Srinivas Ramanathan

Designing an on-demand multimedia service

A quantitative study of techniques for designing a high-performance multiuser multimedia on-demand information service is presented. The problem of maintaining continuity of playback of each media stream in the presence of multiple subscriber requests is formulated, and admission control algorithms that permit a multimedia server to satisfy the maximum number of subscribers simultaneously are presented. A feedback technique in which a multimedia service uses lightweight messages called feedback units that are transmitted back to it by subscribers mediaphones to detect asynchrony among them and steer them back to synchrony thereafter is presented. The feedback technique guarantees synchronous playback of media streams transmitted by the multimedia server to subscribers over metropolitan-area networks.<<ETX>>

Adaptive feedback techniques for synchronized multimedia retrieval over integrated networks

The authors have developed intermedia synchronization techniques for multimedia on-demand retrieval over integrated networks in the absence of global clocks. In these techniques, multimedia servers use lightweight messages called feedback units transmitted by media display sites (such as audiophones and videophones, generically referred to as mediaphones) to detect asynchronies among those sites. They present strategies by which the multimedia server can adaptively control the feedback transmission rate from that mediaphone, so as to minimize the associated overheads without permitting the asynchrony to exceed tolerable limits. They compare the performance of various resynchronization policies such as conservative, aggressive, and probabilistic. Performance evaluation of the feedback techniques indicates that their overheads are negligible; for a typical audio/video playback environment, the feedback frequency was about one in a hundred. >

Architectures for personalized multimedia

Personalized multimedia on-demand services are fast evolving from a symbiosis of storage, network, and content providers. A major obstacle to their practical realization is the unprecedented cost of storage and transmission. Architectures and caching techniques can minimize the costs of delivering personalized multimedia programs across metropolitan networks.<<ETX>>

Communication architectures and algorithms for media mixing in multimedia conferences

The problem of media mixing that arises in teleconferencing applications such as teleorchestra is addressed. The mixing algorithm presented minimizes the difference between generation times of the media packets that are being mixed together in the absence of globally synchronized clocks, but in the presence of jitter in communication delays on packet switched networks. In order to support applications, such as teleorchestra, that involve a large number of participants, hierarchical mixing architectures are proposed, and it is shown that they are an order of magnitude more scalable than purely centralized or distributed architectures. Furthermore, mechanisms for minimizing the delays incurred by mixing in various communication architectures are presented. The mixing algorithms are implemented on a network of workstations connected by Ethernets, and the performance of various mixing architectures is experimentally evaluated. The results reveal the maximum number of participants that can be supported in a conference. >

Load management in distributed video servers

We define and formulate various policies for load management in distributed video servers. We propose a predictive placement policy that determines the degree of replication necessary for popular videos using a cost based optimization procedure based on a priori predictions of expected subscriber requests. For scheduling requests, we propose an adaptive scheduling policy that compares the relative utilization of resources in a video server to determine an assignment of requests to replicas. To optimize storage utilization, we also devise methods for dereplication of videos based on changes in their popularities and in server usage patterns. Performance evaluations indicate that a load management procedure which uses a judicious combination of the different policies performs best for most server configurations. Advances in storage technologies are making high performance video servers a reality. These video servers are being deployed over emerging broadband networks to deliver a variety of interactive, digital video services to thousands of residential subscribers. To meet the scalability requirements in such large deployments, distributed video server architectures are being considered (M. Buddhikot and G. Parulkar, 1995). We propose various methods for load management that are targeted at improving the cost effectiveness of distributed video servers.

Feedback techniques for continuity and synchronization in multimedia information retrieval

Future advances in storage and networking technologies will make it feasible to build multimedia on-demand information servers capable of providing services similar to those of a neighborhood videotape rental store over metropolitan area networks. Such multimedia information servers must not only support retrieval of continuous media units (such as video frames and audio samples), but also preserve synchrony among playback of the different media components constituting a multimedia object. We develop techniques for supporting continuous and synchronous retrieval from multimedia servers. We present feedback techniques by which, during retrieval of multimedia objects from a multimedia server to mediaphones, the multimedia server uses lightweight messages called feedback units transmitted periodically back to it (by mediaphones) to detect impending discontinuities as well as asynchronies at mediaphones. The multimedia server then preventively readjusts media transmission so as to avoid either anomaly, and steers the mediaphones back to synchrony. Given the available buffer sizes at mediaphones and the maximum tolerable asynchrony, we present methods to determine the minimum rate at which feedback units must be transmitted so as to maintain both continuity and synchronization. These feedback techniques remain robust even in the presence of playback rate mismatches and network delay jitter, and their initial simulation for video-audio playback yields a feedback rate of one per 1,000 media units to keep the asynchrony within 250ms, showing that the overhead due to feedback transmission is very small. The constant rate feedback techniques developed in this article form the basis of a prototype on-demand information server being developed at the UCSD Multimedia Laboratory.

