《電子與通信工程專(zhuān)業(yè)英語(yǔ)》課件第4章

Unit4ApproachestoOpticalInternetPacketSwitchingNEWWORDSANDPHRASES

NOTES

EXERCISES

參考譯文

EXTENSIVETEXT

1．Introduction

Theconvergenceoftelecommunicationsanddatacommunicationshascausedaparadigmshiftinthenetworkingenvironment.ThemassiveexplosionintrafficgeneratedbytheInternethasdriventhecurrenttrend,withtheInternetProtocol(IP)becomingthedominantprotocolfordatacommunicationsaswellasrepresenting,inthelongerterm,averystrongcandidatefortheconvergenceofdatacommunicationswithtelecommunications.Intandem,thedevelopmentofwavelength-divisionmultiplexed(WDM)techniquesonpoint-to-pointlinkshavebeguntoutilizethemassiveopticalbandwidthofinstalledsingle-modeopticalfibersmoreefficiently.WDMwasinitiallyarapidsolutiontothesevererouteexhaustionproblemsbroughtaboutbyexponentiallyincreasingtraffic.Thefuturedeploymentofopticalcross-connectedWDMtransportnetworks,initiallyforprotectionandbypassbutultimatelymanagingopticallightpathsdynamicallyinmultipleringormesharchitectures,willpotentiallymodifytheroleofthenetworkfunctionalitiesprovidedbythesynchronousopticalnetwork/synchronousdigitalhierarchy(SONET/SDH)layer[1].Thus,IPoverWDMhasbecomeaveryimportantareaofstudy,encompassingawiderangeofsolutionstosupportingpredominantlyIPtrafficoverWDMopticalpaths.MuchresearcheffortfocusesondevelopinganelegantsolutiontothemismatchbetweenthetransmissioncapacitiesofferedbytheWDMopticallayerandtheprocessingpowerofrouters.IProutersperformfourmaintasks:

(1)

Routing:providingnetworkconnectivityinformationthroughroutingtables.

(2)

Forwarding:definingtheoutputofeachincomingpacket(basedontheroutingtables).

(3)

Switching:directingeachpackettotheproperoutput(definedbytheforwardingprocess).

(4)

Buffering:resolvingcontentionbystoringpacketswhenmorethanonewishestogotothesameoutputatonceduetotheunschedulednatureoftheirarrival.

Currently,theforwardingprocessimpliesmajorthroughputlimitations,withthesizeoftheroutingtablesandfrequencyoftheirupdatesbeingmajorissues.Suchproblemsarecurrentlyaddressedandmanaged,butthetimeneededfortablelookupsetsafundamentallimitonrouterthroughput.Muchworkhasconcentratedonthedevelopmentofdatastructuresandalgorithmsforminimizingthelookuptimegivenroutingtableandmemoryspaceconstraints.

Thereisnodoubtthatmassivestrideshavebeenachievedinhigh-throughputrouterdesigns(>1

Tb/s).Nevertheless,despitetheseimpressiveadvances,thereisstillafearthatelectronicswitchingsystemsexhibitlimitedupgradeflexibility.GiventhatWDMallowscheapandeasyincrementalincreasesofthetransmissionbandwidth,frequentupgradesofthetransport-layertransmissioncapacitycanbeenvisagedtomatchincreasingdemand,inturnplacingheavydemandsontheswitchingprocess[2].

2．Opticalpacketswitching

Anumberofotherapproachestoobviateoramelioratetheforwardingbottleneckarebeingresearched,utilizingopticalpacketorbursttechniques,perhapswithelectronicbuffering,toimplementIPoverWDM.However,thestrategydetailedhereimplementscontentionresolution(buffering)directlyintheopticaldomaintoyieldWDMopticalpacketswitching.Theobjectiveistoshiftthebulkoftheswitchingburdenintotheopticaldomain,permittingcompatiblescalingoftheswitchingcapabilitywithWDMtransmissioncapacity.Thusfar,thisstrategyhasassumedahybridsolution,achievingdecouplingbetweenthethroughputandtherouting/forwardingprocesses.Transmissionandswitchingareexecutedintheopticaldomain,whileroutingandforwardingarecarriedoutelectronically,wheretherelativelycomplexpacketheaderprocessingoccursindependentoftheopticalpayload[3].ThisdecouplingeffectivelypermitstheopticalpacketlayertosupportarangeofnetworkprotocolswhileharnessingthepowerofWDMtransmission.However,itmustbenotedthatthisisalsochanging,withtherecentdemonstrationofrudimentaryheaderprocessingfunctionsdirectlyintheopticaldomain.Theserelievesomeoftheburdenplacedonelectronicprocessing,therebyreducingcontrolsignalsetuptimeandmanaginglatencymoreeffectively.

