Designing an On - Demand Multimedia ServiceP

Future advances in networking coupled with rapid advances in storage technologies will make it feasible to build a multimedia on-demand server capable of providing services similar to those of a neighborhood videotape rental store on a metropolitan-area network. In this paper, we study various admission control policies that permit such a multimedia server to satisfy multiple subscribers simultaneously without violating any of their continuous media playback requirements. We propose a quality proportional policy that retrieves media blocks at a rate proportional on an average to the playback rates of media streams, but uses a staggered toggling technique by which successive numbers of media blocks retrieved are ne-tuned individually to admit and service an optimal number of subscribers simultaneously. This policy permits dynamic additions and deletions of requests in a transparent manner (i.e., without causing discontinuity in the retrieval of any of the existing requests). Performance evaluation shows that the quality proportional policy is an order of magnitude more scalable compared to straightforward admission control policies such as servicing one subscriber per disk head and round robin servicing of subscribers. In order to ensure synchronous retrieval of multimedia objects requested by each of the admitted subscriber, we propose a feedback technique in which a multimedia server uses lightweight messages called feedback units generated by display devices and transmitted back to the server to detect asynchronies during playback. We study various resynchronization policies such as, aggressive, conservative, and probabilistic, and compare their performance for video/audio playback. Whereas aggressive policies perform best only at lower playback rates, conservative policies perform best only at higher playback rates. In contrast, probabilistic policies perform uniformly well at all playback rates. The policies and algorithms for admission control and synchronous retrieval presented in this paper form the basis for a prototype multimedia server being developed at the UCSD Multimedia Laboratory.