The H.32x protocol suite used in most VT systems is defined in standards set by the ITU. The evolution from H.320 (traditionally associated with room systems that operate over ISDN) to H.323 (for desktop systems that operate over a LAN) has paralleled with the work of the Motion Picture Experts Group (MPEG). MPEG (normally associated with streaming video and digital satellite systems) has been adopted and extended by the Organisation Internationale de Normalisation (ISO/IEC). A following generic diagram includes mechanisms that apply to older H.320 systems, current H.323 applications-and even newer hybrid systems that will employ MPEG or wavelet-encoding algorithms.

Note that ITU's H.320 and H.323 architectures have subsets that include video codec, audio codec, data exchange and communications protocol specifications. The newer H.323 suite encompasses most of H.320, but adds much more. For example, H.323 includes H.261, the only video algorithm available to H.320, but adds the option of using H.263, a new video algorithm that is more tolerant of congested LAN or low bandwidth conditions. Similarly, H.323 VT systems must be able to use the G.711 audio algorithm from H.320, but H.323 systems also may use the newer G.723 low bandwidth/medium quality audio algorithm. Look for more details later in this section.

User Interface

The user prepares to open a VT session by launching a Windows application program. Its outer shell is a GUI (graphical user interface) that may present everything in a single screen window or use separate floating windows for local picture, remote picture and conference management. Here is an illustration of the Intel ProShare System 500's GUI ready to place a call. Note that a NetMeeting collaboration window is available.

The following block diagram shows a generic Visual Telephone system implemented within PC-based application software. Today, most conferencing systems-H.320 room systems as well as H.323 desktop units-are built around core PCs hosted by a Windows OS (operating system).