The reader is probably familiar with the Internet, the global computer network everybody nowadays uses to communicate with people all over the world and look up information using the World Wide Web. Less well known is how exactly this Internet works. Fortunately it is not necessary to understand the complex workings to be able to use it. Since the start of the popularization of the Internet (when WWW was invented in 1993), more and more applications of the Internet use graphics, audio and video to enhance the content of the communication. One application that is predicted to grow fast in popularity is multiuser conferencing using audio and video. A serious problem with that kind of application is the bandwidth requirements of real-time interactive audio and video. With just two users communicating, this is already a heavy load for a common Ethernet network. When the video is not transmitted to one user, but to several, the bandwidth requirements increase linearly with each additional user....or doesn't it?
One way to solve the problem of bandwidth shortage is to provide more. Several new network technologies exist that can provide ten times (and more) bandwidth than those currently used. One of them is Asynchronous Transfer Mode (ATM). ATM can use different types of media and can be scaled very well to handle a wide range of bandwidths (from 1 Mbit/s to 10 Gbit/s or higher). One problem with ATM is that it is still in the process of being defined.
But is adding more bandwidth enough? Applications like pay-TV, where only paying viewers can watch, typically have not 10 or 100 viewers, but more like 100,000 or even millions of viewers. Clearly, adding bandwidth is not a realistic solution, it would only solve small scale multi-user problems.
A more scalable solution is ``Multicasting'', a service provided by the network to copy the data you want to transmit to each user that wants to receive it. The network will copy the data only when two destinations cannot be reached over the same link, which makes it a very elegant solution that ideally uses no more bandwidth than a single transmission on each network it needs to be.
For the Internet the first steps to add multicasting support to the existing set of protocols were made as early as December 1985, when the first official document (called a Request For Comment or RFC) was published: ``Host Groups: A Multicast Extension to the Internet Protocol'' (RFC 966).
ATM and multicasting together combines high bandwidth with efficient transmission to multiple destinations. When the Internet factor is integrated as well, high bandwidth multimedia conferencing over the worldwide Internet should become a reality. The prospect of multicasting to millions of viewers is a lot less daunting than sending each user a separate copy from the source.
Given that ATM has support for unidirectional multicast connections, the implementation of IP multicasting over ATM seems a relatively easy step. However, several complications with such an implementation can be identified and need to be addressed.