Withanextensiveopticalpacketlayer,theinterfacetoIPandotherprotocolsiscrucial.Encapsulation,theadditionofdeliveryinformationtothedatabytheopticalpacketlayer,willoccuratinterworkingunits(IWUs)ateachinterfacetotheelectronicclientlayer.EncapsulationpermitsarangeofprotocolssuchasIPandasynchronoustransfermode(ATM)tobemappedintotheopticalpayloads,whichmaybeofeitherfixedorvariableduration.IPhidesthecomplexityofthephysicallayer(includingopticalpacketswitching),providingaunifiedinterfacetohigherlayers,regardlessoftheunderlyingnetworktype.Inadditiontoencapsulation,theIWUscreateheadersforproperroutingwithintheopticalpackerlayer,andmultiplextrafficfromdifferentinputlinksforonwardtransmissioninopticalpacketsforthesamedestination,ensuringanentirelyopticalend-to-endconnectionpath.Opticalpacketsprovideafurthermultiplexingtier,allowingtheaggregationoftrafficflowspriortotransmissionovertheopticallayer,andalsopotentiallyobviatingtheneedforSDHasanadaptationlayerforIPtrafficonWDMlinks.Opticalpacketnetworkthereforeoffersapotentialsolutiontoprovidingbothconnectionlessandconnection-orientednetworkingcapacities,flexibleintermsofbandwidthmanagementandfuture-proofwithregardtobandwidthgrowth.NostandardsexistyetformappingprotocolssuchasIPandATMintotheopticalpacketlayer.

Ashintedabove,therearetwoprincipalapproachestoopticalpacketswitching,bothwithapplicationstotheInternet:

(1)

Employingfixed-lengthopticalpackets,withmanycorrespondingtooneIPdatagram,requiringIWUstofragmentandreassemblethepacketseitherattheedgesofthelayerorontheinputsandoutputsoftheswitch.

(2)

Employingavariable-lengthopticalpacketforeachIPdatagram.

Mostreportedresearchtodatausesfixed-durationopticalpackets.Hence,forthepurposesofthisarticle(inordertoillustratethefunctionalityofanopticalpacket-switchedlayer),thebulkofthesubsequentmaterialwillbeconfinedtofixed-lengthpacketswhereboththeheaderandpayloadareencodedonthesamewavelength.Itisassumedthatthedestinationswitchoutputforeachpacketisderivedfromtheheaderafteropto-electronicconversion;theheadermaythusbeatalowerbitratetoallowitselectronicmanipulation.Duetothenatureofopticalbuffering,thepayloaddurationisfixed,whateveritscontent;thenetworkthroughputisproportionaltopayloadbitratewhichmayvaryfrom10Gb/sandup,witheasyupgradecapability.Aswillalsobecomeapparent,thewavelengthdimensioniscrucialnotonlyfortransmissioncapacitybutalsoinexecutingpracticalcontentionresolution.Thearticlewillendwithsomeforward-lookingconceptsaddressingtherequirementstoswitchandbuffervariable-lengthopticalpackets.

3．Thedesignofopticalpacketswitches

Agenericopticalpacket-switchednodestructureconsistsofthreesubblocks(Fig4.1):

(1)

Aninputinterfaceconsistingofan(optical)synchronizerwhichalignsincomingpacketsinrealtimeagainstaclock.

(2)

Aswitchingcorewhichroutesthepacketstotheirproperoutputsandexecutescontentionresolution.

(3)

Anoutputinterfacewhichinsertsanewheaderandmayhavetoregeneratethedata.

Fig4.1Agenericopticalpacket-switchednodestructure

Packetformatisafundamentalconsiderationinanypackettransmissionsystem,definedbytherequirementsofthelayerwithrespecttonetworkfunctionalities,andisalsocrucialforopticaldomainimplementations[4].Forexample,considerthepacketformatdefinedbytheKEOPSproject,uponconsiderationofthedelay-throughputperformanceoftheopticalpacket-switchedlayerunderdifferenttrafficflowsaswellasconsiderationofthenodeandnetworkroutingrequirements(Fig4.2).Throughoutitisassumedthattimeisdividedintoequaltimeslots,eachcontainingoneopticalpacket,andthepayloadmaycontaindatafrom622Mb/sto10Gb/s.

Fig4.2Anexampleofanopticalpacketformat(asdefinedbyKEOPS)

Insummary,theheaderfieldis14bytes:8forroutinginformation;3foridentificationofpayloadtype;flowcontrolinformation,packetnumberingforsequenceintegritypreservation,andheadererrorchecking.Theformatalsoindirectlyindicatestheneedforopticalsynchronizersastheinputinterfacetothenode.Itisassumedthatwhenpacketsentertheswitch,theirboundariesarealignedsothateachpacketisalignedwithitstimeslot.Suchsynchronizationisgenerallyarequirementforcorrectswitchoperation,achievinglowpacketloss;approachestoachievingpacketsynchronismattheinputstotheswitchesconstituteaseparatetopicandwillnotbediscussedfurther.Thus,2bytesarerequiredtoaidinthesynchronizationprocessas“tag”uniquelyidentifyingthestartofthepayload.Guardbandsaccountforswitchingtimesoftheconstituentopto-electronicdevicesaswellaspayloadpositionjitter.Thepayloaddurationresultsfromatrade-offbetweentransmissionefficiency(thelongerthepayloadduration,thehighertheefficiency)andthepracticallimitationsonoveralllengthoftheopticalfiberdelaylinebuffers.Atthistime,delaylinesaretheonlyviableapproachtoimplementingcontentionresolutiondirectlyintheopticaldomain.

