多媒体通信系统 应用子系统

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.Applicationssuchaskiosks,multimediamail,collaborativeworksystems,virtualrealityapplicationsandothersrequirehigh-speednetworkswithahightransferrateandcommunicationsystemswithadaptive,lightweighttransmissionprotocolsontopofthenetworks.Fromthecommunicationperspective,wedividethehigherlayersoftheMultimediaCommunicationSystem(MCS)intotwoarchitecturalsubsystems:anapplicationsubsystemandatransportsubsystem.ApplicationSubsystemCollaborativecomputing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)。ThecurrentinfrastructureofnetworkedworkstationsandPCs,andtheavailabilityofaudioandvideoattheseend-points,makesiteasierforpeopletocooperateandbridgespaceandtime.Inthisway,networkconnectivityandend-pointintegrationofmultimediaprovidesuserswithacollaborativecomputingenvironment.CollaborativecomputingisgenerallyknownasComputer-SupportedCooperativeWork(CSCW).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[WSM+91]。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:time,userscaleandcontrol[WSM+91].Therefore,thecollaborationspacecanbepartitionedintoathree-dimensionalspace(showninfigure11.1).TimeSynchronousAsynchronousTelephoneTele-MentoringDesktopTeleconfereningCollaborativeSoftwareElectronicMailBulletinBoardDevelopmentCollaborativeDocumentEditingGroupScheduleManageSingleUserMultipleUsers(Group)DistributedControlCentralizedFace-to-FaceMeetingGroupCSCWConferenceRoomMeetingGroupDecisionmakingUserScaleFigure11.1: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。Withrespecttotime,therearetwomodesofcooperativework: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。Theuserscaleparameterspecifieswhetherasingleusercollaborateswithanotheruseroragroupofmorethantwouserscollaboratetogether.Control控制轴上分成集中式和分布式两种.在集中式控制中有一个主席，由他来控制协同工作，而在分布式控制中，每个组成员能控制协同工作中属于自己的任务.Controlduringacollaborationcanbecentralizedordistributed.Centralizedcontrolmeansthatthereisachairman(e.g.,mainmanager)whocontrolsthecollaborativework.Distributedcontrolmeansthateverygroupmemberhascontroloverhis\herowntasksinthecollaborativeworkanddistributedcontrolprotocolsareinplacetoprovideconsistentcollaboration.GroupCommunicationArchitecture组通（GC）是指CSCW环境下多个用户之间的通信，协作可以是同步的，而其中控制可以是集中式或分布式的。Groupcommunication(GC)involvesthecommunicationofmultipleusersinasynchronousoranasynchronousmodewithcentralizedordistributedcontrol.组通信结构由支撑模型、系统模型和接口型[WSM+91]。组通信支持模式包括了通过多点传输网络通信的组通信代理（如图11.2）。它可用于完成组汇合（grouprendezvous）、应用共享和会议管理。Agroupcommunicationarchitectureconsistsofasupportmodel,systemmodelandinterfacemodel[WSM+91].TheGCsupportmodelincludesgroupcommunicationagentsthatcommunicateviaamulti-pointmulticastcommunicationnetworkasshowninFigure11.2.Groupcommunicationagentsmayusethefollowingfortheircollaboration:GroupRendezvous;SharedApplications;Conferencing.MulticastCommunicationNetworkGroupRendezvousCommunication(Transport)SupportConferencingApplicationSharingGroupCommunicationAgentGroupCommunicationAgentFigure11.2:Groupcommunicationsupportmodel.GroupRendezvousGroupRendezvousGroupCommunicationAgentCommunication(Transport)SupportCommunication(Transport)SupportConferencingConferencingApplicationSharingApplicationSharing组通信系统模型基于客户机－服务器(client-server)模式，其中client提供组成员与系统间进行交互的用户界面，server则向用户提供组通信功能，每一服务器都有它的独特的功能。TheGCsystemmodelisbasedonaclient-servermodel.Clientsprovideuserinterfacesforsmoothinteractionbetweengroupmembersandthesystem.Serverssupplyfunctionsforaccomplishingthegroupcommunicationwork,andeachserverspecializesinitsownfunction.组通信接口模型包括用于在组通信支撑模型内交换信息的两种协议，即用户表现协议和组工作管理协议。用户表现协议执行行客户机间的交互，例如发起会议、结束会议、动态加入和离开会议以及发方言权控制等。组工作管理协议说明客户和服务器之间的通信。活动会议登录和会议信息检索等服务都受这些协议的支持。TheGCinterfacemodelincludestwokindsofprotocolsforexchanginginformationwithintheGCsupportmodel:userpresentationprotocolsandgroupworkmanagementprotocols.Userpresentationprotocolsperforminteractionsamongtheclients,suchasopeningaconference,closingaconference,dynamicjoiningandleavingofameetingandfloorpassing,Groupworkmanagementprotocolsspecifythecommunicationbetweentheclientsandtheservers.Servicessuchasregistrationofactiveconferencesandqueriesforfurtherconferenceinformationaresupportedbytheseprotocols.GroupRendezvous组汇合方法允许建立协同工作组会议，以及提供有关目前和将来的组和会议的其他静态动态信息。组汇合工具在用户界面上提供一组会话和会议的活动信息。组汇合有同步和异步方法。Grouprendezvousmethodsallowforsettingupcollaborativegroupmeetingsandprovidingotherstaticanddynamicinformationaboutgroupsandongoingorfuturemeetings.Thegrouprendezvoustoolsprovideasinglesetofsessionandactivityinformationattheuserinterface.Therearesynchronousandasynchronousmethodsforgrouprendezvous.应用共享被认为是支持组通信活动的一种有效机制。应用共享意味着，当应用共享程序（像如编辑器）从任一参加者执行输入，共享对象（例如文档、正文）的所有执行结果都将发送到所有的参