Most videoconferencing products rely on two protocols for their functionality: H.323 and T.120. These protocols make possible videoconferencing and interactive collaboration, respectively.
The H.323 protocol is one of the most widely deployed protocols in videoconferencing. Ratified by the International Telecommunications Union (ITU) in 1996, H.323 defines the technical specifications for audio and visual communication services on LANs. The protocol covers devices that connect directly to the network and devices that use dial-up connections.
H.323 includes three subspecifications: the H.245 call control channel; the Q.391 call signaling channel; and the optional Remote Access Service (RAS) channel, which you can use for registration and bandwidth administration. H.323 uses TCP for the subspecifications' control functions and sends audio and video data streams via the User Datagram Protocol (UDP), which provides the fastest packet delivery for reliable networks. For multipoint conferencing, some systems use IP Multicast, Real Time Protocol (RTP), and UDP to manage audio and video streams.
H.320 covers technical specifications for audio and video communications via ISDN and Switched 56 connections. H.324 defines specifications for communications through V.34 modem connections.
The T.120 specification includes a family of communication and application protocols that support realtime, multipoint data communications. T.120 covers communications over LAN, ISDN, Plain Old Telephone Service (POTS), and Internet connections. The specification became well known when Microsoft integrated it into NetMeeting.