Anopticalpacketswitchhasthreeprincipalfunctions:switching,buffering,and,optionally,headertranslation.InIP-orientedsystems,thelatterfunctionmaybereplacedbyroutingandforwarding,whichwerediscussedearlier.Switchingensuresthateachpacketemergesatthecorrectoutput,dependingontheinformationcontainedinthepacketheader.Althoughfixed-lengthpacketsarrivingontheinputsmustbesynchronized,thereisnocoordinationbetweenpacketstreamsarrivingondifferentinputs.Henceoneormorepacketsmayarriveduringthesametimeslotondifferentinputswishingtogotothesameoutput.Forthisreason,bufferingisrequired,whereoneormorepacketsarestoredwhileothersaretransmittedtothedesiredoutput.

HeadertranslationrepresentsacentralprocessintheATMtransmissionsystemsstrategy,while,asdiscussedpreciously,routingandforwardingareequallyimportantforIPsystems.Althoughtheseschemesoffergreatfunctionalityandflexibility,theyarenotusedineveryopticalpacket-switchingsystemsincedirectopticalheadertranslationisintheearlystagesofdevelopment.Thus,theheaderisusuallyatalowerbitratethanthepayloadtofacilitateelectronicdecodingandinterpretationofheaderinformation.

NEWWORDSANDPHRASES

convergence n. 會(huì)合，會(huì)聚

exhaustion n. 耗竭；耗盡；耗損；衰竭

exponentially adv. 指數(shù)地

mesh n. 網(wǎng)狀，網(wǎng)眼

elegant adj. 優(yōu)良的，雅致的；簡(jiǎn)潔的

stride n. 進(jìn)展，進(jìn)步；步幅

envisaged vt. 正視

obviatevt. 消除，排除(危險(xiǎn)、障礙等)，回避，預(yù)防

ameliorate vt. 改善，改進(jìn)

bottleneck n. 瓶頸

contention n. 爭(zhēng)奪，爭(zhēng)論

hybrid adj. 雜種的，混合的

decouple vt. 解耦，去耦合

harness vt. 治理，利用

rudimentary adj. 根本的，低級(jí)的

latency n. 等待時(shí)間；執(zhí)行時(shí)間

unified adj. 同一的，統(tǒng)一的

future-proof 經(jīng)得起未來(lái)考驗(yàn)的

manipulation n. 操作，處理

withrespectto 關(guān)于，至于

NOTES

[1]Thefuturedeploymentofopticalcross-connectedWDMtransportnetworks,initiallyforprotectionandbypassbutultimatelymanagingopticallightpathsdynamicallyinmultipleringormesharchitectures,willpotentiallymodifytheroleofthenetworkfunctionalitiesprovidedbythesynchronousopticalnetwork/synchronousdigitalhierarchy(SONET/SDH)layer.“initiallyforprotectionandbypassbutultimatelymanagingopticallightpathsdynamicallyinmultipleringormesharchitectures,”是原因狀語(yǔ)，用來(lái)解釋前面網(wǎng)絡(luò)使用的目的?！皃rovidedbythesynchronousopticalnetwork/synchronousdigitalhierarchy(SONET/SDH)layer.”修飾“networkfunctionalities”，它是省略了whichare的后置定語(yǔ)從句。

本句可譯為：光交叉互聯(lián)WDM傳輸網(wǎng)絡(luò)最初是為了保護(hù)和旁路多環(huán)網(wǎng)或網(wǎng)狀網(wǎng)，但最后(演變?yōu)?動(dòng)態(tài)管理光路，它可能改變網(wǎng)絡(luò)功能的角色，這些網(wǎng)絡(luò)功能是由同步網(wǎng)絡(luò)/同步數(shù)字體系層提供的。

[2]GiventhatWDMallowscheapandeasyincrementalincreasesofthetransmissionbandwidth,frequentupgradesofthetransport-layertransmissioncapacitycanbeenvisagedtomatchincreasingdemand,inturnplacingheavydemandsontheswitchingprocess.

