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Audio and video data—commonly referred to as "continuous media"—are characterized by their large data volume and continuous playback time, which typically requires storing them in a compressed form. The most popular digital audio and video compression formats include MPEG-1 (for video compact disks), MPEG-2 (for digital versatile disks), and MPEG-4 (for some wireless devices). Even compressed audio and video files can still represent a sizable data volume, and the challenge is to design and implement computer systems capable of storing, processing, distributing, and accessing these large continuous media files within heterogeneous communications environments.
With the rapid development of the Internet and the emergence of many new applications like e-learning, e-commerce, targeted advertising, digital television, and interactive television, many users will benefit from using continuous media. The accompanying sidebar by Richard H. Veith, " Interactive Video: Thirty Years and Counting," provides a historical context for the shift from dedicated video-on-demand systems to Internet-based media-on-demand systems.
How to provide access to the desired continuous media simultaneously to the millions of users who are connected to the Internet at different places becomes a challenging issue. The key technology improvements, other than the Internet, that address this challenge include

    • new compression methods that reduce data size yet maintain its quality,

    • adequate client-side processing power to facilitate software decoding,

    • the availability of broadband services to deliver high-quality continuous media data,

    • advanced technologies for storing large volumes of data, and

    • the widespread adoption of the client-server architecture to ensure the delivery of content across a broad geographic area.

This special issue explores the latest research on these aspects of continuous media on demand.
In "A Scalable and Reliable Paradigm for Media on Demand," Gavin B. Horn and his colleagues discuss strategies for reliably transmitting unicast, multicast, or broadcast media over the Internet. They describe the use of forward error correction and a replication engine to reduce the number of packets transmitted over the network. "Streaming Technology in 3G Mobile Communication Systems" by Ingo Elsen and his coauthors gives an overview of third-generation mobile communication systems and the problems and challenges of streaming multimedia content. This is an area that is becoming increasingly important as content providers seek to meet the needs of users in the rapidly growing mobile market.
The performance of a continuous media server's storage system typically bounds its scalability. Both the storage space capacity and the actual bandwidth limit the amount of data the server can store. Therefore, a single server can support only a limited number of accesses. To support a large number of concurrent accesses, it is clear that a number of servers must duplicate the data. If these servers are located in the same site, all accesses, regardless of their originated locations, must traverse a longer distance to reach the site. This tends to consume a large amount of backbone bandwidth, potentially creating network congestion problems. One alternative is to conserve network bandwidth by duplicating data in a proxy server—a server located in close proximity to the clients. The content a proxy server stores can be dynamically changed to reflect a local community's needs. In "Proxy Servers for Scalable Interactive Video Support," Husni Fahmi and colleagues discuss the idea of caching hotspots at proxy servers to reduce the load on video servers and decrease the random-seek response time. They also describe a simulation study in which they evaluated the hotspot-caching technique.
"Keyframe-Based User Interfaces for Digital Video" by Andreas Girgensohn, John Boreczky, and Lynn Wilcox investigates the problem of how to find relevant information—for example, a specific video segment where a certain event occurred—in a video stream. The authors describe three visual interfaces to help users identify potentially useful or relevant video segments using keyframes—still images automatically extracted from video footage—and discuss corresponding user studies evaluating different design alternatives. In "Streaming-Media Knowledge Discovery," Jan Pieper, Savitha Srinivasan, and Byron Dom point out the lack of a comprehensive resource list for locating streaming media information on the Internet and describe their work on building effective indexing and classification tools for locating streaming media data on the Internet.
We thank everyone who submitted papers, assisted in the review process, and helped to get the accepted papers into their published format. We hope you enjoy this special issue on media on demand.
Jonathan C.L. Liu is an assistant professor in the Department of Computer and Information Science and Engineering at the University of Florida. His research interests include high-speed networks, multimedia communications, parallel processing, and artificial intelligence. Liu received a PhD in computer science and engineering from the University of Minnesota. He is a member of the ACM and the IEEE. Contact him at
David H.C. Du is a US West Chair Professor in the Department of Computer Science and Engineering at the University of Minnesota and a founding member of Streaming21, a developer of video-streaming technologies enabling broadcast-quality video and audio on a large scale over the Internet. His research interests include high-speed networks, multimedia applications, high-performance computing over workstation clusters, database design, and CAD for VLSI circuits. Du received a PhD in computer science from the University of Washington. He is a member of the ACM and an IEEE Fellow. Contact him at
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