System requirements for VOD
The desire for interactivity and control places considerable demands on the three
main components of the video on demand system: the server, network and client.
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The server has to support an independent video stream to every client, rather
than broadcasting a single stream, and so must be able to read data from disk
storage onto the network at a very high rate. The server must be able to support
a single bit stream for each client attached to it. For MPEG 1 compressed audio
and video for example, this means a bit stream of 1.5 to 4 Mbit/s, while for
MPEG 2, it means between 4 and 8 Mbit/s. To support a hundred concurrent streams,
means a server must be able to support between 150 and 800 Mbit/s. Not only that,
but it must be able to support them with very tight bounds on delay and delay
variation (jitter).
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There are a number of approaches to providing these very high bit rates. They
include using RAID arrays and distributed servers. In a RAID (Redundant Array
of Inexpensive Disks) a number of disks are arranged so that the compressed video
data can be striped across them. Thus, when video data is being retrieved from
one disk, a seek can be executed on another, thus making the aggregate speed
of the disk array much greater than that of a single large disk.
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Video files are very large, yet not all the videos available are likely to be
needed at any one time. A hierarchical method of storage is usually needed, where
less frequently requested titles are stored on slower media, such as CD-ROM juke
box or a tape library and only brought on-line when required.