Video on demand

Video on demand systems are systems which allow users to select and watch videos over a network. There are several types of video on demand system. NVOD systems, or Near Video on Demand systems are systems in which users wanting to watch a film are batched up for the next start time. This is a reasonable model for films which are in high demand, as the video server[?] can simply distribute the film at short intervals, preferably using multicast techniques. NVOD provides users with a video on demand service, but imposes a short latency delay before the film starts.

Some interactive video on demand systems allow the user to pause, fast forward, fast rewind, slow forward, slow rewind, jump to previous/future frame etc. In other words to provide a large subset of VCR functionality. Such systems require more effort on the part of the server, and may also require greater network bandwidth.

It is possible to put video servers on LANs, in which case they can provide very rapid response to users. Video servers can also serve a wider community via a WAN, in which case the responsiveness may be reduced. Nevertheless, it is possible to provide VOD services over a wide area network.

VOD services are now available in parts of the USA, though information about the technology used for distribution is not widely available. Some VOD systems run over ATM networks, and probably some of them are only linear in the number of users, which means that they don't scale up very well to a large user base.

It is possible to implement VOD using methods such as bandwidth skimming[?], which can deliver O(log n) scaling as the number of users increase.