英文文献-科技类-原文及翻译-电子-电气-自动化-通信

1外文文献原文OnthedeploymentofVoIPinEthernetnetworks:methodologyandcasestudyAbstractDeployingIPtelephonyorvoiceoverIP(VoIP)isamajorandchallengingtaskfordatanetworkresearchersanddesigners.Thispaperoutlinesguidelinesandastep-by-stepmethodologyonhowVoIPcanbedeployedsuccessfully.Themethodologycanbeusedtoassessthesupportandreadinessofanexistingnetwork.PriortothepurchaseanddeploymentofVoIPequipment,themethodologypredictsthenumberofVoIPcallsthatcanbesustainedbyanexistingnetworkwhilesatisfyingQoSrequirementsofallnetworkservicesandleavingadequatecapacityforfuturegrowth.Asacasestudy,weapplythemethodologystepsonatypicalnetworkofasmallenterprise.Weutilizebothanalysisandsimulationtoinvestigatethroughputanddelaybounds.Ouranalysisisbasedonqueuingtheory,andOPNETisusedforsimulation.Resultsobtainedfromanalysisandsimulationareinlineandgiveaclosematch.Inaddition,thepaperdiscussesmanydesignandengineeringissues.TheseissuesincludecharacteristicsofVoIPtrafficandQoSrequirements,VoIPflowandcalldistribution,definingfuturegrowthcapacity,andmeasurementandimpactofbackgroundtraffic.原Ke紧yw影or权ds彩:格Ne据tw移or举k拨De怜si亡gn傍，蛇Ne吧tw第or涌k手Ma帐na搁ge捷me束nt铁，列Vo胆IP企，惜Pe荣rf拦or眯ma捧nc枝e矮Ev房al誓ua繁ti滨on已，陕An笔al才ys件is款，楼Si共mu逐la躬ti哈on互，匹OP对NE毁T

勉1搜I讽nt律ro根du鹊ct虫io肉nThesedaysamassivedeploymentofVoIPistakingplaceoverdatanetworks.MostofthesenetworksareEthernetbasedandrunningIPprotocol.Manynetworkmanagersarefindingitveryattractiveandcosteffectivetomergeandunifyvoiceanddatanetworksintoone.Itiseasiertorun,manage,andmaintain.However,onehastokeepinmindthatIPnetworksarebest-effortnetworksthatweredesignedfornon-realtimeapplications.Ontheotherhand,VoIPrequirestimelypacketdeliverywithlowlatency,jitter,packetloss,andsufficientbandwidth.Toachievethisgoal,anefficientdeploymentofVoIPmustensurethesereal-timetrafficrequirementscanbeguaranteedoverneworexistingIPnetworks.WhendeployinganewnetworkservicesuchasVoIPoverexistingnetwork,manynetworkarchitects,managers,planners,designers,andengineersarefacedwithcommonstrategic,andsometimeschallenging,questions.WhataretheQoSrequirementsforVoIP?HowwillthenewVoIP科lo袭ad诞i食mp课ac啄t膨th乞e陆Qo糊S集fo旨r乓cu铺rr椒en讨tl绕y掠ru牲nn票in跃g饱ne挑tw梯or话k粒se桃rv辨ic逐es捎胶an束d抄ap齿pl扒ic横at谢io婚ns庙?颈Wi我ll险m咽y喊ex锄is丢ti匪ng油n睁et烟wo营rk窑s辛up瞒po番rt络V涛oI位P役刊an鸦d杏sa给ti棵sf款y徐th披e欲st拒an钥da逮rd针iz项ed搁Q消oS烂r题eq印ui靠re教me铸nt文s?找I舌f昆so庙,蓬ho突w北冷ma持ny晓V广oI湾P洞ca嫁ll由s奏ca酱n塔th慨e使ne树tw歌or黑k勿su帆pp此or贷t满be窗fo仗re槐u库pg漠ra于di至ng畜抛pr悄em狐at舟ur犯el陵y法an骄y际pa栗rt聚o帆f始th喇e州ex腊is僻ti匙ng六n尚et跑wo逝rk斗h呜ar唱dw各ar醉e?谦任Th现es弦e挖ch尺al狱le材ng归in辱g帽qu东es怨ti免on唤s舰ha纤ve备l弄ed右t最o辣th专e轿de抵ve挥lo阶pm伟en茧t竟糕of慰s负om汗e绢co自mm鼻er赏ci努al畅t泉oo础ls动f隔or垒t厅es信ti孩ng暴t不he悲p材er捧fo转rm邪an施ce垃o县f抓密mu怜lt门im腊ed阴ia玻a岸pp兰li义ca脾ti垒on涝s爆in徐d舅at详a辣ne柄tw斩or愧ks觉.