Restricting Audio Coders

You can configure a list of permitted (allowed) voice coders that can be used for a specific SIP entity (leg). In other words, you can enforce the use of specific coders. If the SDP offer in the incoming SIP message doesn't contain any coder that is configured as an allowed coder, the device rejects the calls (unless transcoding is implemented whereby Extension coders are added to the SDP, as described in Coder Transcoding). If the SDP offer contains some coders that are configured as allowed coders, the device manipulates the SDP offer by removing the coders that are not configured as allowed coders, before routing the SIP message to its destination. The device also re-orders (prioritizes) the coder list in the SDP according to the listed order of configured allowed coders.

For example, assume the following:

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The SDP offer in the incoming SIP message contains the G.729, G.711, and G.723 coders.

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The allowed coders configured for the SIP entity include G.711 and G.729.

The device removes the G.723 coder from the SDP offer, re-orders the coder list so that G.711 is listed first, and sends the SIP message containing only the G.711 and G.729 coders in the SDP.

The allowed coders are configured in the Allowed Audio Coders Groups table. For more information, see Configuring Allowed Audio Coder Groups.

If you assign the SIP entity an Allowed Audio Coders Group for coder restriction and a Coders Group for extension coders (i.e., voice transcoding), the allowed coders take precedence over the extension coders. In other words, if an extension coder is not listed as an allowed coder, the device doesn't add the extension coder to the SDP offer.