PostScript (59 KB)Architecting Video-on-Demand Systems: DAVIC 1.0 and Beyond
Proceedings, International Symposium on Multimedia Communications and Video Coding, Brooklyn, NY, October 1995
Introduction
The culmination and convergence of several different technologies, including video compression and networking, are today making possible the design (and soon the deployment) of commercial interactive video services. Several commercial trials have been announced, are in progress, or are already completed. The multi-disciplinary nature of video-on-demand services, which includes content providers, service providers, network providers, cable TV distribution companies, as well as the computing and consumer electronics industries, has made it extremely difficult to concentrate within a single organization the expertise needed to design and implement such services. Attesting to that is the significant activity that has been created within the industry in terms of alliances and partnerships.
As with any prospective service of such anticipated scale, it is imperative to ensure that open and interoperable systems are used. This not only alleviates the problem of having to design vertical solutions, but also helps to stir healthy competition (technical and economic) within well-defined sub-system domains (e.g., among content providers or network providers). Most importantly, it allows for - and actually encourages - a fast-paced evolutionary path by easing the process with which innovations and new research results can be incorporated: having well-defined interfaces enables the substitution of existing subsystems with more sophisticated or better performing ones, without having to redesign the whole system.
We discuss the design of Video-on-Demand (VoD) systems, focusing on the efforts of the Digital Audio-Visual Council (DAVIC). DAVIC, established in June 1994, is an association of more than 200 industrial and academic organizations from around the world, pursuing the definition of interfaces and protocols for interoperable VoD systems. DAVIC's scope is extremely broad, addressing the complete spectrum of vertical (bottom-up) and horizontal (end-to-end) specifications. It primarily performs a "systems integration" function, interconnecting several different technologies to create a complete and coherent system. As such, it attempts to distill all current experience about the design of VoD systems (actively monitoring and/or collaborating with related activities, such as ISO, ITU, the ATM Forum, IMA, and the IETF), as well as create solutions for problems that still remain unresolved. As a result, several recent developments in standardization activities (both official and industry-based) have been incorporated. This includes the ISO/IEC MPEG-2 and MHEG-5 specifications, ATM Forum and ITU-T specifications on ATM, the recently designed IP Version 6 (most often referred to as IPng, for "next generation"), CORBA 2.0, etc.
DAVIC expects to release its first set of specifications (DAVIC 1.0) in December 1995. A common and very important theme in DAVIC is to define a single solution for each functionality desired, thus eliminating the need to support multiple (and typically conflicting) specifications.
VoD is just one from a number of possible applications that can be supported from a video-capable communication system. In fact, by proper provisioning, a large number of different applications can be accommodated without overburdening the implementation. Furthermore, an open approach allows the introduction of new services that may not have been anticipated by the system's designers.
The structure of the paper is as follows. We first give an overview of a System Reference Model, that defines the basic components of a VoD system. We then describe in detail each of these components, namely the server, the delivery system (core and access networks), and the subscriber terminal, as well the necessary protocols operating between them. We conclude by discussing media encodings and future directions of the DAVIC effort. The description is necessarily brief, and it should be noted that it is based on the current draft status of the DAVIC specifications; the reader is encouraged to consult the actual text of the specifications for more detailed and up-to-date information.