JournalofMobileMultimedia,Vol.5,No.1(2009)012 - PDF document

JournalofMobileMultimedia,Vol.5,No.1(2009)012–028c\rRintonPressNARROWCASTINGFORARTICULATEDPRIVACYANDATTENTIONINSIPAUDIOCONFERENCINGSABBIRALAMR&DDept.,MobileTechnikaInc.Shinjuku-ku,Tokyo162-0845;JapanE
mail:sabbir@mobiletechnika.jpMICHAELCOHEN&JULI´ANVILLEGAS S.Alam,M.Cohen,J.Villegas,andA.Ahmed13andattend—toprovidedistributedprivacyinSIP-basedconferencing.Extensivedevelopmenthasbeencarriedoutintheareaofconferenceandoorcontrol[12][7].Conventionalfeaturesregardingmediaprivacyinconferencesaretypicallylimitedtoschedulingandselectingthespeaker.Advancedconferencingfeaturessuchasadding/deletingparticipants,changinguseragentsormodes(likeswitchingfromadesktoptoamobilephone),changingmedia,authen-ticatingorauthorizingparticipants,grantingprivileges,controllingpresentationofmedia,sidebars,passiveparticipants,whisper/privatemessages,audio-only,andlecturemodearedescribedinRFC4597[9].Mediaprivacyfeaturesallowparticipantstocontroltheirowninformationandtodistributetheirattention,basedonsecrecy,anonymity,andsolitude[17].ACallWhisper[4]featureallowsaparticipanttotalkprivatelytooneormoreparticipantsinagroup.Thiswalkie-talkie-likefeaturecreatesaone-wayvoiceorvideocommunicationforalimitedperiodoftime.Thesessionterminateswhenthecontrollerreleasesthe“PTT”(pushtotalk)button,sosuchasystemisnotpracticalwhenalongersessionortwo-waycommunicationsessionisnecessary.VoiceChat[20][11]allowsparticipantstocreateoneormoreprivateaudioconferences:althoughthecommunicationchannelintheprivatevoicechatgroupprovidestwo-waycommunication,participantscanhearthemainconferenceatlowvolume.Privateconversation[19]offersaprivatevideo,voice,andtextconversationsessioninsideamainconference.ItissimilartoaCallWhisperfeature,butaddstwo-waycommunicationcapabilityandtextmessaging.InaWebExaudioconference,aconferencechairpersoncanselectivelydisablethemicrophonestoallowonlycertainattendeestospeak.An`audio-only'optionallowsamoderatortorevokeandrestorespeakingprivilegestoattendees,sothatmutedattendeescanlistenbutnotspeak.WebExparticipantscanhaveaprivatechatwithsomeoneduringameeting.WhisperCoaching(www.audiocodes.com)allowsasupervisortolistentoamainconferenceconversationwhiletalkingtoaselectedsetofparticipantsattheconference.Theprivacycontrolallowedbytheseapplicationsisratherblunt.Inordertobettercontrolmediaprivacy,weexploretheconceptofnarrowcasting[10],designforprototypingindecentralized[2]andcentralized[1]modelswithacollaborativevirtualenvironment(CVE)[3].Inthisarticle,wedescribeamechanismandinstanceof“MediaServerComponentModel”architectureforpolicy-basedme-diamixingwithacentralizedmediamixerwithinthestandardSIP[15]frameworkformultimediaconferencingsystems.Wehavedenedmediaprivacycommandsanddevelopedapolicyevaluationalgorithm,amediamixinganddeliverymechanismconsideringpolicyconguredbyconferencepar-ticipants.Wedeployfournarrowcastingcommands—mute,deafen,select,andattend—toprovidedistributedprivacy.1.1MediaPrivacyControlandLimitationsMuteisapopularfeatureformediaprivacy.Ithasthreedifferentvarieties:self-mute,PBX-mute,andnarrowcastingmute,juxtaposedinTable1.Self-muteallowsausertowithholdhismediastreamsfromotherparticipants.InPBX-mute,acontrollerdisablesaparticipant'soutgoingmediatootherparticipants.NarrowcastingmutereferstoP2Pcontrolwithwhichaparticipant(controller)canselectanotherparticipant(controllee)todisallowthecontrollee'smediatowardsthecontroller.Acallcenterscenarioprovidesanotherexampleofmediaprivacy:ininstanceswhenarst-tieragentcannotansweracustomer'squestions,theagentmighthaveaprivateside-channelcommuni-cationwithasupervisorasback-upforrealtimecustomersupport,asshowninFig.1(a).PrivacycontrolisinvokedsothattheSupervisor'smediagoesonlytotheagent,nottothecustomer,asshowninFig.1(b).Traditionalconferencingsystemsdonotgenerallyprovidesuchfeatures.Forinstance, S.Alam,M.Cohen,J.Villegas,andA.Ahmed15 