Codecs Page 299 of 910
IP Line Description, Installation and Maintenance

Codec selection

For every virtual trunk call, a codec must be selected before the media path is
opened.
When a call setup with the FastStart procedure is used, the terminating node
selects a common codec and sends the selected codec to the originating node.
For a call modification (media redirection) or for a call setup using the
SlowStart procedure, the codec selection occurs on both nodes. Each node has
two codec lists: its own list and the far-end’s list. To select the same codec on
both nodes, it is essential to use the same codec selection algorithm on both
nodes.
For the codec selection, both the near- and far-end codec lists are retrieved:
• The far-end list is not modified because it is already sorted when it is
received (in FastStart or TCS message).
• The near-end list is sorted and then expanded to include lower payloads,
the same way it is done before sending the codec list in FastStart
message.
The following conditions are met before codec selection occurs:
• There are two codec lists:
— The near-end list is the codec list of the local unit.
— The far-end list is the codec list received from the far end.
• Each codec list can contain more than one payload size for a given codec
type. The codec list depends on the codec configuration.
• Each codec list is sorted by order of preference. The first codec in the
near-end list is the near-end’s most preferred codec and the first codec in
far-end list is the far end’s most preferred codec, and so on.