VOD systems
The challenges mentioned in 1.4 are multiplied when we attempt to provide a multimedia service such as video on demand (VOD), which must have a fast response because of the low tolerance for delay before the audio/video stream must be available to the user. The following is a discussion of the things that need to be considered for a video on demand system.

Video on demand systems are made up of a server on which digitised video is stored, an archive management system, an on-line delivery and asset management system, a user-management system, a network that carries the digital video, and a number of clients that display the video (Figure 0-5). A client can be either a PC or a television set connected to a set top box.


Figure 0-5 VOD

It is important to distinguish between video on demand and broadcast video. There are no broadcasts with video on demand. A video on demand client requests the video, which is then delivered in real-time after only a short delay. There is a single channel from the client right back to the server.

The functionality each viewer desires is similar to that of a VCR system. A viewer might wish to pause, fast-forward or rewind. Additional functions such as frame by frame viewing, zoom and skip to a particular scene may also be available. Frame captures might also be desirable, but copyright issues must first be dealt with by the service provider. These features are of limited use in entertainment video where the video is usually viewed from beginning to end with little or no interruption, but are useful for education and research. For example, students studying a play that has been put on a video server might want to examine a particular scene many times to examine the set, the performance, the lighting and so on. The interactivity of video on demand provides the opportunity for many potential applications.