Supervisor Agent CustomerCallCenter (a) Mixer Supervisor Agent Customer Agent+Customer Supervisor+Customer Agent(b)Fig.1.MediaPrivacy:ACallCenterApplicationScenario PolicyServer F M P1 P2 P3 P4 ConferenceServerLegend:Signaling Mediastreams Mixercontrol Fig.2.TypicalConferenceArchitectureencesystem(Fig.2).ASIPconferenceservercomprisesafocus,policyserver,andmediamixer.Thefocushandlestheconferencecontrol—creating,modifying,andterminatingconferences.Conferencepolicyismanagedbythepolicyserver,whichcancongureamediaserver.Mixinganddistributionofmediastreamsarethemainfunctionsofamediamixer,suchasa“voiceswitch”foraudioconferenc-ing,whichtransmitssomecompositesignalstotherespectiveterminals,assuggestedbythemultiplearrowheadsonthe(dashed)returnvectors.Value-addedservices—suchasmonitoringconferencestatus,participantstatus,andbilling—canbeimplementedinsideoroutsideofthisframework.2.1SIPConferenceModelTherearetwogenericconferencemodels:looselyandtightlycoupled.Inalooselycoupledmodel,thereisneitheracentralpointofcontrolnoraconferenceserver,whereasinatightlycoupledmodel,acentralizedconferencecontrolservermanagestheconferences.Thetightlycoupledconferencingmodelcanbefurtherclassiedintosixdifferenttypesdependingonthelocationofthefocusandthemixer,asillustratedinFig.2,includingtheMediaServerComponentModelusedforourproof-of-concept.ThesemodelsaredetailedbyJ.Rosenberg[13]andY.Choet.al.[5]. 18NarrowcastingforArticulatedPrivacyandAttentioninSIPAudioConferencingTable2.NarrowcastingCommands P1mutesP2P1deafensP2P1selectsP2P1attendsP2 MediaVectors P1 P4 P2 P3 P1 P4 P2 P3 P1 P4 P2 P3 P1 P4 P2 P3 Semantics Blockthemediastreamcomingfromasource.Blockmediastreamsgoingtoasink.Limittheprojectedsoundtoparticularsources.Limitthereceivedsoundtoparticularsinks. Situation Participantwantstoblockmediafromspe-cicparticipants.Participantwantstoblockmediatospecicparticipants.Participantwantstore-ceivemediaonlyfromparticularparticipants.Participantwantstosendmediatoonlyspecicparticipants. FigurativeAvatars MobileIcons 44+4+4+ MediaDistribution 2664111011111113775266401111111113775266411111100113775266410011111113775 20NarrowcastingforArticulatedPrivacyandAttentioninSIPAudioConferencingbyasinglehumanuserofpossiblymultipleiconicrepresentativesinaninterface.Suchcomplicatedsubtletiesarebeyondthescopeofthisarticle.)3.2DeafenDeafenisasink-relatedmediaprivacycommandthatblocksmediastreamstoaselectedparticipant.Forexample,ifBob(P1)wantstosharehismediawitheveryoneinaconferenceexceptAlice(P2),thenAlicewillnotreceiveanystreamsfromBobifBobdeafensAlice.(Transposingtheparticipantsonecanrealizeanequivalentoperation,P2mutesP1.)ThesecondcolumnofTable2showsthemediarelationshipamongfourparticipants.PjnXi=1PiPjPawhenPj=Po;whenPa=Po;nXi=1PiPjotherwise:(3)Againinthisexample,a=1ando=2.3.3Select(Solo)Theprivacycommandselectlimitsreceivedmediatoparticularsources.Forinstance,studentsmightselectateachertoavoiddistractions.P1willreceivemediaonlyfromP2ifP1selectsP2,implicitlymutingthecomplementoftheselection.ThethirdcolumnofTable2showsthemediarelationshipsamongfourparticipants;twovectorsaredisabledinthiscase.PjPowhenPj=Pa;nXi=1PiPjotherwise:(4)3.4AttendAttendistheotherincludingcommandformediaprivacy,limitingreceivedsoundtoaparticularrecipient.IfAliceattendsBob,onlyBobwillhearAlice,sinceotherparticipantsareimplicitlydeafened.TherightmostcolumnofTable2showsthemediarelationshipamongfourparticipants;againtwomediavectorsaresuppressed.PjnXi=1PiPjwhenPj=Po;nXi=1PiPjPaotherwise:(5)4SystemDesignandImplementationThemainrequiredfunctionsformediapolicycongurationandcontrolarepolicyconguration,pol-icyevaluation,andmediamixinganddistribution.TheMediaServerComponentModel(toprightof(Fig.2)selectedforourimplementationcomprisesacentralizedfocus(collocatedwiththepolicyserver),acentralizedmixer,andparticipants.Thearchitecture,elaboratedinFig.6,embedspolicyconguration,mediamixing,andaCVEinterfacewithinaSIPframework.Allthecomponentsinthis 