Demystifying Multimedia Conferencing Over the Internet Using the H.323 Set of Standards

The Telecommunication Sector of the International Telecommunication Union (ITU-T) has developed a set of standards for multimedia conferencing over packet-based networks. These standards are aggregated under a standard umbrella termed Recommendation H.323. Recommendation H.323 describes terminals, equipment, and services for multimedia communication over networks such as the Internet. H.323 terminals and equipment carry real-time voice, video, and data, in any combination thereof. Terminals signal calls using Q.931-derived procedures defined in Recommendation H.225.0. After the call signaling phase, terminals proceed to the call control phase where they exchange capabilities and logical channel information using the H.245 protocol defined in Recommendation H.245. Once the call has been established, audio and video (if supported) are initiated. Both media types use the Real Time Protocol (RTP) defined by the Internet Engineering Task Force (IETF) as their Transport Protocol. Procedures for audio and video packet format and transport are described in Recommendation H.225.0. Recommendation H.323 allows for the use of a variety of video codecs (e.g., H.261, H.263, H.263+) and audio codecs (e.g., G.711, G.723.1). Data collaboration is also allowed using Protocol T.120. We provide an overview of these protocols and explain the H.323 call scenario. Other entities such as gatekeepers, Multipoint Control Units (MCUs), and gateways are also addressed in Recommendation H.323. These entities allow network management, centralized multipoint, and interoperability with other conferencing standards. We explain each of these entities briefly and provide some scenarios of their interaction with the H.323 terminals. IP telephony has become an important driver for packetbased communications. We address the role of H.323 procedures in deploying IP telephony, and describe how new H.323 features such as supplementary services and security facilitate this purpose.