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Traditionally, there have been two ways of deliveirng IP data - Unicast and Broadcast. However neither of these is ideal for broadband multimedia. Using unicast you have to broadcast the data stream once to each user (so if 10 users connect to a server running at 400kbps you need 4000kbps of bandwidth total). This works fine for low bandwidth applications like audio streaming but places too much of a drain for greater data rate - MPEG2 streaming video can easily use 3.5Mbps. Using broadcast you have to broadcast the data to everyone, even if they do not want the data (So if 10 users out of 1000 want a 400kbps stream all 1000 users end up receiving a 400kbs stream)

The solution to this is muticasting - the server sends out a single data stream, and the routers duplicate the data only to subnets that actually want to receive it - so if 10 out of 1000 users want to receive a 400kbps stream, the server only needs 400kbps of outgoing bandwidth, and this 400kbps stream will only be sent to subnets where at least one person wants the data.

Informing the routers which subnets want the data is done by using a multicast routing protocol such as DVMRP, MOSPF or PIM

See also: IP Unicast, IP Broadcast

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