P. Venkat Rangan

Characterizing user behavior and network performance in a public wireless LAN

This paper presents and analyzes user behavior and network performance in a public-area wireless network using a workload captured at a well-attended ACM conference. The goals of our study are: (1) to extend our understanding of wireless user behavior and wireless network performance; (2) to characterize wireless users in terms of a parameterized model for use with analytic and simulation studies involving wireless LAN traffic; and (3) to apply our workload analysis results to issues in wireless network deployment, such as capacity planning, and potential network optimizations, such as algorithms for load balancing across multiple access points (APs) in a wireless network.

Designing file systems for digital video and audio

We address the unique requirements of a multimedia file system such as continuous storage and retrieval of media, maintenance of synchronization between multiple media streams, and efficient manipulation of huge media objects. We present a model that relates disk and device characteristics to the recording rate, and derive storage granularity and scattering parameters that guarantee continuous access. In order for the file system to support multiple concurrent requests, we develop admission control algorithms for determining whether a new request can be accepted without violating the realtime constraints of any of the requests.We define a strand as an immutable sequence of continuously recorded media samples, and then present a multimedia rope abstraction which is a collection of individual media strands tied together by synchronization information. We devise operations for efficient manipulation of multi-stranded ropes, and develop an algorithm for maintaining the scattering parameter during editing so as to guarantee continuous playback of edited ropes.We have implemented a prototype multimedia file system, which serves as a testbed for experimenting with policies and algorithms for multimedia storage. We present our initial experiences with using the file system.

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>>

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.

A window-based editor for digital video and audio

The authors present a window-based editor for manipulating digital video and audio. The editor supports real-time recording, playback, and editing (cut, copy, and paste) of several multimedia objects. Using the X Window system, the authors have implemented the editor on an environment of Sun SPARCstations, and PC-ATs equipped with video compression hardware. The user interface of the multimedia editor consists of a main editing window for each display device, and rope windows, which represent synchronized sequences of digital video and audio being accessed, called ropes. Experience shows that the editor provides a simple and easy to use, but powerful system for multimedia document preparation, and it can act as a basis for supporting applications such as multimedia mail, electronic distribution of television news and video entertainment, etc.<<ETX>>

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.

Admission Control Algorithm for Multimedia On-Demand Servers

Future advances in networking coupled with the rapid advances in storage technologies will make it feasible to build a multimedia on-demand server on a metropolitan-area network. However, the economic viability of such a multimedia on-demand server is dependent on the ability to amortize its operational costs over a large number of clients. In this paper, we present a taxonomy of policies for servicing multiple clients, and propose round robin and quality proportional admission control algorithms for determining whether a retrieval request from a new client can be admitted without violating the real-time requirements of any of the clients already being serviced. We evaluate the performance of various servicing policies, and show that they are an order of magnitude scalable compared to straightforward multiplexing techniques such as servicing one subscriber per disk head.

Video conferencing, file storage, and management in multimedia computer systems

Abstract Technological advances in networking and storage are revolutionizing computers so as to support digital multimedia. In this paper, we present a video conferencing system that can form the basis for supporting most video interactions and services in distributed computing systems. We present powerful paradigms for collaboration such as conferences using multiple channels of video and audio, hierarchically and non-hierarchically related conferences, and develop a model and software architecture for supporting these paradigms in multimedia computer systems. We describe a implementation of the architecture based on a general event-driven message exchange mechanism, and investigate the resulting issues of control synchronization among agents. We have developed a video file server that isintegrated with the conferencing architecture to provide storage and retrieval of full-motion video information within conferences. The conferencing and file systems are integrated at the agent-server level (above the operating systems, but below the user interface). The performance of the system, which has been in operation for several months, is highly satisfactory, showing the feasibility of adding a video dimension to high-performance computing.

Frame-Induced Packet Discarding: An Efficient Strategy for Video Networking

In order to provide efficient frame loss guarantees for video communication over ATM-like fast packet switched networks, we propose a simple to implement, yet effective, strategy called Frame-Induced Packet Discarding (FIPD), in which, upon detection of loss of a threshold number (determined by an applications video encoding scheme) of packets belonging to a video frame, the network attempts to discard all the remaining packets of that frame. Performance simulations are shown to demonstrate the efficacy of the FIPD strategy; networks employing FIPD exhibit close to two-fold increase in the number of video channels that they can support.