“Giventhat…”意為“假設(shè)……則……”，表示某種假設(shè)情況，后面是前面假設(shè)條件成立后的結(jié)果?！癕atch”意為“匹配”?！癲emandson”意為“對(duì)……的要求”。

本句可譯為：假設(shè)WDM允許傳輸帶寬輕易、頻繁地不斷增長(zhǎng)，傳輸層傳輸容量的頻繁升級(jí)則可看做是匹配日益增長(zhǎng)的容量需求，進(jìn)而就會(huì)對(duì)交換過(guò)程施加繁重的需求壓力。

[3]Transmissionandswitchingareexecutedintheopticaldomain,whileroutingandforwardingarecarriedoutelectronically,wheretherelativelycomplexpacketheaderprocessingoccursindependentoftheopticalpayload.“while”引導(dǎo)并列從句，和前面的“Transmissionandswitchingareexecutedintheopticaldomain,”是并列從句；“where”引導(dǎo)定語(yǔ)從句，修飾“electronically”。

本句可譯為：傳輸和交換在光域?qū)崿F(xiàn)，而路由和轉(zhuǎn)發(fā)在電域?qū)崿F(xiàn)，相對(duì)復(fù)雜的分組頭獨(dú)立于光載荷在電域進(jìn)行處理。

[4]Packetformatisafundamentalconsiderationinanypackettransmissionsystem,definedbytherequirementsofthelayerwithrespecttonetworkfunctionalities,andisalsocrucialforopticaldomainimplementations.“providingtheleastsignalattenuationandthehighesttransmissionspeedsofanyfibercabletype.”為分詞短語(yǔ)作結(jié)果狀語(yǔ)，可以理解為whichcanprovidetheleastsignalattenuationandthehighesttransmissionspeedsofanyfibercabletype的縮寫(xiě)。

本句可譯為：分組的格式在任何分組傳輸系統(tǒng)中都是一個(gè)基本的、需要考慮的事項(xiàng)，對(duì)于光域的應(yīng)用也是至關(guān)重要的。分組的格式是由某層根據(jù)網(wǎng)絡(luò)功能的需求決定的。

EXERCISES

I．Translatethefollowingwordsorphases.

IP WDM IWU

SONET/SDH hybridsolution opticalpayload

光分組交換 光電轉(zhuǎn)換 光纖延遲線(xiàn)

傳輸容量

II．TranslatethefollowingparagraphsintoChinese.

(1)

Withanextensiveopticalpacketlayer,theinterfacetoIPandotherprotocolsiscrucial.Encapsulation,theadditionofdeliveryinformationtothedatabytheopticalpacketlayer,willoccuratinterworkingunits(IWUs)ateachinterfacetotheelectronicclientlayer.

(2)

Intandem,thedevelopmentofwavelength-divisionmultiplexed(WDM)techniquesonpoint-to-pointlinkshavebeguntoutilizethemassiveopticalbandwidthofinstalledsingle-modeopticalfibersmoreefficiently.WDMwasinitiallyarapidsolutiontothesevererouteexhaustionproblemsbroughtaboutbyexponentiallyincreasingtraffic.

(3)

Thus,2bytesarerequiredtoaidinthesynchronizationprocessas“tag”uniquelyidentifyingthestartofthepayload.Guardbandsaccountforswitchingtimesoftheconstituentopto-electronicdevicesaswellaspayloadpositionjitter.Thepayloaddurationresultsfromatrade-offbetweentransmissionefficiency(thelongerthepayloadduration,thehighertheefficiency)andthepracticallimitationsonoveralllengthoftheopticalfiberdelaylinebuffers.Atthistime,delaylinesaretheonlyviableapproachtoimplementingcontentionresolutiondirectlyintheopticaldomain.

(4)

Switchingensuresthateachpacketemergesatthecorrectoutput,dependingontheinformationcontainedinthepacketheader.Althoughfixed-lengthpacketsarrivingontheinputsmustbesynchronized,thereisnocoordinationbetweenpacketstreamsarrivingondifferentinputs.Henceoneormorepacketsmayarriveduringthesametimeslotondifferentinputswishingtogotothesameoutput.Forthisreason,bufferingisrequired,whereoneormorepacketsarestoredwhileothersaretransmittedtothedesiredoutput.

參考譯文

第四單元光互聯(lián)網(wǎng)分組交換方法

1．簡(jiǎn)介

無(wú)線(xiàn)電通信和數(shù)據(jù)通信的融合導(dǎo)致了網(wǎng)絡(luò)環(huán)境的模式變遷，Internet產(chǎn)生的大量業(yè)務(wù)推動(dòng)了這種趨勢(shì)。隨著IP協(xié)議正在成為數(shù)據(jù)通信的主要協(xié)議，在較長(zhǎng)的時(shí)期內(nèi)，IP協(xié)議也是代表無(wú)線(xiàn)通信和數(shù)據(jù)通信融合的強(qiáng)大的支持方。一前一后地，點(diǎn)對(duì)點(diǎn)鏈路的WDM(波分復(fù)用技術(shù))的發(fā)展已經(jīng)開(kāi)始利用已有的單模光纖的大帶寬使其利用率更高。最初，WDM為呈指數(shù)增長(zhǎng)的業(yè)務(wù)的嚴(yán)重路由耗盡問(wèn)題提供了快速解決的方法。光交叉互聯(lián)WDM傳輸網(wǎng)絡(luò)最初是為了保護(hù)和旁路多環(huán)網(wǎng)或網(wǎng)狀網(wǎng)，但最后(演變?yōu)?動(dòng)態(tài)管理光路，它可能改變網(wǎng)絡(luò)功能的角色，這些網(wǎng)絡(luò)功能是由同步網(wǎng)絡(luò)/同步數(shù)字體系層提供的。因此，IPoverWDM已經(jīng)成為一個(gè)重要的研究領(lǐng)域，它包括了支持主要的IP業(yè)務(wù)在WDM光鏈路上傳輸?shù)脑S多方法。許多研究工作集中在開(kāi)發(fā)一種優(yōu)良的解決辦法，這種辦法用來(lái)解決WDM光層的傳輸能力和路由器的處理能力之間的不匹配問(wèn)題。IP路由器完成四個(gè)主要的功能：

