The Center for Information Technology (CIT) makes special NIH events, seminars, and lectures available to viewers on the NIH network and the Internet from the VideoCast web site. Videocasting is the method of electronically streaming digitally encoded video and audio data from a server to a client.
VideoCast is often referred to as streaming video. 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. When users request a streaming file they will receive an initial burst of data after a short delay (file latency). While content is being viewed, the streaming server machine and software continues to "stream" data in such a manner that the viewer experiences no break in the content. 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. Multicasting conserves bandwidth (one 128kb stream viewed by 100 clients uses only 128kb of bandwidth), however, clients cannot determine when the data stream starts, pause the stream without losing data, or move forward or backward in the data stream. A client that connects in the middle of a multicast misses half of the data stream.
Unicasts are data streams decoded on a one-to-one basis. Unicasting is a bidirectional communication. Clients send requests to the server, and each connection from client to server uses bandwidth. A 128kb data stream unicast to 100 clients uses 12,800kb (12.8Mb) of bandwidth. When unicasting is deployed for a past event (not a live web cast), clients can determine when the data starts, move forward or backward in the data stream, or pause the stream without missing any data.
Most of the NIH network is multicast enabled. Live events streamed to computers on the NIHnet are mostly multicast, thus saving considerable bandwidth. However, the same live event streamed to computers on the Internet or to non-multicast enabled segments of the NIH network are unicast. On-demand viewing of past events, whether on the NIH network or the Internet are always unicast.
For more information, visit NIH VideoCast Frequently Asked Questions
Center for Information Technology
National Institutes of Health
Bethesda, Maryland 20892
301 594 6248 (v) 301 496 8294 (TDD)