H323

H.323 is an ITU-T standard for multimedia communications over packet-based networks. Originating from the H.32x family, it defines a framework for real-time audio, video, and data conferencing over IP networks, including local area networks, wide area networks, and the Internet. The standard specifies how terminals, gateways, and conferencing devices establish, control, and manage multimedia sessions across heterogeneous networks.

Its signaling and control components include H.225.0 for call signaling and registration and admission, through a potential gatekeeper, and H.245 for media control, capability exchange, and logical channel negotiation. In larger deployments, a gatekeeper provides address resolution, bandwidth management, and call admission control; endpoints may be pure terminals, gateways to PSTN networks, or multipoint control units that manage conferences.

Media streams are typically carried over RTP/RTCP once capabilities are negotiated. The H.323 suite has evolved with extensions such as H.460 for NAT and firewall traversal and various vendor-specific features to improve interoperability with other signaling systems, including gateways to SIP or the PSTN.

Although widely deployed in traditional videoconferencing and legacy VoIP systems, H.323 has seen competition from SIP-based solutions for new deployments. It remains supported by many vendors and is still used in several enterprise and room-system environments, particularly where established H.323-based infrastructure exists.