Hierarchical conferencing architectures for inter-group multimedia collaboration

Advances in computer and communication technologies have stimulated the integration of digital video and audio with computing, leading to the development of various computer-assisted collaborations. In this paper, we propose a multi-level conferencing paradigm (called super conferences) for supporting collaborative interactions between geographically separated groups of users, with each group belonging to possibly a different organization. Hierarchical communication architectures are naturally suited for carrying out media transmission in super conferences. We study the performance of hierarchical communication architectures, and present algorithms for bounding end-to-end delays of real-time media traffic in them. We derive some interesting limits on the number of participants in a group and the number of groups within a super conference, so as not to violate bandwidth and delay requirements of multimedia. At the Multimedia Laboratory at the University of California, San Diego, we have implemented a conferencing system on an environment of Sun SPARCstations and PCATS equipped with digital video and audio processing hardware. As an interesting application of the conferencing system, we have developed a tele-presenter by which users can remotely attend lectures in progress. We present our initial experiences with using the system.

Multimedia information caching for personalized video-on-demand

The synergy between computing and information systems promises to herald a new epoch in which users have access to an entirely new variety of entertainment services that are customized to suit their individual needs. In this paper, we explore the architectural considerations that underlie the efficient realization of personalized video-on-demand entertainment services over metropolitan area broadband networks. To deliver video programmes to users homes at attractive costs, we present intelligent caching strategies that judiciously store media information at strategic locations in the network. For information caching in a metropolitan area network, we devise a simple, yet effective caching strategy that uses a Greedy heuristic to determine when, where and for how long programs must be cached, so as to minimize the cumulative storage and network costs borne by users. Preliminary simulations demonstrate that the Greedy caching strategy performs exceptionally, in most cases providing near-optimal solutions.

Research: Techniques for multimedia synchronization in network file systems

One of the unique features that distinguishes digital multimedia from traditional computer data is the presence of multiple media streams, whose display must proceed in a mutually synchronized manner. The design of techniques for synchronization of multimedia data at the time of storage, and retrieval from network file servers is the subject matter of this paper. We present algorithms by which a file server can create a relative time system and synchronize media units transmitted by different sources on a network to construct a multimedia object. These algorithms stay robust in the absence of global clocks, presence of transmission jitter and generation rate mismatches. We develop a feedback technique using which the file server can detect asynchronies in display devices during retrieval of multimedia objects, and even restore synchrony by deleting or duplicating media units destined for asynchronous destinations. We then present strategies by which the file server can actually predict the time in future when the asynchrony of a device is expected to exceed the permitted bound, and take gradual preventive action to nullify the asynchrony in advance. These algorithms can be generalized to heterogeneous multimedia networks in which there may be variations in sizes of media units generated, differences in network locations of sources and destinations, etc. We are currently implementing these techniques in a digital multimedia on-demand storage server being developed at the UCSD Multimedia Laboratory.

TCP for high performance in hybrid fiber coaxial broad-band access networks

Motivated by the phenomenal growth of the Internet in recent years, a number of cable operators are in the process of upgrading their cable networks to offer data services to residential subscribers, providing them direct access to a variety of community content as well as to the Internet. Using cable modems that implement sophisticated modulation-demodulation circuitry, these services promise to offer a several hundredfold increase in access speeds to the home compared to conventional telephone modems. Initial experiences indicate that cable networks are susceptible to a variety of radio-frequency (RF) impairments that can result in significant packet loss during data communication. In the face of such losses, the transmission control protocol (TCP) that is predominantly used by data applications degrades dramatically in performance. Consequently, subscribers of broad-band data services may not perceive the projected hundredfold increase in performance. We analyze the performance of TCP under different network conditions using simulations and propose simple modifications that can offer up to threefold increase in performance in access networks that are prone to losses. These modifications require only minor changes to TCP implementations at the local network servers alone (and not at subscribers PCs).