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H.323 is an ITU standard that specifies how terminals (PCs), equipment, and services for multimedia communicate over networks that do not provide a guaranteed quality of service (such as the Internet). H.323 terminals and equipment can carry real-time video, voice, and data, or any combination of these elements. Products that use H.323 for audio and video let you connect and communicate with other people over the Internet, just as people using different makes and models of telephones can communicate using the telephone.
H.323 is actually a series of standards that define:
- How calls are established.
- The framework for capability negotiation (the process of getting two machines to communicate with each other).
- How data is transmitted on the wire.
- Default audio and video compressor/decompressor (codec) components. Aaudio and video codecs encode and decode input/output from audio and video sources for communicating between nodes.
For low bandwidth use, the H.323 standard specifies standard codecs for audio (G.723) and for video (H.263) that enable H.323 products to send and receive voice and video images. H.323 also specifies use of the T.120 standard for data conferencing. More than 120 leading companies in the industry publicly announced their intent to support and implement H.323 in their products and services. This broad support establishes H.323 as the leading solution for audio and video conferencing over the Internet.
The H.323 standard includes the following components.
| Component | Description |
|---|---|
| H.225.0 | This standard defines a layer that formats the transmitted video, audio, data, and control streams for output to the network and retrieves the received video, audio, data, and control streams from the network. H.225.0 uses the packet format specified by IETF RTP and RTCP specifications for logical framing, sequence numbering, and error detection as part of audio and video transmissions. Support for RTP and RTCP enables the receiving node to synchronize the received packets in the proper order so the user hears or sees the information correctly. |
| H.245 | The H.245 standard provides the call control mechanism that enables H.323-compatible terminals to connect to each other. |
| H.261 | The H.261 standard specifies the format and algorithm for an alternative, higher bandwidth, video codec used to send or receive video images over network connections. |
| H.263 | This standard specifies the format and algorithm for the default video codec used to send or receive video over low-bandwidth network connections. |
| G.711 | The G.711 standard specifies the format and algorithm for an alternative, higher bandwidth, audio codec used to send or receive voice over network connections. |
| G.723 | Audio codec, for 5.3 and 6.3 kilobits per second (Kbps) modes. |
| G.723.1 | This standard specifies the format and algorithm for the default audio codec used to send or receive voice over low-bandwidth network connections. |
The Windows NetMeeting 3 Resource Kit, available from the Microsoft Windows NetMeeting Web site.