often do not. MCUs eventually find their way into most organizations. They generally appear after the company has been renting bridge space from service bureaus and can thereby make a business case to purchase an MCU. The ideal multipoint conference would provide every location with access to all Video Communications: The Whole Picture available speech, video, and data from all locations. Unfortunately, continuous- presence multipoint conferencing is not historically been feasible because it requires connection-oriented network links between each and every site (as Internet bandwidth grows and quality-of-service improves, this will become a problem of the past). Fast switching provides an alternative to continuous presence. Consequently, video and data information have come to be provided selectively using criteria for determining which site should have the floor. In such cases, selection can be made manually (by, for instance, the conference facilitator) or it can be fully automatic. Figure 13-8. Operator Interface (courtesy of Polycom). MCUs have different operations modes. Most allow chairperson control in which a single conference manager determines who will be displayed on the screen. Others provide a rotating mode in which the MCU cycles through the conference sites, displaying each briefly on the screen. There is also a voice-activated mode in which the MCU fast-switches between sites based on who is speaking. In a voice-activated mode, some systems offer speech-checking algorithms provided for voice-activation to prevent loud noises (sneezes, coughs and doors shutting, for instance) from diverting the camera from the site with the speaker to the site with the loud noise. MCUs must support all the video and audio algorithms present in the conference and, often graphics and still-image annotation. This includes the full H.320 suite. Within that suite the H.231 and H.243 protocols address multipoint conferencing. G.728 audio support is also desirable. Some MCUs limit connection speeds to only certain Px64 mu