Video on demand systems are systems which allow users to select and watch video content over a network as part of an interactive television system. VOD systems are either "streaming", in which viewing can start as the video streams over the Internet (or other network), or "download", in which the program is brought in its entirety to a set-top box before viewing starts.

NVOD systems, or Near Video on Demand systems are streaming 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.

All download and some streaming 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. For streaming systems this requires 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. Streaming 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 streaming VOD services over a wide area network. Download VOD services are practical to homes equiped with cable modems or DSL lines.

The first commercial VOD service took place in Hong Kong around 1990. The technology was not mature, Video CDs were much cheaper, and pay TV was not common in Hong Kong. Hong Kong Telecom lost a large amount of money and the service was shut down after it was acquired by Pacific Century Cyberworks in 2000.

VOD services are now available in all parts of the USA. Streaming VOD systems are available from cable providers (similar to cable modem technology) who use the large downstream bandwidth present on cable systems to deliver movies and television shows. Users can typically pause, fast-forward, and rewind VOD movies due to the low latency and random-access nature of cable technology. The large distribution of a single signal makes streaming VOD impractical for most satellite TV systems; however, Echostar recently announced a plan to offer video on demand programming to PVR-owning subscribers of its DISH Network satellite TV service. The programs are automatically recorded on the PVR, then the users can watch, play, pause, and seek at their convenience. VOD is also quite common in more expensive hotels. Internet download VOD systems are widely available.

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.

Video on Demand Providers

• Comcast has a video-on-demand service in the US.
• Time Warner Cable has a video-on-demand service in the US.
• HomeChoice has a video-on-demand service in the London area of the UK.
• Rogers Cable has a video-on-demand service in Canada.
• MovieBeam a video-on-demand service by Disney.

See also

• Near Video on Demand (NVOD)