H.323 Standards



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What standards apply to H.323?

Call signallng and control

  • H.323 - Packet-based multimedia communications systems
  • H.225 - call control protocol
  • H.235 - security
  • H.245 - media control protocol
  • Q.931 - digital subscriber signalling
  • H.450.1 - Generic functional protocol for the support of supplementary services in H.323

H323 Annexes

  • Annex D - Real Time fax over H.323
  • Annex E - Multiplexed call signalling
  • Annex F - Simple Endpoint Terminal (SET)
  • Annex G - Text SET
  • Annex H - Mobility
  • Annex I - Operation over low QoS Networks
  • Annex J - Secure SET
  • Annex K - HTTP Service Control Transport
  • Annex L - Stimulus Signalling
  • Annex M - QSig Tunneling
  • Annex N - QoS

Audio codecs

  • G.711 - PCM audio codec 56/64 kbps
  • G.722 - audio codec for 7 Khz at 48/56/64 kbps
  • G.723.1 - speech codec for 5.3 and 6.3 kbps
  • G.728 - speech codec for 16 kbps
  • G.729 - speech codec for 8/13 kbps

Video codecs

  • H.261 - video codec for >= 64kbps
  • H.263 - video codec for < 64kbps

Related standards

  • H.320 - The original ISDN videoconferencing standard.
  • H.324 - An extension of H.320 for videoconferencing over PSTN lines
  • T.120 - Real time data conferencing protocol

Where do standards document come from?

The documents covering the H.323 protocol suite are created by the International Telecommunications Union (ITU).

The International Multimedia Teleconferencing Consortium (IMTC) is a
consortium of companies that promote the use of multimedia communications,
hold interoperability events, etc. The membership of the IMTC covers most of the big telcos and companies, and membership is "open" to anyone who can afford the US$US6000 per year membership fee.

All of the ITU standards relating to H.323 can be found here.

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Why isn't the document I need available for download?

There are three good reasons why the document you want may not be available:

  • The ITU charges between $US20 and $US100 for any standards document. Someone has to pay for it...
  • Documents cannot be freely distributed.
  • Once the IMTC has decided upon a standard, it must be ratified by the ITU and then made available. This may take many months.

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Documents available for download

It can be hard to find the document you need on this site. Fortunately, Paul E Jones of Cisco has an excellent web site on which references all of the latest H.323 specifications by name, and contains links (where available) to the appropriate draft specification for download. His site is at:


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Last Modified: 12 May 2004 14:34
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