A Survey of Packet-Loss Recovery Techniques：數(shù)據(jù)包丟失恢復(fù)技術(shù)的研究

ASurveyofPacket-LossRecoveryTechniquesColinPerkins,OrionHodsonandVickyHardmanDepartmentofComputerScienceUniversityCollegeLondon(UCL)London,UKIEEENetworkMagazineSep/Oct,1998OverviewDevelopmentofIPMulticast“Light-weightsession”Scaleto1000’sofparticipantsHowtohandlepacketloss?RepairOverviewThispaper:LosscharacteristicsofMBoneTechniquestorepairlossina‘light-weight’mannerConcentrateonaudioRecommendationsOtherpapers:Fully-reliable(everybitmustarrive),butnotreal-timeReal-time,butdonotincludereceiverbasedapproachesOutlineOverviewMulticastChannelCharacteristicsSenderBasedRepairReceiverBasedRepairRecommendationsIPMulticastChannelCharacteristicsGroupaddressClientreceivesonaddressSendersendstoaddress,withoutknowledgeofclientsLooselycoupledconnections“Extension”toUDPNottwo-wayMakesitscalableAllowsclientstodolocalrepairMulticastroutershareswithunicasttrafficCanhavehighlossOftenMBonerouter2ndrateMBoneLossCharacteristicsMostreceiversinthe2-5%lossrangeSomesee20-50%lossCharacteristicsdiffer,soneedlocaldecisionsMboneJitterCharacteristicsHighjitterIftoolate,willbediscardedandlooklikelossInteractiveapplicationsneedlowlatencyInfluencesrepairschemeMediaRepairTaxonomyMediaRepairSenderBasedReceiverBasedSenderBasedRepairTaxonomyWorkfromrighttoleftUnitofaudiodatavs.apacketUnitmaybecomposedofseveralpacketsOronepacketmayhaveseveralunitsofaudiodataForwardErrorCorrection(FEC)AddextradatatostreamUseextradatatorecoverlostpacketsTwoclasses:Mediaindependent(notmultimediaspecific)Mediadependent(knowledgeofaudioorvideo)MediaIndependentFECGivenkdatapacketsGeneraten-kcheckpacketsTransmitnpacketsSchemesoriginallyforbits(likechecksumsinpacketheaders)AppliedtopacketsSo,forexamplei’thbitofcheckpacket,checksi’thbitofeachassociatedpacketFECCodingXORoperationacrossallpacketsTransmit1paritypacketeveryndatapacketsIf1lossinnpackets,canfullyrecoverReed-SolomontreataspolynomialCanrecoverifanykofnpacketsarriveMediaIndependentFECAdvantagesandDisadvantagesAdvantagesMediaindependentAudio,video,differentcompressionschemesComputationissmallandeasytoimplementDisadvantagesAdddelay(mustwaitforallnpackets)Addbandwidth(causingmoreloss?)AdddecodercomplexitySenderBasedRepairTaxonomyMediaSpecificFECMultiplecopiesofdata“Quality”ofsecondaryframes?MediaSpecificFECSecondaryFrameSendpacketenergyandzerocrossingrate2numbers,sosmallCoarse,buteffectiveforsmalllossbetterthaninterpolatingacrossmissingpacketsLowbit-rateencodedversionofprimaryLowernumberofsamplebitsaudiosample,sayFull-versionofsecondaryEffectiveifprimaryissmall(lowbandwidth)MediaSpecificFECDiscussionTypicaloverhead20-30%forlow-quality[HSK98]

MediaspecificFECcanrepairvariousamountsbytradingoffqualityofrepairMediaindependentFEChasfixednumberofbitsforcertainamountofrepairCanhaveadaptiveFECWhenspeechchangesandcannotinterpolateAddwhenincreaseinloss[PCM00]Delaymorethan1packetwhenburstylossMediaSpecificFECAdvantagesandDisadvantagesAdvantagesLowlatencyOnlywaitasinglepackettorepairMultipleifadaptedtoburstylossesCanhavelessbandwidththanindependentFECDisadvantagesComputationmaybemoredifficultimplementStilladdbandwidthAdddecodercomplexityLowerquality(vs.othermethodsofrepair)SenderBasedRepairTaxonomyInterleavingDispersetheeffectsofpacketlossManyaudiotoolssend1phoneme(40msofsound)InterleavingAdvantagesandDisadvantagesAdvantagesMostaudiocompressionschemescandointerleavingwithoutadditionalcomplexityNoextrabandwidthaddedDisadvantagesDelayofinterleavingfactorinpacketsEvenwhennotrepairing!GainstoqualitycanbemoderateSenderBasedRepairTaxonomyRetransmissionIfdelayslessthan250ms,candoretransmission(EffectiveforLANorfastInternetconnectionButhome-connectionoften500ms+ScalableReliableMulticast(SRM)Hoststime-outbasedondistancefromsenderToavoidimplosionMcastrepairrequest(andrepair)toallAllhostscanreply(timersstopimplosion)RetransmissionDiscussionInatypicalmulticastsession,canhaveeverypacketusuallylostbysomereceiverWillalwaysretransmitatleastonceFECmaysavebandwidthTypically,crossoverpointtoFECbasedonlossrateSomeparticipantsmaynotbeinteractiveUseretransmissionOthersuseFECRetransmissionAdvantagesandDisadvantagesAdvantagesWellunderstoodOnlyaddadditionaldata‘a(chǎn)sneeded’DisadvantagesPotentiallylargedelaynotusuallysuitableforinteractiveapplicationsLargejitter(differentfordifferentreceivers)Implosion(settingtimersdifficult)MediaRepairTaxonomyDonotrequireassistanceofSenderReceiverrecoverasbestitcanOftencalledErrorConcealmentWorkswellforsmallloss(<15%),smallpackets(4-40ms)Notasubstituteforsender-basedRatherusebothReceiverbasedcanconcealwhatisleftMediaRepairSenderBasedReceiverBasedTaxonomyofErrorConcealmentWhenpacketislost,replacewithfill-inSplicingSplicetogetherstreamoneithersideDonotpreservetimingAdvantage“Easy,peazylemon-squeezy”Worksokforshortpacketsof4-16msDisadvantageCrappyforlossesabove3%CaninterferewithdelaybufferingSilenceSubstitutionFillthegapleftbylostpacketwithsilencePreservetimingAdvantageStill“easy,peazylemon-squeezy”Worksgoodforlowloss(<2%)Worksokforshortpacketsof4-16msDisadvantageCrappyforhigherlosses(3%+)Ineffectivewith40mspackets(typical)NoiseSubstitutionHumanpsychsayscanrepairifsound,notsilence(phonemicrestoration)Replacelostpacketwith“whitenoise”LikestaticonradioStillpreservetimingSimilartosilencesubstitutionSendercansend“comfortnoise”soreceivergetswhite-noisevolumerightRepetitionReplacemissingpacketwithpreviouspacketCan“fade”ifmultiplerepeatsovertimeDecreasesignalamplitudeto0Stillprettyeasy,butcanworkbetterAsteptowardsinterpolationtechniques(next)TaxonomyofErrorConcealmentWhenpacketislost,reproduceapacketbasedonsurroundingpackets.InterpolationBasedRepairWaveformsubstitutionUsewaveformrepetitionfrombothsidesoflossWorksbetterthanrepetition(thatusesoneside)PitchwaveformreplicationUserepetitionduringunvoicedspeechanduseadditionalpitchlengthduringvoicedspeechPerformsmarginallybetterthanwaveformTimescalemodifications“Stretch”theaudiosignalacrossthegapGenerateanewwaveformthatsmoothlyblendsacrosslossComputationallyheavier,butperformsmarginallybetterthanothersTaxonomyofErrorConcealmentUseknowledgeofaudiocompressiontoderivecodecparametersRegenerationBasedRepairInterpolationoftransmittedstateState-baseddecodingcantheninterpretwhatstatecodecshouldbeinReducesboundary-effectsTypicallyhighprocessingModel-BasedrecoveryRegenerate‘speech’tofitwithspeechoneithersideVerycomplicated,oftenlanguagedependentSummaryofReceiverBasedRepairQualityincreasedecreasesathighcomplexityRepetitionisat‘knee’incurveOriginalLossRepetitionWaveSubstitution(Boundariesbetter)(BothbadatC)GroupworkCo