戏A租li众st饭o旗f衫th英e言妇av皇ai衔la恭bl雁e馒co里mm蜻er叛ci挽al导t剂oo盏ls堂t血ha地t张su脖pp递or丸t徐Vo枕IP元i公s禽li口st教ed悟i精n软揉[1留,2隐].谊F符or繁t推he霉m衔os迎t漠pa父rt删,蚂th庙es宴e诱to舅ol肠s它us舰e患tw匀o仪co趟mm榴on缸泄ap洞pr狱oa驰ch尖es糊i跑n户as破se丑ss发in翅g奏th苏e箩de毒pl辅oy汁me起nt利o贸f故Vo国IP庭i腥nt喝o誉th吸e尊声ex堡is棉ti妨ng屈n聚et罩wo延rk默.土On笑e炼ap蹦pr编oa也ch巧i葬s丙ba个se搁d宣on饮f缝ir馅st壳p摘er拐fo性rm剧in妄g吓桐ne拨tw月or赴k东me产as知ur盆em步en袭ts膨a赞nd丸t张he蚁n陕pr握ed揭ic安ti厉ng趟t钢he趴n置et袍wo解rk膛陶re馒ad舌in荷es夹s醋fo之r遭su山pp视or徐ti逼ng秃V编oI刃P.顺T听he详p谱re喂di桶ct煎io谦n映of攀t剩he而延ne鬼tw柜or鸦k正re想ad玻in折es叛s族is鞋b放as酿ed策o蠢n侨as肠se员ss绿in干g寿th讨e糟he筐al妻th毫o谜f单腰ne栋tw触or炮k浓el鉴em痛en榜ts采.著Th愚e昼se每co认nd扫a钟pp双ro趋ac娱h勇is鸟b逼as改ed知o阁n转上in螺je话ct悠in传g漏re怖al械V条oI递P把tr及af舟fi烈c佳in处to份e埋xi债st泛in轮g拒ne乎tw沿or洁k盾an弱d兆匆me伪as礼ur母in宣g睁th耐e防re己su扁lt低in看g惰de贼la邻y,抢j粮it套te龟r,轰a领nd孙l槽os泡s.峡未Ot勾he削r污th经an伴t视he艰c吧os尾t突as雷so吉ci啊at挂ed赛w册it评h加th燃e市co虎mm郑er推ci冷al裤t暑oo嗽ls兰,该剂no晚ne镜o否f吓th尘e泄co识mm驳er悟ci县al础t字oo山ls醋o保ff率er朋a访c荐om团pr烈eh悟en贫si盘ve运迅ap路pr哈oa渐ch颗f窄or井s挠uc鸡ce竿ss知fu幕l翼Vo狼IP兔d里ep像lo搜ym制en鲁t.骆毙I骄n绵pa哥rt笛ic桥ul圾ar蒸,眼观no矿ne外g达iv握es遇a觉ny阴p驾re特di宗ct惯io唤n急fo佣r幸th汇e董to捐ta千l灵nu蓄mb象er艇o垄f潮ca陷ll元s翠th齿at犹毒ca设n摧be川s脆up污po甩rt泳ed德b别y池th狼e甩ne拳tw缝or泰k求ta惠ki洁ng功i塘nt绘o斗ac债co技un薪t灶杜im米po续rt辉an肺t予de悬si双gn家a合nd梁e公ng摊in肌ee衰ri页ng幻f蔑ac左to盛rs讨.挨Th先es奏e筝fa抬ct治or衡s侮塌in目cl屠ud难e斤Vo返IP父f鸟lo武w兰an晨d量ca惯ll骨d悠is谈tr蚊ib幕ut虽io滑n,以f缝ut梯ur遵e烘gr拨ow修th劳蓄ca礼pa钩ci替ty信,筛pe吊rf鼠or诚ma烛nc第e涝th览re尽sh薯ol秤ds仁,斩im绍pa卡ct受o社f蝇Vo堡IP庆o蚁n奉干ex毅is各ti拌ng绩n歌et弯wo启rk沙s移er淹vi光ce污s绝an闪d膛ap姜pl预ic涌at奉io侦ns碧,朗an尘d嘴im禾pa宝ct混竹ba饲ck买gr胖ou阅nd于t患ra糟ff受ic多o饲n纱Vo椒IP振.错Th言is闭p拆ap闻er选a朴tt草em抹pt王s接to巩a环dd崖re踏ss怠信th先os花e灵im楼po舟rt翼an稳t残fa累ct斗or苏s宜an遍d救la奔yo玻ut掘a候c俯om袭pr映eh时en竞si斤ve叉骄me迅th贯od民ol唱og斑y投fo蛛r也a仪su海cc坟es兵sf皇ul塑d锤ep喊lo款ym而en危t竖of跟a坝ny扣m求ul专ti星me冰di易a度旷ap罩pl疯ic考at谁io像n锋su伴ch冒a宇s炒Vo鼠IP偷a摘nd牵v展id圈eo印喊co尺nf镰er钢en阀ci统ng至.芳史Ho吴we侧ve关r,醉t躬he探p竟ap大er刑f腿oc额us柴es梦o漏n雄Vo帐IP烫a鹿s乘th讨e灵ne凯w铅se进rv姓ic蛙e介of我员in露te波re白st佛t档o婆be方d脸ep粥lo轨ye保d.然T扰he普p这ap愿er差a虑ls兰o男co竟nt闻ai器ns材m种an袍y燥us姐ef摧ul遗骑en州gi绞ne攻er鞋in危g遥an吉d究de仇si牛gn糠g衣ui唯de裙li晓ne膛s,具a见nd动d芬is给cu苏ss集es牵m聪an株y燃肺pr照ac殖ti仪ca敏l厅is缎su枣es陕p庆er关ta拢in仁in惯g优to直t击he观d是ep绑lo扯ym置en宪t落of疮V脂oI然P.牺T首he辫se变存is拜su膀es鄙i草nc凭lu半de讽c涉ha岔ra心ct思er胆is而ti架cs情o火f杂Vo蛋IP锄t蓬ra由ff龙ic嘉a体nd幸Q自oS寻璃re球qu丝ir放em福en掏ts担,愚Vo确IP销f茂lo们w睡an书d娇ca留ll地d夹is雷tr骡ib银ut恼io须n,猜d箩ef廉in旦in荒g帜悠fu球tu植re牵g器ro乱wt麦h粱ca闭pa值ci依ty归,邻an沟d绒me溜as授ur雪em演en染t毒an胸d漆im讯pa脏ct企o似f舅王ba晋ck闻gr劝ou剂nd尖t北ra贩ff村ic期.捆As士a仙c本as谋e卡st衡ud都y,键w眯e狗il惧lu胜st房ra此te编h剑ow最o爪ur尸旱ap诵pr私oa译ch融a熄nd菠g旺ui浩de肉li镰ne槽s涝ca农n母be叶a肤pp凳li承ed澡t圾o鉴a做ty居pi米ca攀l迹ne攻tw宗or风k评古of表a辞s遵ma萝ll号e昨nt之er交pr丑is市e.挣鸭Th亚e热re渠st支o皱f盈th滥e喝pa赖pe鸣r满is远o歼rg枕an搅iz螺ed对a惨s药fo冬ll寇ow扛s.