Status of H.324—the videoconferencing standard for the Public Switched Telephone Network and mobile radio

The International Telecommunications Union (ITU) has prepared a series of draft recommendations defining a multimedia video-conferencing/videophone terminal transmitting over the Public Switched Telephone Network (PSTN). Recommendation H.324 defines the overall system structure and therefore the device commonly known as an H.324 terminal. The author outlines the key technical characteristics of the entire set of five draft recommendations–H.324, G.723, H.263, H.246, and H.223. Of particular interest is the new H.263 video coding algorithm, which outperforms the H.261 video coder by approximately 2 or 3 to 1. The H.263 coder employs a wide range of advanced coding techniques, including half-pel motion compensation, 8 × 8-pel motion vectors, over-lapped block motion compensation, and PB frames. It is likely that the H.263 recommendations will be used for other applications at higher bit rates as well as for the H.324 PSTN. The ITU has also begun work to adapt the H.324 PSTN terminal for mobile radio applications. The primary effort is the addition of error control measures to the multiplexer (and possibly the source coders) to address the more challenging error characteristics in the mobile environment.