H.323

H.323
Name	H.323
Developer	International Telecommunication Union
Introduced	1996
Osi layer	Application layer
Ports	1720 (H.225.0), dynamic RTP/RTCP
Based on	Q.931, H.245

H.323. H.323 is a suite of ITU standards that defines protocols for providing audio-visual communication sessions over packet-switched networks, including the early Internet. Originally designed for videoconferencing over LANs, it was a foundational protocol for Voice over IP before the widespread adoption of SIP. The standard encompasses call signaling, media control, and the transport of audio, video, and data between endpoints.

Overview

The International Telecommunication Union's Telecommunication Standardization Sector first ratified the H.323 standard in 1996, with subsequent versions expanding its capabilities. It was developed as part of the broader H.32x family of standards for multimedia communications over different networks. Early commercial implementations were heavily promoted by companies like Microsoft with its NetMeeting application and Cisco Systems in its VoIP gateways. The protocol suite was instrumental in enabling early enterprise videoconferencing systems and interoperability between different vendors' equipment, forming a crucial bridge between traditional PSTN telephony and emerging IP networks.

Protocol architecture

H.323 is not a single protocol but an umbrella specification that incorporates several other ITU-T recommendations. Core signaling is handled by H.225.0, which is based on Q.931 from ISDN, and H.245 for negotiating media channels. Media transport itself uses the RTP and RTCP, as defined by the IETF. The architecture defines several key components: Terminals (endpoints), Gatekeepers for address translation and bandwidth management, Gateways for interconnection with other networks like the PSTN, and MCUs for hosting multipoint conferences.

Call setup and control

A basic point-to-point call in H.323 begins with a discovery or registration phase with a Gatekeeper using the RAS protocol. Call signaling, using H.225.0, establishes a connection between endpoints, similar to the process in an ISDN network. Once the call is set up, H.245 control messages are exchanged on a separate logical channel to negotiate media capabilities, open RTP/RTCP streams for audio and video, and manage the call. For conferences involving more than two parties, a MCU centrally manages the signaling and media mixing, coordinating streams from all participating terminals.

Codecs and media

The standard mandates support for specific audio and video codecs to ensure baseline interoperability. For audio, G.711 PCM is required, with other codecs like G.723.1, G.729, and G.722 being widely supported for better compression. Video support originally centered on H.261, with the more advanced H.263 and later H.264 becoming common for higher quality. The T.120 series of recommendations can be used alongside H.323 for data collaboration and application sharing, enabling features like shared whiteboards and file transfer during a session.

Security features

Later versions of H.323 incorporated stronger security mechanisms. H.235 is the umbrella standard for security within the suite, defining profiles for authentication, integrity, and encryption. It supports the use of AES and Triple DES for media encryption, and HMAC for signaling message integrity. Security can be implemented between endpoints or between an endpoint and a Gatekeeper, helping to secure calls against eavesdropping and tampering, which was a growing concern for enterprise deployments.

Related standards and alternatives

H.323 is directly related to other ITU-T standards like H.320 for ISDN and H.324 for PSTN circuits. Its primary competitor, developed by the IETF, is the SIP, which is generally considered more flexible and web-friendly. Other significant alternatives include the MGCP and its successor Megaco/H.248, which are often used in conjunction with SIP in large carrier networks. While largely supplanted by SIP for VoIP and many new services, H.323 remains in use in certain legacy videoconferencing systems and specialized carrier interconnects.

Category:ITU-T recommendations Category:Videotelephony Category:Voice over IP