首S庙ec抢ti惊on脊2弟渐pr怀es卖en天ts甲a踏t绳yp嘱ic燃al恋n遵et鉴wo屿rk分t字op谣ol应og江y赴of偶a续s隐ma战ll闹e圾nt迷er盈pr微is染e彩to宽晌be姐u勿se絮d斗as级a刘c庙as域e巴st洽ud液y桃fo网r恭de航pl纳oy个in玉g考Vo蜂IP虏.击Se林ct套io厕n误3她趴ou症tl御in秋es脱p均ra寺ct巷ic斯al国e奸ig嫁ht郑-s击te暑p棉me印th些od等ol撞og垃y寸to迅d患ep友lo荣y置假su蜂cc鲁es纤sf针ul辽ly莫V咸oI昨P裙in胶d外at郊a包ne塞tw脂or待ks偶.他Ea逝ch蛋s龙te骡p伐is喇d监es悦cr葵ib漆ed召朗in扫c然on债si抗de果ra炉bl蓄e酿de三ta赞il腊.零Se够ct乱io吸n泥4钱de它sc察ri驱be侦s先im应po段rt嫩an极t严de末si旬gn辽贺an灶d奔en埋gi简ne吨er锋in啄g久de赶ci筐si乎on吨s摆to钉b卫e庄ma肥de蒸b插as揪ed枪o护n查th丽e凡an左al承yt鼻ic贩蹄an弱d么si吨mu曾la旧ti签on辟s旁tu个di决es欣.括Se伙ct搭io牺n自5些co需nc督lu紧de红s商th州e典st肥ud帖y克an虾d炮利id赢en叼ti荣fi痕es耻f睁ut沸ur寿e多wo催rk期.2ExistingnetworkFig.1illustratesatypicalnetworktopologyforasmallenterpriseresidinginahigh-risebuilding.Thenetworkshownisrealisticandusedasacasestudyonly;however,ourworkpresentedinthispapercanbeadoptedeasilyforlargerandgeneralnetworksbyfollowingthesameprinciples,guidelines,andconceptslaidoutinthispaper.ThenetworkisEthernet-basedandhastwoLayer-2Ethernetswitchesconnectedbyarouter.TherouterisCisco2621,andtheswitchesare3ComSuperstack3300.Switch1connectsFloors1and2andtwoservers;whileSwitch2connectsFloor3andfourservers.EachfloorLANisbasicallyasharedEthernetconnectingemployeePCswithworkgroupandprinterservers.ThenetworkmakesuseofVLANsinordertoisolatebroadcastandmulticasttraffic.AtotaloffiveLANsexist.AllVLANsareportbased.Switch1isconfiguredsuchthatithasthreeVLANs.VLAN1includesthedatabaseandfileservers.VLAN2includesFloor1.VLAN3includesFloor2.Ontheotherhand,Switch2isconfiguredtohavetwoVLANs.VLAN4includestheserversforE-mail,HTTP,Webandcacheproxy,andfirewall.VLAN5includesFloor3.AllthelinksareswitchedEthernet100MbpsfullduplexexceptforthelinksforFloors1–3whicharesharedEthernet100Mbpshalfduplex.3Step-by-stepmethodologyFig.2showsaflowchartofamethodologyofeightstepsforasuccessfulVoIPdeployment.Thefirstfourstepsareindependentandcanbeperformedinparallel.Beforeembarkingontheanalysisandsimulationstudy,inSteps6and7,Step5mustbecarriedoutwhichrequiresanyearlyandnecessaryredimensioningormodificationstotheexistingnetwork.Asshown,bothSteps6and7canbedoneinparallel.Thefinalstepispilotdeployment.3.1.VoIPtrafficcharacteristics,requirements,andassumptionsForintroducinganewnetworkservicesuchasVoIP,onehastocharacterizefirstthenatureofitstraffic,QoSrequirements,andanyadditionalcomponentsordevices.Forsimplicity,weassumeapoint-to-pointconversationforallVoIPcallswithnocallconferencing.FordeployingVoIP,agatekeeperorCallManagernodehastobeaddedtothenetwork[3,4,5].Thegatekeepernodehandlessignalingforestablishing,terminating,andauthorizingconnectionsofallVoIPcalls.AlsoaVoIPgatewayisrequiredtohandleexternalcalls.AVoIPgatewayisresponsibleforconvertingVoIPcallsto/fromthePublicSwitchedTelephoneNetwork(PSTN).Asanengineeringanddesignissue,theplacementofthesenodesinthenetworkbecomescrucial.Wewilltacklethisissueindesignstep5.OtherhardwarerequirementsincludeaVoIPclientterminal,whichcanbeaseparateVoIPdevice,i.e.IPphones,oratypicalPCorworkstationthatisVoIP-enabled.AVoIP-enabledworkstationrunsVoIPsoftwaresuchasIPSoftPhones.Fig.3identifiestheend-to-endVoIPcomponentsfromsendertoreceiver[9].Thefirstcomponentistheencoderwhichperiodicallysamplestheoriginalvoicesignalandassignsafixednumberofbitstoeachsample,creatingaconstantbitratestream.Thetraditionalsample-basedencoderG.711usesPulseCodeModulation(PCM)togenerate8-bitsamplesevery0.125ms,leadingtoadatarateof64kbps.ThepacketizerfollowstheencoderandencapsulatesacertainnumberofspeechsamplesintopacketsandaddstheRTP,UDP,IP,andEthernetheaders.Thevoicepacketstravelthroughthedatanetwork.Animportantcomponentatthereceivingend,istheplaybackbufferwhosepurposeistoabsorbvariationsorjitterindelayandprovideasmoothplayout.Thenpacketsaredeliveredtothedepacketizerandeventuallytothedecoderwhichreconstructstheoriginalvoicesignal.WewillfollowthewidelyadoptedrecommendationsofH.323,G.711,andG.714standardsforVoIPQoSrequirements.Table1comparessomecommonlyusedITU-Tstandardcodecsandtheamountofone-waydelaythattheyimpose.ToaccountforupperlimitsandtomeetdesirablequalityrequirementaccordingtoITUrecommendationP.800,wewilladoptG.711ucodecstandardsfortherequireddelayandbandwidth.G.711uyieldsaround4.4MOSrating.MOS,MeanOpinionScore,isacommonlyusedVoIPperformancemetricgiveninascaleof1–5,with5isthebest.However,withlittlecompromisetoquality,itispossibletoimplementdifferentITU-Tcodecsthatyieldmuchlessrequiredbandwidthpercallandrelativelyabithigher,butacceptable,end-to-enddelay.Thiscanbeaccomplishedbyapplyingcompression,silencesuppression,packetlossconcealment,queuemanagementtechniques,andencapsulatingmorethanonevoicepacketintoasingleEthernetframe.3.1.1.End-to-enddelayforasinglevoicepacketFig.3illustratesthesourcesofdelayforatypicalvoicepacket.Theend-to-enddelayissometimesreferredtobyM2EorMouth-to-Eardelay.G.714imposesamaximumtotalone-waypacketdelayof150msend-to-endforVoIPapplications.In[22],adelayofupto200mswasconsideredtobeacceptable.Wecanbreakthisdelaydownintoatleastthreedifferentcontributingcomponents,whichareasfollows(i)encoding,compression,andpacketizationdelayatthesender(ii)propagation,transmissionandqueuingdelayinthenetworkand(iii)buffering,decompression,depacketization,decoding,andplaybackdelayatthereceiver.3.1.2.BandwidthforasinglecallTherequiredbandwidthforasinglecall,onedirection,is64kbps.G.711codecsamples20msofvoiceperpacket.Therefore,50suchpacketsneedtobetransmittedpersecond.Eachpacketcontains160voicesamplesinordertogive8000samplespersecond.EachpacketissentinoneEthernetframe.Witheverypacketofsize160bytes,headersofadditionalprotocollayersareadded.TheseheadersincludeRTP+UDP+IP+Ethernetwithpreambleofsizes12+8+20+26,respectively.Therefore,atotalof226bytes,or1808bits,needstobetransmitted50timespersecond,or90.4kbps,inonedirection.Forbothdirections,therequiredbandwidthforasinglecallis100ppsor180.8kbpsassumingasymmetricflow.3.1.3.OtherassumptionsThroughoutouranalysisandwork,weassumevoicecallsaresymmetricandnovoiceconferencingisimplemented.Wealsoignorethesignalingtrafficgeneratedbythegatekeeper.Webaseouranalysisanddesignontheworst-casescenarioforVoIPcalltraffic.Thesignalingtrafficinvolvingthegatekeeperismostlygeneratedpriortotheestablishmentofthevoicecallandwhenthecallisfinished.Thistrafficisrelativelysmallcomparedtotheactualvoicecalltraffic.Ingeneral,thegatekeepergeneratesnoorverylimitedsignalingtrafficthroughoutthedurationoftheVoIPcallforanalreadyestablishedon-goingcall.Inthispaper,wewillimplementnoQoSmechanismsthatcanenhancethequalityofpacketdeliveryinIPnetworks.AmyriadofQoSstandardsareavailableandcanbeenabledfornetworkelements.QoSstandardsmayincludeIEEE802.1p/Q,theIETF’sRSVP,andDiffServ.Analysisofimplementationcost,complexity,management,andbenefitmustbeweighedcarefullybeforeadoptingsuchQoSstandards.Thesestandardscanberecommendedwhenthecostforupgradingsomenetworkelementsishighandthenetworkresourcesarescarceandheavilyloaded.3.2.VoIPtrafficflowandcalldistributionKnowingthecurrenttelephonecallusageorvolumeoftheenterpriseisanimportantstepforasuccessfulVoIPdeployment.BeforeembarkingonfurtheranalysisorplanningphasesforaVoIPdeployment,collectingstatisticsaboutofthepresentcallvolumeandprofilesisessential.Sourcesofsuchinformationareorganization’sPBX,telephonerecordsandbills.Keycharacteristicsofexistingcallscanincludethenumberofcalls,numberofconcurrentcalls,time,duration,etc.Itisimportanttodeterminethelocationsofthecallendpoints,i.e.thesourcesanddestinations,aswellastheircorrespondingpathorflow.Thiswillaidinidentifyingthecalldistributionandthecallsmadeinternallyorexternally.Calldistributionmustincludepercentageofcallswithinandoutsideofafloor,building,department,ororganization.Asagoodcapacityplanningmeasure,itisrecommendedtobasetheVoIPcalldistributiononthebusyhourtrafficofphonecallsforthebusiestdayofaweekoramonth.ThiswillensuresupportofthecallsatalltimeswithhighQoSforallVoIPcalls.Whensuchcurrentstatisticsarecombinedwiththeprojectedextracalls,wecanpredicttheworst-caseVoIPtrafficloadtobeintroducedtotheexistingnetwork.Fig.4describesthecalldistributionfortheenterpriseunderstudybasedontheworstbusyhourandtheprojectedfuturegrowthofVoIPcalls.Inthefigure,thecalldistributionisdescribedasaprobabilitytree.Itisalsopossibletodescribeitasaprobabilitymatrix.Someimportantobservationscanbemadeaboutthevoicetrafficflowforinter-floorandexternalcalls.Forallthesetypeofcalls,thevoicetraffichastobealwaysroutedthroughtherouter.ThisissobecauseSwitchs1and2arelayer2switcheswithVLANsconfiguration.Onecanobservethatthetrafficflowforinter-floorcallsbetweenFloors1and2imposestwicetheloadonSwitch1,asthetraffichastopassthroughtheswitchtotherouterandbacktotheswitchagain.Similarly,Switch2experiencestwicetheloadforexternalcallsfrom/toFloor3.3.3.DefineperformancethresholdsandgrowthcapacityInthisstep,wedefinethenetworkperformancethresholdsoroperationalpointsforanumberofimportantkeynetworkelements.Thesethresholdsaretobeconsideredwhendeployingthenewservice.Thebenefitistwofold.First,therequirementsofthenewservicetobedeployedaresatisfied.Second,addingthenewserviceleavesthenetworkhealthyandsusceptibletofuturegrowth.Twoimportantperformancecriteriaaretobetakenintoaccount.Firstisthemaximumtolerableend-to-enddelay;andsecondistheutilizationboundsorthresholdsofnetworkresources.Themaximumtolerableend-to-enddelayisdeterminedbythemostsensitiveapplicationtorunonthenetwork.Inourcase,itis150msend-to-endforVoIP.Itisimperativetonotethatifthenetworkhascertaindelaysensitiveapplications,thedelayfortheseapplicationsshouldbemonitored,whenintroducingVoIPtraffic,suchthattheydonotexceedtheirrequiredmaximumvalues.Asfortheutilizationboundsfornetworkresources,suchboundsorthresholdsaredeterminedbyfactorssuchascurrentutilization,futureplans,andforeseengrowthofthenetwork.Properresourceandcapacityplanningiscrucial.Savvynetworkengineersmustdeploynewserviceswithscalabilityinmind,andascertainthatthenetworkwillyieldacceptableperformanceunderheavyandpeakloads,withnopacketloss.VoIPrequiresalmostnopacketloss.Inliterature,0.1–5%packetlosswasgenerallyasserted.However,in[24]therequiredVoIPpacketlosswasconservativelysuggestedtobelessthan10.Amorepracticalpacketloss,basedonexperimentation,ofbelow1%wasrequiredin[22].Hence,itisextremelyimportantnottoutilizefullythenetworkresources.Asrule-of-thumbguidelineforswitchedfastfull-duplexEthernet,theaverageutilizationlimitoflinksshouldbe190%,andforswitchedsharedfastEthernet,theaveragelimitoflinksshouldbe85%[25].Theprojectedgrowthinusers,networkservices,business,etc.mustbealltakenintoconsiderationtoextrapolatetherequiredgrowthcapacityorthefuturegrowthfactor.Inourstudy,wewillascertainthat25%oftheavailablenetworkcapacityisreservedforfuturegrowthandexpansion.Forsimplicity,wewillapplythisevenlytoallnetworkresourcesoftherouter,switches,andswitched