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Video on demand (VOD) or audio video on demand (AVOD) systems allow users to select and watch/listen to video or audio content on demand. VOD systems either stream content through a set-top box, allowing viewing in real time, or download it to a device such as a computer, digital video recorder, personal video recorder or portable media player for viewing at any time. The majority of cable- and telco-based television providers offer both VOD streaming, such as pay-per-view, whereby a user buys or selects a movie or television program and it begins to play on the television set almost instantaneously, or downloading to a DVR rented from the provider, for viewing in the future. Download and streaming video on demand systems provide the user with a large subset of VCR functionality including pause, fast forward, fast rewind, slow forward, slow rewind, jump to previous/future frame etc. These functions are called trick modes. For disk-based streaming systems which store and stream programs from hard disk drive, trick modes require additional processing and storage on the part of the server, because separate files for fast forward and rewind must be stored. Memory-based VOD streaming systems have the advantage of being able to perform trick modes directly from RAM, which requires no additional storage or CPU cycles on the part of the processor. 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. Download VOD services are practical to homes equipped with cable modems or DSL connections. Servers for traditional cable and telco VOD services are usually placed at the cable head-end serving a particular market as well as cable hubs in larger markets. In the telco world, they are placed in either the central office, or a newly created location called a Video Head-End Office (VHO).
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