Video on demand is sometimes compared to an electronic video rental store. The user (customer) selects any one of a large number of available videos and takes it home to view. Only with video on demand, the selection is made at home using the television set’s remote control, and the video starts immediately. No trip to the store is needed. Needless to say, implementing video on demand is a wee bit more complicated than describing it. In this section, we will give an overview of the basic ideas and their implementation.
Is video on demand really like renting a video, or is it more like picking a movie to watch from a 500-channel cable system? The answer has important technical implications. In particular, video rental users are used to the idea of being able to stop a video, make a quick trip to the kitchen or bathroom, and then resume from where the video stopped. Television viewers do not expect to put programs on pause.
If video on demand is going to compete successfully with video rental stores, it may be necessary to allow users to stop, start, and rewind videos at will. Giving users this ability virtually forces the video provider to transmit a separate copy to each one.
On the other hand, if video on demand is seen more as advanced television, then it may be sufficient to have the video provider start each popular video, say, every 10 minutes, and run these nonstop. A user wanting to see a popular video may have to wait up to 10 minutes for it to start. Although pause/resume is not possible here, a viewer returning to the living room after a short break can switch to another channel showing the same video but 10 minutes behind. Some material will be repeated, but nothing will be missed. This scheme is called near video on demand. It offers the potential for much lower cost, because the same feed from the video server can go to many users at once. The difference between video on demand and near video on demand is similar to the difference between driving your own car and taking the bus.
Watching movies on (near) demand is but one of a vast array of potential new services possible once wideband networking is available. The general model that many people use is illustrated . Here we see a high-bandwidth (national or international) wide area backbone network at the center of the system.
Connected to it are thousands of local distribution networks, such as cable TV or telephone company distribution systems. The local distribution systems reach into people’s houses, where they terminate in set-top boxes, which are, in fact, powerful, specialized personal computers.
