StateDuration SuccessTsucTfrmTSIFSTACKCollisionTcolTfrmTSIFSTACKDeferredTdefTfrmTSIFSTACKBacko TbkfrBOrTslot2CWrTslot Table1Durationcalculationsforthefourstatesof80211DCFframespacep ID: 609218
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Figure1:IEEE802.11DCFtiming.transmission,thisaddeddelaycandegradethequalityofreceivedvideowhenplayoutdeadlinesareviolated.There-fore,itisimportanttounderstandthetypesofdelayscausedbyDCFinordertoestimatenetworkconditionspertinenttotheoperationofDRAS.264.Thefollowingsubsectionsdiscussthesedetails.2.1802.11DCFOperationInDCF,eachstation(STA)monitorsthewirelessmediumforactivityinintervalsoftimeslots(Tslot)[4].WhenthemediumissensedtobeidleforaspeciedperiodofTDIFS,STAsarepermittedtotransmit.However,toavoidcollisionsduetosimultaneoustransmissionsfrommultipleSTAs,anexponentialbacko mechanismisemployedwherebyaran-dombacko (BO)timeslotischosenfromacontentionwindow(CW).ACWismadeupofanintegralnumberoftimeslots,whereitsinitialsizedependsonthestandard,anddoublesinsizeaftereachsuccessivecollision.WhentheCWreachesapre-denedmaximumvalue,itremainsatthisvalueuntilasuccessfultransmissiontakesplaceortheretrylimitforthepacketisreached(i.e.,dropped).Fig.1showsanexampletimingoftheIEEE802.11DCFfromtheperspectiveofaSTAsharingthemediumwithotherSTAs.ASTAcanbeclassiedtobeinoneoffourstates:success,collision,backo ,anddeferred,whicharealsoindicatedinFig1.Initially,followingaDIFStimeperiod,theSTAobtainsarandombacko valueof11andstartsthecountdownprocess,whichrepresentsthebacko state.After5timeslots,anotherSTAissensedaccessingthemediumandthustheSTAmustdeferaccessandtransi-tionstothedeferredstate.WhenthemediumbecomesfreeandaDIFSintervalhaspassed,theSTAcontinuesinthebacko statefortheremaining6timeslots.TheSTAthengainsaccesstothemediumandtransmitswithoutacolli-sion,representingthesuccessstate.Followingasuccessfultransmission,thesameprocessforanewpacketbeginswithanewrandombacko valueof3.However,afteraDIFSintervalandadurationof3timeslots,theSTAseizesthemediumwhileanothernodeissimultaneouslyaccessingthemedium,puttingtheSTAinthecollisionstate.Table1providesthetimingcalculationsforeachstateundertheassumptionthatallpacketsareofequallength.Basedonthisassumption,thesuccess(Tsuc),collision(Tcol),anddeferral(Tdef)timesareequal.Thedurationsofthesestatescoverthetimeasenderbeginstransmissionofapackettothetimeitexpectstoreceiveanacknowledgement(ACK).AsshowninTable1,thisisthesumofthetimerequiredtotransmitaMAClayerdataframe(Tfrm),theshortinter- StateDuration SuccessTsuc=Tfrm+TSIFS+TACKCollisionTcol=Tfrm+TSIFS+TACKDeferredTdef=Tfrm+TSIFS+TACKBacko Tbkf(r)=BOrTslot2(CWrTslot) Table1:Durationcalculationsforthefourstatesof802.11DCF.framespaceperiod(TSIFS),andthetimerequiredtotrans-mitanACKframe(TACK).Thetimingcalculationforthebacko state(Tbkf)onlydependsontherandomtimeslotthatischosenfortherthretry,BOr.2.2TheH.264CodecH.264isahighlyecientcodingstandardthatusespredic-tivemethodstoreconstructvideosequences.Anencodedvideoconsistsofasequenceofgroupofpictures(GOP),whichisasetofcodedpicturesthatspeciestheorderofI-,P-,andB-frames.TheinterdependenciesbetweenframescanleadtoerrorpropagationwithinaGOPsequencewhenpacketlossoccurs.Framescancontainamixtureofdi erentmacroblock(MB)types,whereeachMBisa1616-pixelre-gion.MBsarelabelledaccordingtotypesofreferencesmadeforprediction.Forexample,aB-frameholdsMBsthatarebi-predicted;however,itmayalsocontainintra-predictedMBs.AP-framecontainsMBsthatarepredictedfrompastframesandmayalsocontainintra-predictedMBs.I-framescontainonlyintra-predictedMBsanddonotreferenceotherframes.AnHDvideoframeencodedusingH.264istypicallysubdi-videdintomultipleslices.SlicesareclassiedbythetypesofMBstheycontain.TheveslicetypessupportedbytheH.264standardareI(andIDR),P,B,SP,andSI.AnIDRorInstantaneousDecoderRefreshsliceisanI-slicethatpre-ventsthedecoderfromreferencingearlierslicesandalwaysoccursatthestartofanewGOP[12].SPandSIarespe-cialtypesofslicesthatareenabledthroughtheExtendedProleoftheH.264standard.OurdiscussionisfocusedontheI/IDR-,P-,andB-slicesastheyaresupportedbynearlyallH.264proles,andtherefore,readilyavailableinencodedvideos.H.264o