(1)路由：通過(guò)路由表提供網(wǎng)絡(luò)連接信息。

(2)轉(zhuǎn)發(fā)：定義每個(gè)輸入分組的輸出端口(基于路由表)。

(3)交換：把每個(gè)分組輸出到正確的輸出端口(由轉(zhuǎn)發(fā)過(guò)程定義)。

(4)緩存：由于分組到來(lái)的隨機(jī)性，當(dāng)多個(gè)分組想要同時(shí)通過(guò)同一出口時(shí)，可通過(guò)存儲(chǔ)分組解決沖突。

目前，隨著路由表尺寸的變大和更新頻率問(wèn)題，轉(zhuǎn)發(fā)過(guò)程成為了限制吞吐量的主要問(wèn)題。這些問(wèn)題目前已得到解決，但是路由表的吞吐量(依然)基本受限于路由表查詢(xún)時(shí)間。大量的工作集中在開(kāi)發(fā)數(shù)據(jù)結(jié)構(gòu)和算法，這些數(shù)據(jù)結(jié)構(gòu)和算法(的作用)是為了最小化路由表的查詢(xún)時(shí)間和存儲(chǔ)空間限制。

毫無(wú)疑問(wèn)，在大吞吐量路由器設(shè)計(jì)上已經(jīng)取得了巨大的進(jìn)展(大于1

Tb/s)。不過(guò)，盡管取得了這些驚人的進(jìn)步，但人們?nèi)匀粚?duì)電子交換系統(tǒng)有限的升級(jí)靈活性存有疑慮。假設(shè)WDM允許傳輸帶寬輕易、頻繁地不斷增長(zhǎng)，傳輸層傳輸容量的頻繁升級(jí)則可看做是匹配日益增長(zhǎng)的容量需求，進(jìn)而就會(huì)對(duì)交換過(guò)程施加繁重的需求壓力。

2．光分組交換

許多其他的解決或消除轉(zhuǎn)發(fā)瓶頸的方法正在進(jìn)行研究。利用光分組或突發(fā)技術(shù)，或者與電緩存一起來(lái)實(shí)現(xiàn)IPoverWDM。但是，這里講述的策略是直接在光域解決沖突(緩存)來(lái)讓步于WDM光分組交換。其目的是把交換負(fù)擔(dān)轉(zhuǎn)移到光域，允許交換能力和WDM傳輸容量之間進(jìn)行有比例的協(xié)調(diào)。因此這種策略可認(rèn)為是一種混合解決方法，它解除了吞吐量和路由/轉(zhuǎn)發(fā)過(guò)程之間的耦合關(guān)系。傳輸和交換在光域?qū)崿F(xiàn)，而路由和轉(zhuǎn)發(fā)在電域?qū)崿F(xiàn)，相對(duì)復(fù)雜的分組頭獨(dú)立于光載荷在電域進(jìn)行處理。這種去耦合方式使得光分組層能有效地支持很多網(wǎng)絡(luò)協(xié)議，同時(shí)也利用了WDM的傳輸能力。但是需要注意的是：(這種情形)隨著最近的一些演示情況在發(fā)生變化，這些演示中，有一些分組頭頭部處理功能可初步直接在光域進(jìn)行，這減輕了電處理的負(fù)擔(dān)，因而也有效地減少了控制信號(hào)的建鏈時(shí)間和管理時(shí)間。

有了一個(gè)擴(kuò)展的光分組層，則它與IP協(xié)議和其他協(xié)議的接口就是至關(guān)重要的。在每個(gè)互連單元(IWU)與電客戶(hù)層的接口處光分組層進(jìn)行傳輸信息的疊加，即封裝。封裝允許大量的協(xié)議數(shù)據(jù)比如IP和ATM數(shù)據(jù)映射到光載荷中，這些光載荷的持續(xù)時(shí)間可能是固定或可變的。IP隱藏了物理層的復(fù)雜性(包括光分組交換)，對(duì)高層提供統(tǒng)一的接口，而不管底層網(wǎng)絡(luò)是什么類(lèi)型的。除了封裝功能，互連單元(IWU)也為(光分組)在光分組層的正確路由生成頭部，同時(shí)為了轉(zhuǎn)發(fā)而復(fù)用來(lái)自不同輸入鏈路的、去往同一目的地址的分組業(yè)務(wù)，從而保證了一個(gè)全光的端到端的鏈路(的正確運(yùn)行)。光分組提供了一個(gè)更高的復(fù)用等級(jí)，它允許業(yè)務(wù)流在光層傳輸之前進(jìn)行聚合，潛在地消除了對(duì)SDH的需求(SDH是IP業(yè)務(wù)在WDM鏈路上傳輸時(shí)需要的適配層)。因此，光分組網(wǎng)絡(luò)提供了一種潛在的解決辦法：能傳輸無(wú)連接和面向連接業(yè)務(wù)，并且具有靈活的帶寬管理能力和經(jīng)得起未來(lái)考驗(yàn)的帶寬增長(zhǎng)能力。目前還沒(méi)有制定IP和ATM數(shù)據(jù)映射到光分組層的標(biāo)準(zhǔn)。

