientupperandlowercircuits,strongmaintenance,control,andmanagementfunctions,unifiedstandards,andhighertransmission.Theadvantagesofhigh-speedservices,etc.,canwellmeettheneedsoftherapiddevelopmentofcommunicationnetworks.Sofar,SDHhasobtainedunprecedentedapplicationanddevelopment.Intermsofstandardization,aseriesofrecommendationsthathavebeenestablishedandwillbeestablishedhavebasicallycoveredallaspectsofSDH.Ithasbeguntobewidelyusedintrunknetworks,long-distancenetworks,relaynetworks,andaccessnetworks.Inaddition,researchandapplicationarealsoactivelycarriedoutinopticalfibercommunication,microwavecommunication,andsatellitecommunication.
Inrecentyears,on-demandTV,multimediaservices,andotherbroadbandserviceshavesprungup,providingabroadspaceforSDHapplicationsinaccessnetworks.TheadvantagesofSDHtechnologyappliedtotheaccessnetworkare:1)Forlarge-scaleenterpriseusersthatrequirehighreliabilityandhigh-qualityservices,SDHcanprovideidealnetworkperformanceandservicereliability.2)Thescopeofnetworkmanagementcanbeextendedtotheuserside,simplifyingmaintenancework.3)UtilizingtheinherentflexibilityofSDHenablesnetworkoperatorstoprovideuserswithlong-termandshort-termbusinessneedsfasterandmoreeffectively.
Fromatechnicalpointofview,therelativebandwidthrequirementoftheaccesslayerisrelativelysmall,anditneedstoprovideEthernet,TDM,andpossiblyATMandotherintegratedservicetransmission.BasedontheSDHsystemandcapableofprovidingEthernetandATMtransmissionandprocessingsystems(includingTDM,EthernetandATMinterfaces,andevenEthernetandATMswitchingmodules)willbethemainmethodtosolvetheaccesslayertransmission.ThismethodcanbecheapProvidehigh-qualityprivateline,ATM,Ethernetandotherservicesaccess,transmissionandprotectiononaserviceprovisionpoint(POP).
Asthebackbonetransmissioncapacitycontinuestoincrease,theaccesscapabilitiesofthemetrotransmissionnetworkarealsodiversified.However,IP-basednetworkservicesarestillunpredictable,whichrequiresthetransmissionnetworktohavebetteradaptivecapabilities,andthisadaptivecapabilityisnotonlytheadaptabilityofthenetworkinterfaceornetworkcapacity,butalsorequirestheautonomyofnetworkconnections.adaptability.Ingeneral,low-cost,flexibleandfastcompletionofserviceaccessandserviceconvergencefromtheoperator'sendofficetotheuserendarethemainrequirementsforthefuturemetropolitanareanetworkaccesssystem.
Simplyspeaking,thiswayofusingSDHtotransmitEthernetandotherservicesistomaptheservicesofdifferentnetworklevelstoeachtimeslotoftheSDHcircuitthroughtheVCcascademethod,andtheSDHThenetworkprovidesacompletelytransparenttransmissionchannel,whichisanintegratedwholefromtheperspectiveofphysicallayerequipment.Thissolutioncangreatlyreducethescaleofinvestment,reduceequipmentfootprint,reducepowerconsumption,andtherebyreducetheoperatingcostsofnetworkoperators.Atthesametime,theabilitytoprovidemultipleservicescanalsoenablenetworkoperatorstoquicklydeploynetworkservices,increaseservicerevenue,andenhancemarketcompetitiveness.
Tosumup,SDHhasbecomethemainstreamoftransmissionnetworkdevelopmentwithitsobviousadvantages.ThecombinationofSDHtechnologyandsomeadvancedtechnologies,suchasopticalwavelengthdivisionmultiplexing(WDM),ATMtechnology,Internettechnology(PacketoverSDH),etc.,makeSDHnetworksmoreandmoreimportant.SDHhasbeenincludedintheapplicationprojectsofhigh-speedcommunicationnetworksinthe21stcenturybyvariouscountries.Itisarecognizeddevelopmentdirectionofdigitaltransmissionnetworksinthetelecommunicationsindustryandhasgreatcommercialprospects.