ontinuing to develop, extensions to H.320. H.320 now belongs to a family of standards known as H.32X. In this family, along with H.320, are H.310, H.321, H.322, H.323 and H.324. The H.32X family is truly a family. Each group of specifications recognizes the other members and each includes similar functionality, albeit with differences, which are largely due to the unique characteristics of the networks, they were designed to operate over. H.310 H.310 targets HDTV-class video quality (e.g., CCIR-601 and above) using the International Standards Organization (ISO) Motion Picture Experts Group's MPEG- 2 video compression algorithm (we address MPEG 2 in more detail in Appendix B). The H.310 specification employs the MPEG-2 coding system to convert an analog video signal for transmission over a cell-relay-based digital network, specifically, a network based Asynchronous Transfer Mode (ATM). IBM did much of the preliminary work on the H.310 standard, and worked closely with the ITU-T to refine it to the point of approval. Standards for Videoconferencing H.310 is made up of three sub-parts. One is H.262 ("Information Technology-Generic Coding of Moving Pictures and Associated Audio Information") better known as MPEG-2. Another is H.222.0 ("Information Technology-Generic Coding of Moving Pictures and Associated Audio Information: Systems"). The MPEG-2 standard includes a system layer. The system layer allows MPEG-2 to keep data types synchronized and multiplexed in a common serial bit stream. H.220 adapts H.310 to work within the system layer confines. The third sub- part is H.222.1, "Multimedia Multiplex and Synchronization for Audiovisual Communication in ATM Environments." The MPEG-2 system layer specifies a format for transmitting audiovisual information over a data communications network. The format, known as a transport stream, relies on clock reference information to maintain synchronization between related audio and video streams. H.222.1 describes how MPEG-2-encoded transport streams flow over ATM. H.310 allows v