The H.323 Umbrella
H.323 is a suite of protocols defined by the International Telecommunication Union (ITU)
for multimedia conferences over LANs. The H.323 protocol was designed by the ITU-T
and was initially approved in February 1996. It was developed as a protocol that provides
IP networks with traditional telephony functionality. Today, H.323 is the most widely
deployed standards-based voice and videoconferencing standard for packet-switched networks.
The protocols specified by H.323 include the following:
■ H.225 Call Signaling: 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 an H.323 gatekeeper.
■ H.225 Registration, Admission, and Status: Registration, admission, and status
(RAS) is the protocol between endpoints (terminals and gateways) and gatekeepers.
RAS is used to perform registration, admission control, bandwidth changes, status,
and disengage procedures between endpoints and gatekeepers. A 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.245 Control Signaling: H.245 control signaling is used to exchange end-to-end
control messages governing the operation of an 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
■ Audio codecs: An audio codec encodes the audio signal from a microphone for
transmission by 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 supported, as specified in the ITU–T G.711 recommendation
(coding audio 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) might also be supported.
■ Video codecs: A video codec encodes video from a camera for transmission by the
transmitting H.323 terminal and decodes the received video code on a video display
of 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.
In Cisco IP Communications environments, H.323 is widely used with gateways, gatekeepers,
and third-party H.323 clients, such as video terminals. Connections are configured
between devices using static destination IP addresses.
10 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
Note Because H.323 is a peer-to-peer protocol, H.323 gateways are not registered with
Cisco Unified Communications Manager as an endpoint is. An IP address is configured in
the Cisco UCM to confirm that communication is possible.