問題：
What is video on demand (VOD)?
答案：
True VOD implies completely random access to video. Users may access the video they want and when they want it. This is synonymous with dialing a video from a data bank and not having to go to a video rental store. In contrast, near-VOD often implies a set of TV channels showing the same movie, but with shifted starting times. Owing to the demanding nature of the application in sense of data capacity, compression techniques are needed. The bit rates applied in some VOD projects are comparable to that of CD-based video, which provides a reasonable picture quality and makes delivery possible by means of ADSL over copper cables of a length commonly found in telephony. The Asymmetric Digital Subscriber Line (ADSL) technology is typically used on distances up to about 5 to 6 km at 2 Mbps.
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