One of the ways communication can be characterized is by the number of
receivers targeted by a sender. Traditional communication modes have been
one-to-one or unicast, and one-to-all or broadcast. Among
these two extremes we find multicast, the targeting of a single
data stream to a selected
set of receivers, which may or may not include the sender.
This communication model supports applications where data
and control are split amongst multiple actors, such as updates to
replicated databases, contacting one of a group of distributed servers, and
interprocess communication among co-operating processes.
Multimedia applications communicate using a collection of information formats, such as text and graphics, which can be classified as discrete or time independent media, and audio and video, which can be classified as continuous or time dependent media. Sound and moving images are natural forms of communication between humans, thus support for continuous media is an important goal for enhancing the scope of communications applications. The proliferation of cost effective audio and video hardware for existing workstations, increasingly provided as standard equipment, along with the availability of high speed networks providing the necessary bandwidth for continuous media communications, is expected to establish multimedia applications as standard collaboration tools.
Since multimedia enhances the abilities of computer-based communication in supporting human collaboration, it should be expected that such applications can greatly benefit by the use of multicast capabilities. A canonical example of such interactions arises in video conferencing[1][2]; in addition, groupware[3] and computer supported co-operative work (CSCW)[4] applications, which naturally fit into the multicast model, can be made more effective by incorporating audio and video capabilities.
Efficient multicasting is a fundamental issue for the success of group applications; while in the past it has been regarded as a feature of limited use, mostly provided as an afterthought, it has now been recognized as a very desirable service[5][6] for emerging high speed networks. Since a major market for the emerging Broadband Integrated Services Digital Network (B-ISDN) is expected to be selective video distribution[7][8], with selective multicasting taking the place of indiscriminate broadcasting so as to reduce the waste of resources caused by transmitting too many channels to a limited number of receivers, the interaction between multimedia and multicasting is of special importance.
In this paper we give an overview of the issues arising from the interaction of multicasting and multimedia communications and attempt to present some proposed solutions in perspective. While multicasting and multimedia are by themselves important research topics, we present an overview of the relevant issues first and then show how interrelations between them can pose additional problems for the designer. We also show that there are special case solutions that are of great importance in this context. The problem at hand can be summarized as the efficient transmission of large volumes of multiple, time correlated, delay sensitive streams of continuous media, to a dynamic group of destinations, using a packet switched network.
The remainder of the paper is organized as follows: Section ii discusses the nature and needs of applications communicating continuous media over a packet network, giving an overview of continuous media characteristics, required network support and, last, a presentation of hierarchical coding techniques. Section iii presents the general problems related to multicasting in a packet network, such as group dynamics, routing support, dealing with feedback control and hardware support for multicasting. Section iv brings multimedia and multicasting together, discussing host and network heterogeneity, resource reservations, and extending the previous discussions on routing and feedback control in this context.