Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 252
Multicasting Type

There are two types of data transmission normally associated with multicasting: real-time and nonreal-time.

Real-time transmission is the ability to work with the information while it is being transmitted. Two examples of this are video and voice transmissions. Nonreal-time transmission is the ability to transfer the information for use at a later time, directly after the transfer or days later. Examples include CBT files, kiosk, or any type of data file. Both types of transmission have their purposes in the multicast arena. However, nonreal-time trans missions are more applicable today because their use is not immediate.

Real-time transfers such as video and voice are still in the experimental stage. Transferring information across a network that is packet switched without some capability for priority is experimental. In some environments, it works rather well (on a single subnet), but when transported across an internet, the quality deteriorates and therefore the interest in the products wanes. There are protocols that are experimental (RSVP) that should assist real-time multicast protocols by providing bandwidth to the application instead of to the network.

Some real-time multicast transfers use a simplex approach that makes it “real near” real-time—they buffer the incoming data for about 30 seconds and start the play. While the information is being used, the application continues to buffer the incoming data for playback. Not perfect, but it’s better than nothing.