H.323 is an umbrella specification, because it includes a various other ITU standards. The components under H.323 architecture are terminal, gateway, gatekeeper and multipoint control unit (MCUs).

Terminals represent the end device of every connection. It provides real time two way communications with another H.323 terminal, GW or MSU. This communication consists of speech, speech and data, speech, and video, or a combination of speech, data and video.

Gateways establish the connection between the terminals in the H.323 network and with the terminals belong to networks with different protocol stack such as the traditional PSTN network or SIP, Megaco end points.

Gatekeepers are responsible for translating between telephone number and IP addresses. They also manage the bandwidth and provide mechanism for terminal registration and authentications. Gatekeeper also provide services such as call transfer, call forwarding etc.

MCUs are taking care of establishing multipoint conferences. It consists of a mandatory Multipoint Control, which is for call signaling and conference control, and an optional Multipoint Processor., which is for switching/mixing of media stream, and sometimes real-time transcoding of the received audio/video streams.

There are five types of information exchange enabled in the H.323 architecture:

• Audio (digitized) voice
• Video (digitized)
• Data (files or image)
• Communication control (exchange of supported functions, controlling logic channels, etc.)
• Controlling connections and sessions (setup and tear down)

The H.323 was first published in 1996 and the latest version (v5) was completed in 2003.

Call control and signaling

H.225.0: Call signaling protocols and media stream packetization (uses a subset of Q.931 signaling protocol)

H.225.0/RAS: Registration, Admission and Status

H.245: Control protocol for multimedia communication

Audio processing:

G.711: Pulse code modulation of voice frequencies

G.722: 7 kHz audio coding within 64 kb/s

G.723.1: Dual rate speech coders for multimedia communication transmitting at 5.3 and 6.3 kb/s

G.728: Coding of speech at 16 kb/s using low-delay code excited linear prediction

G.729: Coding of speech at 8kps using conjugate-structure algebraic-code-excite linear-prediction

Video processing:

H.261: Video codecs for audiovisual services at Px64kps.

H.263: Video coding for low bit rate communication.

Data conferencing:

T.120: This is a protocol suite for data transmission between end points. It can be used for various applications in the field of Collaboration Work, such as white-boarding, application sharing, and joint document management. T.120 utilizes layer architecture similar to OSI model. Top layer (T.126, T.127) are based on the services of layer layers (T.121, T.125).

Media transportation:

RTP: Real time Transport Protocol

RTCP: RTP Control Protocol

Security:

H.235: Security and encryption for H.series multimedia terminals.

Supplementary services:

H.450.1: Generic functions for the control of supplementary services in H.323

H.450.2: Call transfer

H.450.3: Call diversion

H.450.4: Call hold

H.450.5: Call park and pick up

H.450.6: Call waiting

H.450.7: Message waiting indication

H.450.8: Names Identification services

H.450.9: Call completion services for H.323 networks

The following figure illustrate the structure of the key protocols in the H.323 architecture.

Details of each protocols will be discussed in separate documents.

H.323: ITU-T VOIP Protocols Overview