Cisco Cisco IOS Software Release 12.2(15)T Guide Du Développeur

An H.323 terminal is an endpoint in the LAN that participates in real-time, two-way communications 
with another H.323 terminal, gateway, or multipoint control unit (MCU). A terminal must support audio 
communication and can also support audio with video, audio with data, or a combination of all three.

H.323 terminals must support the following standards and protocols:

H.245—An ITU standard used by the terminal to negotiate its use usage of the channel. The H.245 
control channel provides in-band reliable transport for capabilities exchange, mode preference from 
the receiving end, logical channel signaling, and control and indication. Part of the capabilities 
exchange includes specifying which coder-decoders (CODECs) are available. Recommended audio 
CODECs include G.711, G.722, G.723, G.723.1, G.728, and G.729. Recommended video CODECs 
include H.261 and H.263.

H.225.0—An ITU standard that uses a variant of Q.931 to set up the connection between two H.323 
endpoints.

RAS—(Registration Admission Status) A protocol used to communicate with the H.323 gatekeeper. 

RTP and RTCP—(Real-Time Transport Protocol and Real-Time Control Protocol) Protocols used to 
sequence the audio and video packets. The RTP header contains a time stamp and sequence number, 
allowing the receiving device to buffer as much as necessary to remove jitter and latency by 
synchronizing the packets to play back a continuous stream of sound. RTCP controls RTP and 
gathers reliability information and periodically passes this information onto session participants. 

Internet

H.323 terminal

Corporate LAN

H.323 terminal

H.323 terminal

Cisco gatekeeper

MCU

Cisco
proxy

Gateway

S6910

Router

H.320 terminal
(over ISDN)

H.324 terminal
(over POTs)

Speech only
(Telephone)

Real-time

network

Telephone

network