Page 296 of 910 Codecs
553-3001-365 Standard 4.00 August 2005
— If the G.729AB codec is configured, the G.729A codec and the
G.729AB codec are registered with the Call Server.
— If the G.723 codec is configured, the G.723 codec is registered with
the Call Server.
Example 1
G.711 a-law, G.729A, G.729AB, and G.723.1 are configured.
The Voice Gateway registers G.711 a-law, G.711 mu-law, G.729A, and
G.729AB.
Example 2
G.711 mu-law, G.729A, and G.723.1 are configured.
The Voice Gateway registers G.711 a-law, G.711 mu-law, and G.729A.
Example 3
G.711 mu-law and G.723.1 are configured.
The Voice Gateway registers G.711 a-law, G.711 mu-law, and G.723.1.

Codec negotiation

For every virtual trunk call, a common codec must be selected for the call.
This is known as codec negotiation. Codec negotiation for virtual trunk calls
is performed through the H.323 FastStart and Terminal Capability Set (TCS)
messages.
For a call setup with the FastStart procedure, the originating node sends its
codec list in the FastStart element in the SETUP message to the terminating
node. For a call setup using the SlowStart procedure or for a call modification
(media redirection), each node sends its codec list in the TCS message to the
other node.