For us broadcasters, multicast means transmitting multiple program streams over one RF channel, but in the Internet world, multicast -- or, more precisely, IP multicast -- means something different. Robert X. Cringely has a very good overview of IP multicast technology in his PBS column and argues that it's the next big thing. He writes:
... IP Multicast is the not-so-simple carriage of the same digital signal to thousands or millions of people at the same time. This is as opposed to unicast, which can also serve millions of people but requires millions of parallel video streams to do so. ...
So, in unicast, every viewer of your video or listener to your audio requires a separate stream from the source. Think of it as a multi-stemmed bush or shrub. In IP multicast, you send one out and it branches remotely. Think of it as a tree. Cringely continues:
... Here's a very simple explanation for the way that IP Multicast is supposed to work. Seinfeld episode #60, The Junior Mint (which happens to be the third most popular Seinfeld episode of all time according to some Internet poll) is assigned the Class D multicast address of 224.1.2.3. If you want to watch that episode you click on it in some client application that "subscribes" to that address. When the show is made available on a server anywhere on a part of the net that supports multicast, you will start to receive it. All the routers between here and there look for multicast subscriptions and enable them. If no other customer at your ISP wants to see The Junior Mint, then the video isn't carried on your subnet nor is any of it cached locally. No bandwidth is used. But if one person does want to see The Junior Mint, then it is held to some extent in a local cache and available for all local subscribers. ...
Link: PBS. Thanks to Stephen Hill for the tip. --Dennis
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