多媒体应用的支持系统已从本地系统发展到了分布式系统。例如公用信息查询台、多媒体电子邮件、协同工作系统、分布式虚拟现实等应用都需要高速网络、高传输率和带有适当的传输协议的通信系统。从通信角度看，我们将多媒体通信系统MCS分成两大子系统：应用子系统和传输子系统。Theconsiderationofmultimediaapplicationssupportstheviewthatlocalsystemsexpandtowarddistributedsolutions.Applicationssuchaskiosksmultimediamailcollaborativeworksystemsvirtualrealityapplicationsandothersrequirehigh-speednetworkswithahightransferrateandcommunicationsystemswithadaptivelightweighttransmissionprotocolsontopofthenetworks.FromthecommunicationperspectivewedividethehigherlayersoftheMultimediaCommunicationSystemMCSintotwoarchitecturalsubsystems:anapplicationsubsystemandatransportsubsystem.ApplicationSubsystemCollaborativecomputing现在工作站和PC机都配有很强的音频和视频功能若把它们用高速网络联在一起，那么就为人们提供了一个协同计算环境，协同计算通常又称为计算机支持的协同工作CSCW。ThecurrentinfrastructureofnetworkedworkstationsandPCsandtheavailabilityofaudioandvideoattheseend-pointsmakesiteasierforpeopletocooperateandbridgespaceandtime.Inthiswaynetworkconnectivityandend-pointintegrationofmultimediaprovidesuserswithacollaborativecomputingenvironment.CollaborativecomputingisgenerallyknownasComputer-SupportedCooperativeWorkCSCW.CollaborativeDimensions电子化的协同工作系统可根据时间、用户范围和控制这三个参数进行分类WSM91。这样协同工作系统可以被划分一个三维空间（如图11.1所示）。Electroniccollaborationcanbecategorizedaccordingtothreemainparameters:timeuserscaleandcontrolWSM91.Thereforethecollaborationspacecanbepartitionedintoathree-dimensionalspaceshowninfigure11.1.ConferenceRoomMeetingFace-to-FaceMeetingGroupCSCWTelephoneDesktopSynchronousGroupTeleconfereningDecisionTele-MentoringCollaborativemakingSoftwareTimeElectronicDevelopmentCentralizedAsynchronousMailCollaborativeDocumentGroupEditingBulletinBoardScheduleControlDistributedManageSingleUserMultipleUsersGroupUserScaleFigure11.1:Dimensionsofcollaborativecomputing.Time从图中可以看出，在时间轴上有两种模式。即同步模式和异步模式。异步协同工作说明处理活动在同时发生。Withrespecttotimetherearetwomodesofcooperativework:asynchronousandsynchronous.Thesynchronouscooperativeworkhappensatthesametime.UserScale用户轴上的参数说明是否为单用户协同工作，或者是两个用户以上的组与另一组协同工作。Theuserscaleparameterspecifieswh