正如上面所提到的，有兩種主要的近似于光分組交換的方法，都可以應(yīng)用到Internet中：

(1)使用固定長(zhǎng)度的光分組，很多個(gè)這樣的分組對(duì)應(yīng)一個(gè)IP數(shù)據(jù)報(bào)，需要IWU在層間或交換機(jī)的輸入和輸出端進(jìn)行分段和重組。

(2)每個(gè)IP數(shù)據(jù)報(bào)使用一個(gè)不定長(zhǎng)度的光分組。

大部分已經(jīng)報(bào)道的研究都使用固定長(zhǎng)度的光分組。因此，為了說(shuō)明光分組層的功能，下面的資料都限于固定長(zhǎng)度分組，分組的頭部和載荷都使用同樣的波長(zhǎng)來(lái)編碼。假設(shè)每個(gè)分組的目的交換端口是光/電轉(zhuǎn)換后從頭部獲取的，那么，為了使得電操作(成為)可能，其頭部的速率應(yīng)當(dāng)較低。由于光緩存的天然特性，載荷的持續(xù)時(shí)間是固定的，不管它的內(nèi)容是什么，網(wǎng)絡(luò)的吞吐量和負(fù)載比特率是成比例的。負(fù)載比特率的范圍可從10Gb/s到更高，具有很大的上升空間。顯而易見(jiàn)，波長(zhǎng)的個(gè)數(shù)不但對(duì)傳輸容量是至關(guān)重要的，而且對(duì)在實(shí)際運(yùn)用時(shí)的沖突解決也是至關(guān)重要的。為了交換和緩存變長(zhǎng)光分組，本文將提出一些有前瞻性的概念來(lái)分析解決這些需求，并以這些概念結(jié)尾。

3．光分組交換機(jī)的設(shè)計(jì)

一個(gè)通用的光分組交換機(jī)的節(jié)點(diǎn)結(jié)構(gòu)包括三個(gè)子部分(見(jiàn)圖4.1)：

(1)輸入接口，包括一個(gè)用來(lái)對(duì)準(zhǔn)實(shí)時(shí)時(shí)鐘的輸入分組(光的)同步裝置。

(2)交換核心，用來(lái)把分組路由到它們正確的輸出端口，并執(zhí)行沖突解決。

(3)輸出接口，它插入新的分組頭，并且可再生數(shù)據(jù)。

圖4.1通用光分組交換機(jī)節(jié)點(diǎn)結(jié)構(gòu)分組的格式在任何分組傳輸系統(tǒng)中都是一個(gè)基本的、需要考慮的事項(xiàng)，對(duì)于光域的應(yīng)用也是至關(guān)重要的。分組的格式是由某層根據(jù)網(wǎng)絡(luò)功能的需求決定的。例如，KEOPS工程定義的分組格式既考慮了不同業(yè)務(wù)流情況下的光分組交換層的延遲-吞吐量性能，也考慮了節(jié)點(diǎn)和網(wǎng)絡(luò)路由的需要(見(jiàn)圖4.2)。自始至終它都假設(shè)時(shí)間被分成相等的時(shí)隙，每個(gè)時(shí)隙容納一個(gè)光分組，并且有效載荷可能包含從622Mb/s到10Gb/s速率的數(shù)據(jù)。

圖4.2光分組交換格式例子(KEOPS的定義)概要地說(shuō)，頭域有14個(gè)字節(jié)：8個(gè)字節(jié)是路由信息，3個(gè)字節(jié)是有效載荷的類(lèi)型標(biāo)識(shí)、流量控制信息、分組序號(hào)完整性保護(hù)編碼和頭部錯(cuò)誤校驗(yàn)。分組格式間接地表明了對(duì)光同步裝置作為節(jié)點(diǎn)輸入接口的需要。假設(shè)當(dāng)分組進(jìn)入交換機(jī)時(shí)，它們的邊界是對(duì)準(zhǔn)的，那么每個(gè)分組和它的時(shí)隙也是對(duì)準(zhǔn)的。一般來(lái)說(shuō)，這樣的同步過(guò)程對(duì)于正確的交換操作和低的分組丟失率是必需的。在輸入端到交換機(jī)之間同步的方法可以構(gòu)成一個(gè)單獨(dú)的命題，這里不作深入討論。因此，在同步過(guò)程中，使用2個(gè)字節(jié)作為“標(biāo)簽”特別標(biāo)識(shí)有效載荷的開(kāi)始來(lái)幫助達(dá)到同步。保護(hù)頻帶負(fù)責(zé)組成光電設(shè)備的交換時(shí)間以及有效載荷位置的抖動(dòng)。有效載荷持續(xù)時(shí)間源于傳輸效率(持續(xù)時(shí)間越長(zhǎng)，效率越高)和光纖延遲線(xiàn)緩存總長(zhǎng)的實(shí)際限制之間的折中平衡。此時(shí)，光纖延遲線(xiàn)是唯一能直接在光域解決沖突的可行辦法。