-Ethernetlinks.However,keepinmindthispercentageinpracticecanbevariableforeachnetworkresourceandmaydependonthecurrentutilizationandtherequiredgrowthcapacity.Inourmethodology,thereservationofthisutilizationofnetworkresourcesisdoneupfront,beforedeployingthenewservice,andonlytheleft-overcapacityisusedforinvestigatingthenetworksupportofthenewservicetobedeployed.3.4.PerformnetworkmeasurementsInordertocharacterizetheexistingnetworktrafficload,utilization,andflow,networkmeasurementshavetobeperformed.Thisisacrucialstepasitcanpotentiallyaffectresultstobeusedinanalyticalstudyandsimulation.Thereareanumberoftoolsavailablecommerciallyandnoncommerciallytoperformnetworkmeasurements.Popularopen-sourcemeasurementtoolsincludeMRTG,STG,SNMPUtil,andGetIF[26].AfewexamplesofpopularcommerciallymeasurementtoolsincludeHPOpenView,CiscoNetflow,LucentVitalSuite,PatrolDashBoard,OmegonNetAlly,AvayaExamiNet,NetIQVivinetAssessor,etc.Networkmeasurementsmustbeperformedfornetworkelementssuchasrouters,switches,andlinks.Numeroustypesofmeasurementsandstatisticscanbeobtainedusingmeasurementtools.Asaminimum,trafficratesinbitspersecond(bps)andpacketspersecond(pps)mustbemeasuredforlinksdirectlyconnectedtoroutersandswitches.Togetadequateassessment,networkmeasurementshavetobetakenoveralongperiodoftime,atleast24-hperiod.Sometimesitisdesirabletotakemeasurementsoverseveraldaysoraweek.Onehastoconsidertheworst-casescenariofornetworkloadorutilizationinordertoensuregoodQoSatalltimesincludingpeakhours.Thepeakhourisdifferentfromonenetworktoanotheranditdependstotallyonthenatureofbusinessandtheservicesprovidedbythenetwork.Table2showsasummaryofpeak-hourutilizationfortrafficoflinksinbothdirectionsconnectedtotherouterandthetwoswitchesofthenetworktopologyofFig.1.Thesemeasuredresultswillbeusedinouranalysisandsimulationstudy.外文文献译文以太网网络传送调度：方法论和案例分析摘要对网络数据研究者和设计师来说，IP或语音IP调度是一项重大而艰巨的任务。本文概述的准那么和循序渐进的方法，解释了怎样在IP上成功调度传送语音。该方法可用于评估的支持，并准备用在现有的网络。此前购置并部署的网络设备，这种方法预算出了在保证现有网络效劳质量要求和日后足够扩充能力根底上的网络调用次数。作为一个研究的课题，我们把这种方法在一个典型的小型企业网上得到逐步应用。我们运用分析和模拟吞吐量和延迟区域。我们的分析基于排队理论，并且OPNET用于模拟。理论分析和模拟结构比拟一致。此外，本文谈论了许多设计和工程问题。这些问题包括网络通信的特征和效劳质量要求，网络流程和呼叫分配，定义未来增长容量，测定后台通信的影响。关键词：网络设计，网络管理，VoIP，性能评估，分析，模拟，OPNET1绪论最近大量的网络调度在数据网中占有相当的比例。其中大局部基于以太网和运行IP协议。不少网络管理员发现把语音和数据网合二为一非常有吸引力和具本钱效应。这将更易于运行、管理和维护。然而，人们应该认识到IP网络目的是为了非实时应用效劳；另一方面，网络要求带有低延迟、低抖动、低丢包率和充足的带宽。为了到达这一目标，必须保证在现有或新的IP网络中完成实时通信要求的高效网络调度。当在现有的网络部署譬如VoIP这样的新网络效劳，许多网络架构师、经理、方案师、设计师和工程师面临着共同的目标和挑战。什么是网络的效劳质量要求？新的网络负荷怎样冲击了当今运行中的网络效劳和应用的质量？我们现有的网络将支持网络和将满足标准的效劳质量要求吗？如果那样，在过早升级任何现有的网络硬件的零件前，能支持多少个的VoIP网络？这些富挑战性的问题导致了用于测试网络多媒体数据应用性能的商业工具的开展。支持网络的商用工具名单如表[1,2]。很大程度上，这些工具使用两种共同的方法把VoIP部署入现有的网络。一种方法根据第一次执行的网络测量和然后预测支持VoIP的网络应该就绪情况。