The protocols specified by H.323 are listed below. H.323 is independent of the packet network and the transport protocols over which it runs and does not specify them (see Figure 3).

• audio CODECs
• video CODECs
• H.225 registration, admission, and status (RAS)
• H.225 call signaling
• H.245 control signaling
• real-time transfer protocol (RTP)
• real-time control protocol (RTCP)

Figure 3. H.323 Terminal-Side Protocol Stack

Audio CODEC

An audio CODEC encodes the audio signal from the microphone for transmission on the transmitting H.323 terminal and decodes the received audio code that is sent to the speaker on the receiving H.323 terminal. Because audio is the minimum service provided by the H.323 standard, all H.323 terminals must have at least one audio CODEC support, as specified in the ITU–T G.711 recommendation (audio coding at 64 kbps). Additional audio CODEC recommendations such as G.722 (64, 56, and 48 kbps), G.723.1 (5.3 and 6.3 kbps), G.728 (16 kbps), and G.729 (8 kbps) may also be supported.

Video CODEC

A video CODEC encodes video from the camera for transmission on the transmitting H.323 terminal and decodes the received video code that is sent to the video display on the receiving H.323 terminal. Because H.323 specifies support of video as optional, the support of video CODECs is optional as well. However, any H.323 terminal providing video communications must support video encoding and decoding as specified in the ITU–T H.261 recommendation.

H.225 Registration, Admission, and Status

Registration, admission, and status (RAS) is the protocol between endpoints (terminals and gateways) and gatekeepers. The RAS is used to perform registration, admission control, bandwidth changes, status, and disengage procedures between endpoints and gatekeepers. An RAS channel is used to exchange RAS messages. This signaling channel is opened between an endpoint and a gatekeeper prior to the establishment of any other channels.

H.225 Call Signaling

The H.225 call signaling is used to establish a connection between two H.323 endpoints. This is achieved by exchanging H.225 protocol messages on the call-signaling channel. The call-signaling channel is opened between two H.323 endpoints or between an endpoint and the gatekeeper.

H.245 Control Signaling

H.245 control signaling is used to exchange end-to-end control messages governing the operation of the H.323 endpoint. These control messages carry information related to the following:

• capabilities exchange
• opening and closing of logical channels used to carry media streams
• flow-control messages
• general commands and indications

Real-Time Transport Protocol

Real-time transport protocol (RTP) provides end-to-end delivery services of real-time audio and video. Whereas H.323 is used to transport data over IP–based networks, RTP is typically used to transport data via the user datagram protocol (UDP). RTP, together with UDP, provides transport-protocol functionality. RTP provides payload-type identification, sequence numbering, timestamping, and delivery monitoring. UDP provides multiplexing and checksum services. RTP can also be used with other transport protocols.

Real-Time Transport Control Protocol

Real-time transport control protocol (RTCP) is the counterpart of RTP that provides control services. The primary function of RTCP is to provide feedback on the quality of the data distribution. Other RTCP functions include carrying a transport-level identifier for an RTP source, called a canonical name, which is used by receivers to synchronize audio and video.