Feedback Techniques for Intra-Media Continuity and Inter-Media Synchronization in Distributed Multimedia Systems

Future advances in networking and storage will make it feasible to build multimedia on-demand servers that provide services similar to those of a neighborhood videotape rental store over metropolitan area networks such as B-ISDN. Such multimedia servers can support real-time retrieval of multimedia objects by users onto their ISDN videophones and audiophones for playback. The design of techniques and protocols for providing continuous and synchronous access to multimedia services constitutes the subject matter of this paper. In future integrated networks, mediaphones that possess bare minimum capability to playback media but which lack the sophistication to run elaborate time synchronization protocols may be connected directly to the network. We present rate-based feedback strategies by which, during retrieval, a multimedia server uses light-weight messages called feedback units transmitted periodically by mediaphones, to accurately estimate the playback instants of media units. Using these estimates, the multimedia server detects impending playback discontinuities due to buffer overruns or starvations at mediaphones, and preventively readjusts media transmission so as to avoid either of these anomalies. Given the available buffer sizes at mediaphones, we present methods by which a multimedia server can determine the minimum rate at which feedback units must be transmitted to it by the mediaphones, so as to maintain continuity of media playback. In order to guarantee synchronous playback at mediaphones, we first propose a bounded buffering technique which uses buffering limitations at the slave mediaphones to automatically enforce bounds on the asynchrony among mediaphones. Although simple and easy to implement, this technique may entail a large average asynchrony, in order to avoid which we propose a multiple feedback synchronization technique. We present initial performance comparisons of the effectiveness of these synchronization techniques. The techniques for intra-media continuity and inter-media synchronization presented in this paper form the basis of a prototype multimedia on-demand server being developed at the UCSD Multimedia Laboratory. To appear in The Computer Journal, Special Issue on Distributed Multimedia Systems, March, 1993