From Requirements to Services: A Study on Group Communication Support for Distributed Multimedia Systems MPG-95-10

In recent years computers have developed very rapidly from simple processing machines to sophisticated communication systems employing multiple media. Computers are imcreasingly used for all kinds of man-machine and interpersonal communication and co-operation. Many of these applications involve multiple communicating entities as well as multimedia. However, in general no sufficient support for multimedia group communication is yet available. In this paper we first analyse existing systems offering group support at different levels of the system and communication architecture. Subsequently a study on multimedia group application requirements is described. It is mainly concerned with highly interactive applications such as CSCW. The study is based on the examination of existing systems, system scenarios and ethnographical studies. To describe the multitude of characteristics and requirements of these applications in a structured and comprehensive way a set of group application characteristics is defined. Following this, general application requirements are discussed in detail. We concentrate on the special needs of multimedia group applications for the support of multipeer communication and the management of groups. However, to complete the discussion other, more general requirements are introduced as well. In the second part of the paper a set of services offered at an application platform to support group communications is introduced. Because of the complexity of the requirements not all potential required services for every individual application can be considered. To overcome this and to offer flexible, easily extensible and adaptable services we propose a modular service structure based on the objectoriented paradigm. Two services, the group data transmission service and the group management service, are specified in more detail. The group data transmission service offers support for the establishment and maintenance of multiple connections between a group of senders and a group of receivers. This includes joint QoS management of the individual connections. The service resides on top of a connection oriented multicast transport protocol which itself relies on multicast provided in the network layer by protocols like RSVP or ST-II. The task of the group management service is twofold. First, it offers the necessary functions to manage and administer application level groups and their members. Second, it retains the knowledge about the group, its dependencies and relations over layer boundaries to offer effective and efficient group communication. Therefore the task and the location of a group manager in the system architecture is not bounded to any particular layer. It is required by service users located at different places in the architecture. Finally the paper discusses related work in the areas of telecommunications and multimedia conferencing.