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.
Základní principy
Úroveň informační struktury přijatá SDH se nazývá modul synchronního transportuSTM-N(SynchronousTransportMode,N=1,4,16,64), nejzákladnější modulForSTM-1,čtyřiSTM-1synchronnímultiplexníformaSTM-16,plexníSTM-1four1form a čtyři STM-16synchronní multiplexní formaSTM- 64.Dokonce čtyři STM-64synchronní multiplexování tvoří STM-256;SD používá strukturu blokového rámce pro přenášení informací, každý snímek je složen z 9 řad svislých a 270 × N sloupců bajtů v horizontále, každý bajt obsahuje 8 bitů a celýdivý rám je přiložen jako jeden rám ,Oblast STM-Nužitečné zatíženíaSprávaUnitPointer(AUPTR)oblast.Oblast nad hlavou sekce se používá hlavně pro síťový provoz, správu, údržbu a přiřazování, aby bylo zajištěno, že informace lze přenášet normálně a flexibilně a je rozdělena na RegenerátorSekce nad hlavou (RSOH) a vícenásobný (MSpaSectionSectionOs) ve skutečnosti se používá pro informační služby.Přední bajt kanálu používaný pro údržbu a správu kanálů ;ukazatel řídící jednotky se používá k označení přesné polohy prvního bajtu informací v oblasti užitečného zatížení v rámech STM-N, aby bylo možné při příjmu správně oddělit užitečné zatížení. Přenos SDH rámem je uspořádán v sériových kódových tocích zleva doprava a 0 0 x 1 x 1 až 1 každý fram. mes se přenáší za sekundu. Pro STM-1 je každéČíslo v rámci bitů 8 bitů×(9×270×1)=19440 bitů, pak přenosová rychlost STM-1 je 19440×8000=155,520 Mbit/s;a přenosová rychlost STM-4 je 4×155. =622,080 Mbit/s; Přenosová rychlost STM-16 je 16 × 155,520 (nebo 4 × 622,080) = 2488,320 Mbit/s.
WhenSDHtransmitsservicesignals,theframesofvariousservicesignalstoenterSDHmustgothroughthethreestepsofmapping,positioningandmultiplexingasfollows:
(1)MappingistocombinevariousThesignaloftherateisfirstloadedintothecorrespondingstandardcontainer(C)afterthecoderateisadjusted,andthenthechanneloverhead(POH)isaddedtoformthevirtualcontainer(VC).Thedeviationoftheframephaseiscalledtheframeoffset.
(2)Polohování je proces přijímání informací o posunutí rámu do podřízené jednotky (TU) nebo do jednotky řízení (AU). K dosažení toho projde funkcí ukazatele podřízené jednotky (TUPTR) nebo ukazatele řídící jednotky (AUPTR).
(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.
Za účelem podpory přenosu různých služeb SDH určila, že nízkorychlostní multiplexování dosáhne vysokých rychlostí a vysokorychlostní multiplexování dosáhne vyšších rychlostí, aby bylo možné získat různé komunikační rychlosti. První úroveň signálu synchronního přenosu určená SDH je STS-1 (Space Transportation System), rychlost STS-1 je 51,840 Mb/sa STS-1 je získána multiplexováním vícenásobným nízkorychlostním signálem T1, T2 a T3. ,T2=6,312Mbps,T3=44,736Mbps.Tříkanálový STS-1 je multiplexován a odesílán do základního synchronního přenosového modulu STM1pro přenos.Rychlost STM1 je 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 je přísně synchronizováno, což zajišťuje, že celá síť je stabilní a spolehlivá, s malým počtem chyb a snadno se používá a upravuje.
(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)Mechanismus úpravy bodu je složitý
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)Vliv rozsáhlého používání softwaru na zabezpečení systému
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.