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As an alternative to SIP, H.323 is a popular standard for real-time audio and video conferencing among end systems on
the Internet. The H.323 gatekeeper is a device similar to an SIP register.
The H.323 standard is an umbrella specification that includes the following specifications:
- A specification for how endpoints negotiate common aufio/video encodings.
- A specification for how audio and video chunks are encapsulated and sent over the network.
- A specification for how endpoints communicate with their respective gatekeepers.
- A specification for how Internet phones communicate through a gateway with ordinary phones in the public circuit-switched
telephone network.
Each H.323 endpoint must support the G.711 speech compression standard. G.711 uses PCM to generate digitized speech
at either 56 kbps or 64 kbps. Although H.323 requires every endpoint to be vioce capable, video capabilities are optional.
H.323 is a comprehensive umbrella standard, which, in addition to the standards and protocolsmandates a H.245 control
protocol, a Q.931 signaling channel, and anRAS protocol for registration with the gatekeeper.
Some of the most important differences between H.323 and SIP:
- H.323 is a complete, vertically integrated suite of protocols for multimedia conferencing: signaling, registration, admission
control, transport, and codes.
- SIP addresses only session initiation and management and is a single component.
- H.323 comes from the ITU, whereas SIP comes from the IETF and borrows many comcepts from the Web, DNS, and Internet e-mail.
- H.323 is large and complex. Sip uses the KISS principle: keep it simple, stupid.
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