broadcast : Java Glossary

broadcast

Broadcasting means sending the same content simulaneously to a large number of people, often a live event. Video on demand means you send the same content to many people, but not in lock step. Each person viewing can be watching a different part of the content. VOD (Video On Demand) is much more challenging to handle large numbers of clients at once. It requires more server bandwidth.

Low level Internet protocols support multicasting — the ability to send a packet to more than on recipient. You might think this could be used by a TV station website to provide an Internet version of its broadcast, needing only the bandwith to support one channel. However, life is not that simple. Broadcasting to the Internet at large is not feasible for two main reasons:

Specifying who is to get each packet. You can’t very well attach a list of 10,000 recipients to each packet. They would swamp the packet payload itself.
It would be too much of a temptation for abuse to let loose one packet into the Internet that generated hundreds of thousands all spreading out to tens of thousands of destinations.

So multicasting is mainly confined to a single LAN (Local Area Network). There you can send the message to everyone, (which you do anyway), and have each station figure out for itself whether to act on it. Each station might have several addresses.

You might ask, how does BitTorrent work, or how do live events on the Internet get broadcast, and how does YouTube work, or your other favourite broadcasting. You have to study each protocol individually, and unfortunately much of this sort of thing is a trade secret.

One trick is to use the clients listening to relay information to other clients, forming a tree. This reduces the bandwidth on the server. All packets are directed to a single other node on that tree. Usually the packets are exchanged with UDP (User Datagram Protocol) rather than my setting up a TCP/IP (Transmission Control Protocol/Internet Protocol) socket connection.

BitTorrent
LAN
multicasting
TCP/IP
UDP

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