光分組交換有三個(gè)主要功能：交換、緩存和頭部翻譯(可選)。在基于IP的系統(tǒng)中，后面的功能(頭部翻譯)可能被路由和轉(zhuǎn)發(fā)所替換，這個(gè)問(wèn)題前面討論過(guò)了。交換依靠包含在分組頭中的信息來(lái)保證每個(gè)分組能能被傳送到正確的輸出端。盡管到達(dá)輸入端的定長(zhǎng)分組一定是同步過(guò)的，但是來(lái)自不同輸入端的分組流也達(dá)不到(完全)一致。因此，一個(gè)或多個(gè)分組可能在同一時(shí)隙到達(dá)不同輸入端，并去往相同的輸出端。由于這個(gè)原因，緩存是必需的，在這里一個(gè)或多個(gè)分組被存儲(chǔ)起來(lái)，而其他的分組被傳送到希望的輸出端口。

頭部翻譯功能在A(yíng)TM傳輸策略中是一個(gè)重要的過(guò)程，而路由和轉(zhuǎn)發(fā)功能在IP系統(tǒng)中也是同樣重要的，這點(diǎn)前面已討論過(guò)了。盡管這些機(jī)制提供了重要的功能和靈活性，但是并沒(méi)有體現(xiàn)在每個(gè)光分組交換系統(tǒng)中，因?yàn)橹苯拥墓夥纸M頭部翻譯功能還處在發(fā)展的初級(jí)階段，還有很長(zhǎng)的路要走。因此，為了方便電解碼和頭部信息的翻譯，頭部的比特率通常比有效載荷的比特率低。

EXTENSIVETEXT

OpticalBurstSwitchingfortheNextGenerationInternet

Currently,ourdemandsfornetworkbandwidthincreasedaily.WeaccesstheInternettocheckemail,readthenews,checkstockquotes,takeclassesonline,finddrivingdirections,playgamesonline,buyflowersandthislistgoesonandon.Inordertomeetourever-increasingnetworkbandwidthneeds,onesolutionistodesignandbuildtheNextGenerationInternetwithanopticalcoreinfrastructure,withfastconnectionprovisioningtimeandunprecedentedhighdataratesof100terabitspersecondandhigher.Anopticalnetworkisbuiltbyinterconnectingvariousopticalswitcheswithwavelength-divisionmultiplexing(WDM)fibers,i.e.,fibersthatcansimultaneouslytransmitdataoverdifferentwavelengths.Manyoftoday’scommercialopticalnetworks,however,donotutilizetheWDMtechnologyefficiently.Oneexampleisthepopularsynchronousopticalnetwork(SONET)rings,whichconsistofSONETadd/dropmultiplexers(ADM)linkedwithWDMfiberssoastoformaring.Inthiscase,WDMissimplyusedasatransportmedium.Thatis,thedataistransmittedoptically,buttheopticalsignalisterminatedateveryADM.ThesignalisthentranslatedtoelectronicsforprocessingandthentranslatedbacktoanopticalsignalforforwardingtothenextADM.TheproblemisthemismatchbetweentheelectronicprocessingspeedsandtheopticaltransmissionratesthatleadtoanundesirablebottleneckateachSONETnode.Toresolvethebottleneckproblem,anall-opticalWDMnetworkwherethedatatravelsfromthesourcetothedestinationentirelyintheopticaldomainhasbeenproposed.Withrespecttothecurrentstateofthetechnology,anOpticalBurstSwitched(OBS)Networkisoneofthemostpromisingall-opticalarchitecturesfortheNextGenerationInternet.Itefficientlysupportsthetransmissionofburstytrafficoveranall-opticalinfrastructure.OBSisstillbeingdevelopedandithasnotbeenstandardizedyet.ThisarticledescribesthemainfeaturesofanOBSnetwork,itsbenefitsaswellasitschallenges.

