by A.B. Covell  Designing a Videoconferencing Solution Mix And Match Videoconferencing facilitates communication for business purposes, which means more than just point-to-point, one-on-one conferencing. In many cases, you or your users will want to conference with many other users at many other sites. For ISDN conferencing, a Multipoint Control Unit (MCU) will enable such conferences. The MCU can service many conference participants, and MCUs can be cascaded to facilitate very large conferences, though signals will degrade so cascading is typically limited to one level. MCUs receive audio and video from multiple sources and send it back out to conference participants. The audio is typically mixed and low-level background sound is eliminated in the process. Video is typically handled in one of two ways. First, the video sent back to participants is the video of the site that has had the highest level of audio , so the most recent "speaker" has his or her video transmitted and holds the floor. Second, the video is mixed with individual frames for each of four or eight sites in a quadrant. This scheme is referred to as continuous presence, since video from nonspeaking sites persists. When there are more sites than video frames available, an algorithm uses most recent audio data to determine the most active sites, and includes video from those sites in the available quadrants. The first MCUs handled video and audio only. More recent offerings support multipoint data conferencing in support of video/audio, using the T.120 standards. Some vendors have taken to calling the newer data capable products, Multimedia Conference Servers (MCS). MCU and MCS services for ISDN conferences can be obtained from a number of service providers, including most telecom carriers. If you plan to use an MCU only occasionally, then this is a great option. If you expect to need MCU/MCS services on a regular, ongoing basis, you should investigate purchase of an MCU from any of a number of vendors, including PictureTel and Videoserver. Of course, MCS/MCU units are also available for packet-based H.323-based conferencing from vendors such as RadVision, PictureTel and Intel. White Pine's Enhanced CU-SeeMe offers a variation of the MCS/MCU for Internet videoconferencing, called a reflector. A CU-SeeMe Reflector is set up to reflect back all video sources from all CU-SeeMe clients that connect to the reflector, creating sort of a video chat service. Reflector controls allow the Reflector administrator to set limits on connect time, incoming bandwidth and other characteristics for Reflector conference participants. CU-SeeMe Reflectors can be daisy-chained, and CU-SeeMe users can connect in receive-only mode as "lurkers," so that large numbers of Internet users can participate in an Internet Videoconference event. The Multicast Backbone, or MBONE, is another Internet tool that enables group interaction. The MBON E uses special multicast IP addresses, routing and protocols to enable a single stream of packets to service large numbers of users. While the MBONE primarily has been a tool of the research community, momentum is building for more widespread use of the MBONE as a bandwidth-efficient Internet communications tool. But the complexity of implementing MBONE access and the vagaries of Internet transmission will continue to deter many. While your initial forays into videoconferencing will probably focus on one-on-one interactions, it is important to understand the multipoint tools available for the systems and architecture you're considering, since it's very likely you'll be called on to arrange for or deploy multipoint services. Designing a Videoconferencing Solution Typical Videoconferencing Applications Characteristics Of A Successful Videoconference Solution Summary of ITU Videoconferencing Standards Videoconference System Alternatives Network Considerations Mix and Match Sample Scenarios Putting It All Together Videoconferencing Web Resources Print This Page E-mail this URL