Videoconferences can be set up for one-to-one communication, one-to-many, or many-to many. As stated above, using ISDN for anything but one-to-one is relatively expensive at present especially for a project such as this one. However, using LAN technology it is possible at no cost to create many-to-many conferences (and hence also one-to-many) using the so-called multicast technology.Multicasting is a networking solution to the problem created when several people need to communicate. For example, as illustrated in Figure 1.3, if three people needed to communicate via video, each person would have to open two channels to talk to each of the other two participants. Thus six channels would be required for the conference. This, of course, would double to twelve if audio communication was also required. If there were four people in the conference then 12 video and 12 audio channels would be required. In general if n people were in the conference then n(n-1) channels would be required for each medium of communication. It can be seen that multiway conferencing would soon totally clog up a network if implemented in this way. The solution devised for this problem is called multicasting and it simply means that each participant in a conference opens only one channel of communication for each medium and the network nodes take care of the distribution of the data to all other participants.
It should be stressed that multicasting merely reduces the problems since the network still has to carry a huge amount of traffic where there are large numbers of users communicating with each other. A more detailed description of how multicasting is used with the ShowMe software is given in Appendix 3.
Multicast operating system patches are available for all major Unix workstation platforms and are delivered as standard on some platforms (e.g. SUN Solaris 2.3 and later). We did carry out some experiments with the multicasting facilities supported by Showme, LBL and IVS but these were not central to our study since the Helpdesk interactions we looked at tended to be one-to-one. However, we could envisage scenarios where three-way conferencing would be very useful. For example, a user may have contacted an advisor who then wished to bring one or more specialists in on the consultation.