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1、w 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。 w 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。w The consideration of multimedia applications supports the view that local systems expand toward distributed solutions. Applications such as kiosks, multimed
2、ia mail, collaborative work systems, virtual reality applications and others require high-speed networks with a high transfer rate and communication systems with adaptive, lightweight transmission protocols on top of the networks. w From the communication perspective, we divide the higher layers of
3、the Multimedia Communication System(MCS) into two architectural subsystems: an application subsystem and a transport subsystem. Application SubsystemwCollaborative computing w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)。w The current infrastructure of networked
4、 workstations and PCs,and the availability of audio and video at these end-points,makes it easier for people to cooperate and bridge space and time. In this way, network connectivity and end-point integration of multimedia provides users with a collaborative computing environment. Collaborative comp
5、uting is generally known as Computer-Supported Cooperative Work (CSCW).wCollaborative Dimensions w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）。 w Electronic collaboration can be categorized according to three main parameters: time, user scale and control WSM+91. Therefore,
6、 the collaboration space can be partitioned into a three-dimensional space (shown in figure 11.1).TimeSynchronousAsynchronousTelephoneTele-MentoringDesktopTeleconfereningCollaborative SoftwareElectronic MailBulletin BoardDevelopmentCollaborative Document EditingGroup Schedule ManageSingle UserMultip
7、le Users (Group)DistributedControlCentralizedFace-to-Face MeetingGroup CSCWConference Room MeetingGroup DecisionmakingUser ScaleFigure 11.1: Dimensions of collaborative computing.Time w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。 w With respect to time ,there are two modes of cooperative work:
8、 asynchronous and synchronous. The synchronous cooperative work happens at the same time.w User Scalew 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。 w The user scale parameter specifies whether a single user collaborates with another user or a group of more than two users collaborate together.w Control
9、w 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. w Control during a collaboration can be centralized or distributed. Centralized control means that there is a chairman (e.g., main manager) who controls the collaborative work. Distributed control means that every group membe
10、r has control over hisher own tasks in the collaborative work and distributed control protocols are in place to provide consistent collaboration.w Group Communication Architecture w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。w Group communication (GC) involves the communication of multiple
11、users in a synchronous or an asynchronous mode with centralized or distributed control .w 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（group rendezvous）、应用共享和 会议管理。w A group communication architecture consists of a support model, system model and interface model WSM+91 .
12、 The GC support model includes group communication agents that communicate via a multi-point multicast communication network as shown in Figure 11.2. Group communication agents may use the following for their collaboration:Group Rendezvous; Shared Applications; Conferencing.Multicast Communication N
13、etworkGroup RendezvousCommunication (Transport) SupportConferencingApplication SharingGroup Communication AgentGroup Communication AgentFigure 11.2: Group communication support model.Group RendezvousGroup RendezvousGroup Communication AgentCommunication (Transport) SupportCommunication (Transport) S
14、upportConferencingConferencingApplication SharingApplication Sharingw 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。w The GC system model is based on a client -server model. Clients provide user interfaces for smooth interaction between group member
15、s and the system. Servers supply functions for accomplishing the group communication work, and each server specializes in its own function.w 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。wThe GC int
16、erface model includes two kinds of protocols for exchanging information within the GC support model: user presentation protocols and group work management protocols. User presentation protocols perform interactions among the clients, such as opening a conference, closing a conference, dynamic joining and leaving of a meeting and floor passing, Group work management protocols specify the communication between the clients and the servers. Services such as registration of ac
