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.