预测的网络情况是根据网络要素来估计的。第二种是根据参加到现有网络中VoIP的实时通信信息，然后测出延时时间、时基误差和丢包率。除相关的商业工具费用外，没有其它工具能够提供全面兼容的成功网络调度方法。特别是，无任何一个预测能给出网络可以支持的呼叫次数已提到设计和工程的议事日程上来。这些因素包括网络流程和呼叫分配、今后增长容量、性能极限、VoIP对现有网络效劳和应用以及后台通信能力的冲击。本文尝试研究这些重要因素，规划出一个支持像网络和视频会议系统的全面可行方法。无论如何，本文集中论述了网络效劳调度新方法。文章同样包含了许多工程和设计指南，同时也讨论了许多与网络相关的实际问题。这些问题包括网络的通信和效劳质量要求，网络数据流向和呼叫分配，定义今后的增长容量以及我们的方法和纲领怎样应用在像小型的企业网这样的典型网络中。其余局部作如下安排：第二局部论述了小型企业网的典型网络拓扑如何在网络中实现调度。第三局部大致描述了网络数据网调度实用的八个步骤。每一步骤都有详细的说明。第四局部介绍了基于分析和仿真研究得出的重要设计结论和工程结论。第五局部谈到了今后研究还需做的具体工作。2现有的网络互联网数据库打印机效劳器一个小型企业网的逻辑图路由器交换机1交换机2工作组效劳互联网数据库打印机效劳器一个小型企业网的逻辑图路由器交换机1交换机2工作组效劳器文件效劳器层2图1一个小型企业网的逻辑图图1是在大厦中一个的小型企业网的典型网络拓扑图。图中所示真实网络情况仅用于研究目的；然而，我们文中提出的原理、框架和概念等工作成果将会更容易被大型网络采纳。这些网络都是通过一个路由连接的两个第二层以太网交换机。路由是卡西欧2621，交换机是3ComSuperstack3300。交换机1连接第一、二层和两个效劳器；交换机2连接第三层和四个效劳器。每一层的本地局域网就是一个连接有个人电脑的工作组和打印机效劳器的共享以太网。网络利用虚拟局域网来隔离播送和组播拥塞。总计有五个局域网存在。所有虚拟局域网基于端口划分。交换机一配置三个虚拟局域网。虚拟局域网一包括数据库和文件效劳器。虚拟局域网二包括层一。虚拟局域网三包括层二。另一方面，交换机二配置为含有两个虚拟局域网。虚拟局域网四包括邮件效劳器、超文本协议、网页和缓存代理以及防火墙。虚拟局域网五包括层三。除了层1-3是100Mbps的半双工以太网其余链接都是100Mbps速率全双工的以太网。3步进的方法论说明方法步骤的流程图模拟前面网络评估和更改前端调度分析说明方法步骤的流程图模拟前面网络评估和更改前端调度分析图SEQ图表\*ARABIC1说明方法步骤的流程图图2表示了一个成功的网络调度八步骤流程图。前面四步是独立的，可以并行运行。在封装分析和模拟研究前，作为前面网络评估和更改的第5步必须先执行。如图，步骤6和7可以并行执行。最后一步是前端调度。3.1网络通信特征、要求和假想要介绍像网络这样的效劳，首先要概括出它通信的本质、效劳质量要求和其它原因或设备问题。出于简单考虑，我们假定在一个没有呼叫协商的点对点对话网络呼叫。对网络调度来说，网络节点或呼叫管理器节点必须添加到网络[3,4,5]。网络节点处理用于建立、终止和受权所有网络呼叫的连接，同时也要处理外部的呼叫。网络负责转换网络呼叫到公共交换网(或反向)。作为一个工程和设计课题，如何放置这些网络中的节点变得至关紧要。在步骤5中我们会谈到如何处理这个问题。其它硬件要求包括网络客户终端(例如网络机或典型的个人电脑又或是含有激活网络的工作站)可以是独立的设备。激活网络的工作站运行着像IP这样的软件。编码解码解包封装编码解码解包封装图SEQ图表\*ARABIC2网络端到端器件图3表示了一个端对端网络的发送和接收器件。第一个器件是编码器，它用于周期性采集原始声音信号和分配固定的比特位给每一个样本，产生一个常速率的比特流。传统的样本采集编码器G.711利用脉冲编码调制来产生每0.125ms的8位比特的样本，从而产生64kbps的数据速率。压缩包随着编码器压入一定数量的语音标本到信息包，然后参加实时位置、用户数据报、网际协议和以太网报头等信息。语音信号包随之流向数据网络。在接收端有一个重要的元件，它是为了吸收变调的声音和抖动失真的录音回放缓冲器，同时也是为了提供平滑过度的释放。然后数据包被传送至解压器，最终复原成原始声音信号。我们会采用大量被推荐网络的H.323、G.711、和G.714效劳质量保证标准。表1标准ITU-T编码器和其默认值A/D转换延迟标准ITU-T编码器和其默认值A/D转换延迟标准ITU-T编码器和其默认值表1对使用国际电信联盟标准的多媒体数字信号编解码器和大量强制要求的单声道的延迟标准作了一个比拟。为了到达上层限制要求和满足国际电信联盟推荐的质量要求标准P.800,我们采用G.711u编解码器标准来满足延时和带宽要求。G.711u产生4.4等级MOS。MOS，意思为一种经常用在网络性能指标中的评价标准，有1-5个等级，第5级为最好。然而，为了折衷小小的质量问题，最有可能执行每次呼叫需求带宽更少、相关程度稍高、更能接受的端到端的延迟时间的ITU-T编解码器的不同标准。这将通过用密集的、静默压缩的、隐蔽的小包丧失数、队列管理技术和把超过一个声音信号包封装在单个以太网帧中。3.1.1单声道的端到端延迟图3阐述了典型语音包延迟的缘由。这端到端的延迟有时归结于M2E或口到耳的延迟。G.714提出了单声道最大总数不超过150ms的网络端到端应用。在[22]提到不超过200ms的延迟是可以接受的。我们能够通过以下最少三种起作用的元件降低延迟：(i)发送端的编码、压缩和解压延迟；(ii)传播、运输和网络中的排队延迟；(iii)接收端的缓冲、解压、解码、录音重放延迟。3.1.2单个呼叫的带宽单个呼叫的带宽要求，一个参数就是64kbps。G.711多媒体编解码器每个语音包20ms的采样。因此，每秒必需有50个这样的数据包。每个包中包含160语音样本目的是为了到达8000每秒的采样率。每个包通过单个以太网帧传送。每个包有160字节大小，再加上协议各层的报头信息。这些报头包括12+8+20+26对应大小的RTP+UDP+IP+Ethernet信息。因此，总共有226字节(或1808位)的信息需要每秒传送50次，或一次传送90.4kbps。对于