ersabstractionofthebitstreamintwolayers:theVideoCodingLayer(VCL)andtheNetworkAbstractionLayer(NAL).TheVCLreferstotheactualcompressedvideothatresultsfromapplyingH.264compressiontech-niques(prediction,motioncompensation,variablelengthcoding,etc.).TheNALwasintroducedtosupportthepacket-basednatureofexistingnetworksanddictateshowtheoutsideworld(i.e.,routers,NICs,networkprotocols)workswithH.264-encodedvideo,withoutneedingtoknowthedetailsandspecicationsofthecompressedvideobeingtransported[8].WhentheH.264syntaxismappedtoframesequences,eachslicecorrespondstoaparticularregionofaframe.Fig.2showsanexampleframethatcontainssomemissingslicesduetopacketloss.Sliceboundariesareindicatedbythesolidbluelines,anditcanbeclearlyseenthatthereare8slices.Notethatdecoderstypicallyperformerrorconceal-ment(EC)techniqueswheninformationismissingtohide Figure6:ActualMAClayerend-to-enddelayandpredicteddelaysfordi erentretryattempts.andincludingtheretryattemptinquestionsuchthateachpredictiontakesintoaccountdelaysofpreviousattempts.Fig.6showshowthepredicteddelaysfromEq.4fordi er-entretryattemptscomparetotheactualMAClayerend-to-enddelays(indicatedinblue).ThedelaysareforMAClayerframestransportinga10MbpsH.264bitstreamwiththreeadditionalconstantbitrateinterferencestreams.Asareference,thenumberofretriesneededforsomeoftheactualend-to-enddelaysarepointedoutonthegure.Thepre-dicteddelaysinFig.6showthatEq.4tendstounder-predictrealdelaysexperiencedattheMAClayer.Forexample,incaseswherefourretriesareneeded,thecorrespondingpre-dicteddelays(indicatedbysolidblacklines)arealwayslessthantheactualdelayexperienced.Theonlycasewherethedelayisover-predictedisforveretries(indicatedbysolidpinkline).Sinceover-predictionsresultintheselectionofaretrylimittoolowforsuccessfuldelivery,DRAS.264em-ployssomeleniencyintheretryassignmentprocess.Thus,ratherthanusingthehardlimitresultingfromthecompar-isonTpktdly(r)Nslcpkt�DfrminFig.4,DRAS.264assignsonehighervaluetor.5.SIMULATIONSTUDYOursimulationstudywasconductedusingtheOpenEvalua-tionFrameworkforMultimediaOverNetworks(OEFMON)developedattheKoreaAdvancedInstituteofScienceandTechnology(KAIST)[9].OEFMONisbuiltuponamul-timediacomponentDirectShowandanetworksimulatorQualNet[13].Together,theyprovidevisualizationoftheunderlyingnetworkdetailsandon-the- ydisplayofsentandreceivedvideos.OEFMONrequiresthefollowinginputs:ArawvideoleinYUVformat,aQualNetscenariole,aQoSmappingparameterle,andaDirectShowgraph.Thethreeoutputsgeneratedarethereceivedrawvideole,asenderlog,andareceiverlog,whichareusedforoineanalysistocomputePSNR,throughput,delay,andpacketlossratioamongothermetrics.Thesimulationsetupinvolvesnodalarrangementsofwhatwouldnormallybefoundamongstneighboringapartments.Fig.7showsthistypeofarrangementwithfourpairsofstreamingSTAs.Alldistancesbetweenstreamingpairsfall Figure7:Experimentalsetup.within3m,whichisareasonableviewingrangeinhomenetworks.Notethatthisviewingrangecanvary;however,thisdoesnotimpactperformance.Aslongastransmitandreceivepairsresidewithinthecarriersenserangeofeachother,whichcanbeasmuchas100m,therewillbeinterfer-enceamongvideostreams.ThestreamingpairshowninredrepresentstheprimaryvideostreamforwhichDRAS.264isimplemented.Allremainingstreamssimulatebackgroundtracasaconstantbitrate(CBR).An802.11a/gnetworkwithbandwidth54MbpsinQualNet5.0.2isusedforsimulation.Threetestclips(SonyBravia,Heliboarding,andAfricanCats)areusedtorepresenttheprimaryvideostreams.Theseclipsareencodedusingthex264opensourceH.264encoder[3]MainProle,Level4.1at1080p@30fpswithanaveragebitrateof10Mbps.Thelengthoftestclipsrangefrom315framesto372frames.Inaddition,thestreamingprotocolusedisRTPoverUDP,andallCBRtracis10Mbpsresultinginatotalaggregatebitrateof40Mbpstoinducecongestioninthenetwork.Thebackgroundstreamsstartonesecondaftertheprimaryvideostreamisinitiatedandcontinueuntiltheendofsimulationwhenallthepacketsfromtheprimaryvideostreamhavebeensent.Furthermore,thecurrentversionofOEFMONislimitedtosingle-sliceencoding,thusanentireframeisencompassedinasinglesliceforthetestvideo.Notethatthesevideoclipshavecertaincharacteristicsthatneedtobeconsideredforproperstreaming.Forexample,whenstreamedona54Mbpswirelesslinkwithoutconges-tion,theinitialstartupdelaysare58ms,85ms,and260msfortheclipsSonyBravia,Heliboarding,andAfricanCats,respectively,toavoidmissedplayoutdeadlines.Thesere-quirementsareadirectresultofthebitratevariability(andthusjitter)oftheencodedbit-streams,whereframesizevariesfrom73.9Kbto3.3Mb.ThehighstartuptimeforAfricanCatsisduetoit'shighframesizecoecientofvari-ation(CoV)[14].5.1ResultsFig.8showsPSNRvaluesofthereceivedvideosinreferencetotheoriginalundistortedvideoforDRAS.264(redlines)andthedefault802.11MACprotocol(bluelines).Themax-imumPSNRvalueof111dBrepresentsaperfectlyreceivedframe.APSNRvalueof37dBisconsidered\excellent"quality[7]andisdepictedbyadashedgreenlineineachgraph.