22NarrowcastingforArticulatedPrivacyandAttentioninSIPAudioConferencingTable3.PolicyMatrixP=[pij] P1P2:::Pn P1P1(P1P2):::P1(P1Pn) P2(P1P2)Pn(P1Pn) P2P2(P2P1):::P2(P2Pn) P1(P2P1)Pn(P2Pn) ............... PnPn(PnP1)Pn(PnP2)::: P1(PnP1)P2(PnP2) Sendingrequeststoamediamixerfornecessarymediamixing:Aftercompilingthemediapoli-cies,thesystemdetermineswhichmediastreamsneedtobemixedanddeliveredtowhom.UsingstandardMSCMLthepolicyserverinstructsthemediamixertoperformthenecessarymixing.4.3MediaMixingandDistributionThemediaserverreceivesMSCMLrequestsfromapolicycongurationserver.Accordingtotheaccu-mulatedstate,itperformsthenecessarymixinganddeliversthesestreamstosubscribedparticipants.Themaximumnumberofmixes,thepowersetoftheparticipantsexcludingtheemptyanduniversalsets,isn1Xi=1nCi=2n2:(6)Therefore,forn=3,4,5,themaximumnumberofmixeswouldbe6,14,and30,respectively.However,dependingonparticipants'mediaprivacyrequests,theactualnumberofmixesmightbefewer.Fig.7illustratesnarrowcastingmediadistributionamongfourparticipantswhenP1mutesP2anddeafensP4.Allparticipantssendtheirmediatothemediamixer.Themediaservermixesonlythenecessarystreamsanddeliversthembacktotheappropriaterecipients.4.4MixingCongurationandObservationOurprototypeenvironmentcomprisesaSIPserver(BEAWebLogicSIPServer),anapplicationserver(BEAWebLogicWorkshop),amediaserver(Dialogic/CantataSnowshoreIPMediaServer),andfourSIPclients(X-lite).Weimplementednarrowcastingcommandsmute,deafen,attend,and(partially)select,integratingtheselterfunctionsintotheapplicationserver.Fig.8showsthecontrolandmediastreamsamongaparticipant,applicationserver,andmediamixerwhenapplyinganarrowcastingcommand.ThefollowingtraceshowstheMSCMLcodesentfromthepolicyconguration(application)servertothemediamixerwhenP1deafensP2.Ineachblock,therstchunkistheSIPheadersandthesecondchunk,thebody,istheMSCMLpayload.P1makesaprivategroupwithP3andP4soP1,P3, 24NarrowcastingforArticulatedPrivacyandAttentioninSIPAudioConferencing Participants ApplicationServer MediaServer 1.SIP 2.RTP 3.SIP 4.MSCML 5.RTPFig.8.CommunicationFlowBetweenSIPEntities:Adefaultconguration(1.)establishesanormalsession(2.),butitcanbeadjusted(3.)torecongure(4.)themixesreturnedtotheparticipants(5.).Content-Length:281?xmlversion="1.0"?�.15;銇MediaServerControlversion="1.0"�.15;銇responserequest="configure_leg"code="200"text="OK"�.15;銇teamid="sip:P1@192.168.1.11"numteam="2"�.15;銇teammateid="sip:P3@192.168.1.11"/�.15;銇teammateid="sip:P4@192.168.1.11"/�.15;銇/team�.15;銇/response�.15;銇/MediaServerControl�.15;銇TheMSCMLcongurationandaudibilityareshowninTable4whenP1deafensP2.4.5NarrowcastinginVirtualEnvironmentsVirtualenvironmentsarecharacterized,incontrasttogeneralmultimediasystems,bytheexplicitno-tionoftheposition(locationandorientation)oftheperspectivepresentedtorespectiveusers.Oftensuchvantagepointsaremodeledbythestandpointsanddirectionsofobjectsinavirtualspace.Theserepresentativesmightbemoreorlesssymbolic(abstracticons)orgurative(avatars),butactasdele-gatesofhumanusers.Iconsandavatarscandeifyembodiedvirtuality,treatingabstractpresenceasauserinterfaceobject,symbolsandmanifestationsofsourcesandsinks.Wehavedevelopedworksta-