Streaming files are not downloaded, but rather are broadcast in a manner similar to television broadcasts . The videos are processed by a compression program into a streaming format and delivered in a staggered fashion to minimize impact upon the network and maximize the experience of the content for the viewer. The two most commonly-used streaming methods are multicasting and unicasting. Multicasts are single data streams read by any number of clients with the appropriate decoder. Multicasting is a unidirectional communication.
Clients simply listen to a data stream.
A client that connects in the middle of a multicast misses half of the data stream. Unicasting is a bidirectional communication. Clients send requests to the server, and each connection from client to server uses bandwidth. Most of the NIH network is multicast enabled. Live events streamed to computers on the NIHnet are mostly multicast, thus saving considerable bandwidth.