Improving bandwidth efficiency on video-on-demand

ImprovingBandwidthEfficiencyonVideo-on-DemandServers†StevenW.CarterandDarrellD.E.Long‡DepartmentofComputerScienceUniversityofCalifornia,SantaCruzSantaCruz,CA95064Abstract.Video-on-demand(VOD)servershavealimitedamountofbandwidthwithwhichtoserviceclientrequests.ConventionalVODserversdedicateauniquestreamofdataforeachclient,andthatstrategycanquicklyallocatealloftheavailablebandwidthontheserver.Wedescribeasystemcalledstreamtappingthatallowsclientsto“tap”intoexistingstreamsontheVODserver.Byusingexistingstreamsasmuchaspossible,clientscanreducetheamountofnewbandwidththeyrequire,andthatallowsmoreclientstousetheserveratonce,reducingclientlatency.Streamtappinguseslessthan20%ofthebandwidthrequiredbyaconventionalVODserverforpopularvideos,anditperformsbetterthanmanyotherstrategiesdesignedtoimproveVODservers.Keywords:Video-on-demand–Efficiency–Bandwidth1IntroductionAtsomepointinthefuture,video-on-demand(VOD)willallowclientstoturnontheirtelevisionsets,connecttoaVODserverbywayofatelevisionset-topbox(STB),selectavideofromtheserver’svideolibrary,andthenbeginviewingthevideoinstantaneously.Thatpointisstillsometimeoff.AlthoughnumerouscompanieshaveconductedVODtrialsandsponsoredmarketteststhatshowthepublicisveryreceptivetotheideaofVOD(Perry,1996;McCarthy,1997),thesecompanieshavenotbeenabletocreateacommerciallyavailablesystem.Thereason,almostunanimously,iscost.TimeWarner,forexample,spentover$250milliononitsFullServiceNetwork(whichprovidedVODandmore)beforeendingtheprojectbecausethesystemjustwasnotyeteconomicallyviable(Goldstein,1997).Sowhileitisalwaysimportanttoimproveefficiency,itiscriticalwithVOD.Strategiesthatcanmakeexistinghardwaremoreefficientandreducetheneedforadditionalhardware(andcost)maybeenoughtohelpVODcompaniessucceedwheretheyarecurrentlyfailing.Onesuchstrategyiscalledstreamtapping(CarterandLong,1997),anditimprovesthebandwidthefficiencyofaVODserver.Streamtappingaccomplishesthisbyallowingclientsto“tap”intoanystreamofdataontheVODserverthatisdisplayingdatatheclientcanuse.Byusingexistingstreamsasmuchaspossible,clientsrequiretheirownstreamofdataforshorterperiodoftime,makingthemlessofaburdentotheVODserver,andtheexistingstreamsareabletoservicemoreclients,makingthemmoreefficient.However,sinceclientSTB’swillbeabletoreceivedataoutoforder,theywillrequireasmallamountofbufferspaceinwhichtostorethedatauntilitisneeded.Thisbufferspaceneednotbelarge,anditshouldnotaddgreatlytothepriceoftheSTB.†ThisextendsworkpreviosulyreportedinSixthInternationalConferenceonComputerCommunicationsandNetworks(CarterandLong,1997).‡ThisresearchwassupportedbytheOfficeofNavalResearchunderGrantN00014–92–J–1807.TrueProactiveClientClientSavesonversusversusBandwidthBufferSizeNetworkSystemNearVODReactive(streams)(minutes)BandwidthConventionalTrueReactive10NoBatchingTrueReactive10YesDelayedBatchingNearReactive10YesStaggeredBroadcastingNearProactive10YesPyramidBroadcastingNearProactive2–330YesSkyscraperBroadcastingNearProactive35–40YesHarmonicBroadcastingNearProactive4–640YesPiggybackingTrueReactive10YesIntervalCachingTrueReactive10NoAsynchronousMulticastingNearReactive3+10–30YesStreamTappingTrueReactive2–410–30YesTable1:Asummaryofkeypointsforseveralefficiency-improvingsystems.Aconventionalsystem,ontheotherhand,doesnotmakeefficientuseoftheVODserver.Infact,itdoesnotallowanysharingofdatabetweenclients;eachclientsimplyreceivesitsownuniquestreamofdata,andthisstrategycanquicklyallocateallofthestreamsavailableontheVODserver.Whenmeasuringefficiency,wewillusetwometrics:•Serverbandwidth:theaveragenumberofstreamsusedbytheVODserver.Alternatively,wemayuseaverageservicetime(AST)whichistheaveragebandwidth,instreams,usedbytheVODservermultipliedbytheamountoftimetheserverhasbeenrunninganddividedbythenumberofclients.•Clientlatency:theaverageamountoftimeaclientmustwaitbeforeitsrequestisservicedbytheVODserver.Insomewaysthisisthemostimportantmetricofthetwobecauseitistheonlyoneclientswillsee.WeclassifyVODserversintwoways.AserverprovidestrueVODifitneverdelaysaclientrequestwhenithasthebandwidthavailabletoserviceit.Iftheserverdelaysrequests,thenitprovidesnearVOD.AservercanalsoprovideinteractiveVODifitallowsclientstomanipulatethedisplayofthevideothroughtheuseofVCRcontrolssuchaspause,fastforward,andrewind.Wehavenotyetmodifiedstreamtappingtosupportinteractivity,butitisatopicweplantopursueinthefuture.AVODservercanalsobereactiveorproactive.Iftheserverrequiressomeamountofaprioriinformationaboutthevideosandtheirpopularitythenitiscalledproactive.Forexample,withsomesystemstheserviceprovidermustreserveaspecificamountofbandwidthinadvanceforpopularvideos.Thatmeanstheprovidermustknowwhichvideosaregoingtobepopularandhowmuchbandwidthshouldbereservedforeach.Ifnoinformationaboutthevideosisrequired,theserveriscalledreactive.WecontendthattrueVODisbetterthannearVOD,andareactivesystemisbetterthanaproactiveone.Theseclassificationsrepresent,respectively,theidealnatureofVODandthesimplestformofmaintenance.Streamtappingisatrue,reactiveVODsystem.2RelatedWorkOverthepastfiveyears,severalgroupsofresearchershavepresentedsystemsforimprov