1．TheOBSbasics

AnOBSnetworkconsistsofcorenodesandend-devicesinterconnectedbyWDMfibersasshowninFig4.3.AnOBScorenodeconsistsofanopticalcrossconnect(OXC),anelectronicswitchcontrolunit,androutingandsignalingprocessors.AnOXCisanon-blockingswitchthatcanswitchanopticalsignalfromaninputporttoanoutputportwithoutconvertingthesignaltoelectronics.TheOBSenddevicesareelectronicInternetProtocol(IP)routers,asynchronoustransfermode(ATM)switches,orframerelayswitches,equippedwithanOBSinterface.EachOBSend-deviceisconnectedtoaningressOBScorenode.Theend-devicecollectstrafficfromvariouselectronicnetworks(suchasATM,IPandframerelay).ItsortsthetrafficperdestinationOBSend-deviceaddressandassemblesitintolargervariable-sizeunits,calledbursts.Foreachburst,theend-devicealsoconstructsacontrolpacket,whichcontainsinformationabouttheburst,suchastheburstlengthandtheburstdestinationaddress.Thiscontrolpacketisimmediatelysentalongtherouteoftheburstanditiselectronicallyprocessedateachnode.

Fig4.3TheOBSnetworkarchitecture

Thefunctionofthecontrolpacketistoinformthenodesoftheimpendingdataburstandtosetupanend-to-endopticalpathbetweenthesourceandthedestination.Afteradelaytime,knownastheoffset,theend-devicealsotransmitstheburstitself.Thebursttravelsasanopticalsignalovertheend-to-endopticalpathsetupbyitscontrolpacket.Thisopticalpathistorndownafterthebursttransmissioniscompleted.ThisseparationofthecontrolinformationandtheburstdataisoneofthemainadvantagesofOBS.Itfacilitatesefficientelectroniccontrolwhileitallowsforagreatflexibilityintheformatandtransmissionrateoftheuserdata.Thisisbecausetheburstsaretransmittedentirelyasanopticalsignal,whichremainstransparentthroughoutthenetwork.Ingeneral,thetimeittakesthecontrolpackettoreachthedestinationend-deviceisequaltotheend-to-endpropagationdelayplusthesumofalltheprocessingdelaysatalltheintermediatecorenodes.Ontheotherhand,thetimeittakesforabursttoreachthedestinationend-deviceisonlyequaltotheend-to-endpropagationdelay.ThereasonisthattheburstistransmittedasanopticalsignalthattraversestheOBSswitcheswithoutanyprocessingorbufferingdelays.Inviewofthis,thetransmissionofaburstisdelayedbyanoffset.ThiswayitalwaysarrivesatanOBSnode,afteritsswitchcontrolunithashadthechancetoprocessthecontrolpacketassociatedwiththeburstandconfiguretheopticalswitchfabric.Theoffset,therefore,isafunctionofthenumberofnodesthatthecontrolpackethastotraverseendtoend.

2．Burstaggregationalgorithm

Theburstaggregationalgorithmattheend-devicescangreatlyimpacttheoverallOBSnetworkoperationbecauseitsetstheburstcharacteristicsand,therefore,itshapestheburstarrivaltraffic.Thealgorithmhastoconsiderthefollowingparameters:apre-settimer,amaximumburstlengthandaminimumburstlength.Thetimerdetermineswhentheend-deviceistoassembleitscollectedtrafficintoanewburst.Themaximumandtheminimumburstlengthparametersshapethesizeofthebursts.Weneedtosetamaximumburstlengthsinceverylongburstsholdontotheresourcesofthenetworkforalongtime.Thus,theycausetheunfairlossofotherbursts.Ontheotherhand,theminimumburstlengthisalsonecessarybecauseveryshortburstsmaygiverisetotoomanycontrolpackets.ThissituationcanoverloadthecontrolunitoftheOBSnode.Theburstaggregationalgorithmmayusebit-paddingifthereisnotenoughdatatoassembleaminimumsizeburst.OnewaytoprovideclassesoftrafficinOBSistoimplementpriorityqueuesattheedgeofthenetworkduringtheburstaggregation.Basedontheclassofservice,theend-devicessorttheupperlayertrafficintodifferentqueues.Asaresult,eachend-devicewillhaveC*Npriorityqueues,whereCisthenumberofserviceclassesandNisthenumberofpossibledestinations.Anappropriateschedulingalgorithmguaranteesthatthesequeuesareservedaccordingtotheirpriority.

3．Signaling,routingandwavelengthallocation

Signalingisanimportantaspectinanynetwork.Itspecifieshowtheconnectionsareestablishedanditdetermineswhetherornottheresourcesareutilizedefficiently.InmostOBSvariants,thesignalingofconnectionsisaccomplishedusingaone-waysignalingscheme:theburstistransmittedafteranoffsetwithoutanyknowledgeofwhethertheopticalpathhasbeensuccessfullyestablishedend-to-end,asillustratedinFig4.4.

Fig4.4One-waysignaling

End-devicesAandBareconnectedviatwocoreOBSnodes.TheverticallinesinFig4.4representatimelinesoastoshowtheactionstakenbyeachnode.End-deviceAtransmitsacontrolpackettoitsingressOBSnode.Thecontrolpacketisprocessedattheingressnode.Iftheconnectioncanbeaccepted,itisforwardedtothenextnode.Thecontrolpacketisreceivedby