H.323 Background

H.323 is part of a family of real-time communication protocols developed under the auspices of the ITU, the H.32x family. Each protocol in the family addresses a different underlying network architecture e.g. a circuit switched network, B-ISDN, LAN with QoS, and LAN without QoS (H.323). All borrow heavily from the original H.320's structure and modularity.

H.323 is not an individual protocol, but rather a complete, vertically-integrated suite of protocols that defines every component of a conferencing network: terminals, gateways, gatekeepers, MCUs and other feature servers. Amongst others, H.323 uses:

• Q.931 - call setup
• H.225 call signalling
• H.245 - exchanging terminal capabilities and creation of media channels
• RAS - registration and admission control
• RTP/RTCP - sequencing audio and video packets
• G.711/712 - codec specification
• T.120 - data conferencing

All these protocols - dozens of back-and-forth messages - must be negotiated to set up a simple point-to-point voice call.

This is all in contrast to SIP, a simple protocol that specifies only what it needs to. For example, SIP works with RTP but does not mandate it.