通信专业英语On the deployment of VoIP in Ethernet networksWord格式.docx

通信专业英语On the deployment of VoIP in Ethernet networksWord格式.docx

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methodologyandcasestudy

Abstract

DeployingIPtelephonyorvoiceoverIP（VoIP）isamajorandchallengingtaskfordatanetworkresearchersanddesigners.Thispaperoutlinesguidelinesandastep-by-stepmethodologyonhowVoIPcanbedeployedsuccessfully.Themethodologycanbeusedtoassessthesupportandreadinessofanexistingnetwork.PriortothepurchaseanddeploymentofVoIPequipment,themethodologypredictsthenumberofVoIPcallsthatcanbesustainedbyanexistingnetworkwhilesatisfyingQoSrequirementsofallnetworkservicesandleavingadequatecapacityforfuturegrowth.Asacasestudy,weapplythemethodologystepsonatypicalnetworkofasmallenterprise.Weutilizebothanalysisandsimulationtoinvestigatethroughputanddelaybounds.Ouranalysisisbasedonqueuingtheory,andOPNETisusedforsimulation.Resultsobtainedfromanalysisandsimulationareinlineandgiveaclosematch.Inaddition,thepaperdiscussesmanydesignandengineeringissues.TheseissuesincludecharacteristicsofVoIPtrafficandQoSrequirements,VoIPflowandcalldistribution,definingfuturegrowthcapacity,andmeasurementandimpactofbackgroundtraffic.

Keywords:

NetworkDesign，NetworkManagement，VoIP，PerformanceEvaluation，Analysis，Simulation，OPNET

1Introduction

ThesedaysamassivedeploymentofVoIPistakingplaceoverdatanetworks.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?

HowwillthenewVoIPloadimpacttheQoSforcurrentlyrunningnetworkservicesandapplications?

WillmyexistingnetworksupportVoIPandsatisfythestandardizedQoSrequirements?

Ifso,howmanyVoIPcallscanthenetworksupportbeforeupgradingprematurelyanypartoftheexistingnetworkhardware?

Thesechallengingquestionshaveledtothedevelopmentofsomecommercialtoolsfortestingtheperformanceofmultimediaapplicationsindatanetworks.AlistoftheavailablecommercialtoolsthatsupportVoIPislistedin[1,2].Forthemostpart,thesetoolsusetwocommonapproachesinassessingthedeploymentofVoIPintotheexistingnetwork.OneapproachisbasedonfirstperformingnetworkmeasurementsandthenpredictingthenetworkreadinessforsupportingVoIP.Thepredictionofthenetworkreadinessisbasedonassessingthehealthofnetworkelements.ThesecondapproachisbasedoninjectingrealVoIPtrafficintoexistingnetworkandmeasuringtheresultingdelay,jitter,andloss.Otherthanthecostassociatedwiththecommercialtools,noneofthecommercialtoolsofferacomprehensiveapproachforsuccessfulVoIPdeployment.Inparticular,nonegivesanypredictionforthetotalnumberofcallsthatcanbesupportedbythenetworktakingintoaccountimportantdesignandengineeringfactors.ThesefactorsincludeVoIPflowandcalldistribution,futuregrowthcapacity,performancethresholds,impactofVoIPonexistingnetworkservicesandapplications,andimpactbackgroundtrafficonVoIP.ThispaperattemptstoaddressthoseimportantfactorsandlayoutacomprehensivemethodologyforasuccessfuldeploymentofanymultimediaapplicationsuchasVoIPandvideoconferencing.However,thepaperfocusesonVoIPasthenewserviceofinteresttobedeployed.Thepaperalsocontainsmanyusefulengineeringanddesignguidelines,anddiscussesmanypracticalissuespertainingtothedeploymentofVoIP.TheseissuesincludecharacteristicsofVoIPtrafficandQoSrequirements,VoIPflowandcalldistribution,definingfuturegrowthcapacity,andmeasurementandimpactofbackgroundtraffic.Asacasestudy,weillustratehowourapproachandguidelinescanbeappliedtoatypicalnetworkofasmallenterprise.Therestofthepaperisorganizedasfollows.Section2presentsatypicalnetworktopologyofasmallenterprisetobeusedasacasestudyfordeployingVoIP.Section3outlinespracticaleight-stepmethodologytodeploysuccessfullyVoIPindatanetworks.Eachstepisdescribedinconsiderabledetail.Section4describesimportantdesignandengineeringdecisionstobemadebasedontheanalyticandsimulationstudies.Section5concludesthestudyandidentifiesfuturework.

2Existingnetwork

Fig.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-stepmethodology

Fig.2showsaflowchartofamethodologyofeightstepsforasuccessfulVoIPdeployment.Thefirstfourstepsareindependentandcanbeperformedinparallel.Beforeembarkingontheanalysisandsimulationstudy,inSteps6and7,Step5mustbecarriedoutwhichrequiresanyearlyandnecessaryredimensioningormodificationstotheexistingnetwork.Asshown,bothSteps6and7canbedoneinparallel.Thefinalstepispilotdeployment.

3.1.VoIPtrafficcharacteristics,requirements,andassumptions

ForintroducinganewnetworkservicesuchasVoIP,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-enddelayforasinglevoicepacket

Fig.3illustratesthesourcesofdelayforaty