In CUCM codec selection is based on regions. CUCM will check the regions for originating and destination endpoints and, accordingly, allocate the configure inter-region/intra-region codec.
CUCM will apply the codec based on Max Audio Bit Rate. It won't select specific codec (e.g. G.711 or G.729). For example, if the max audio bit rate configured is 64 kbps (G.722, G.711) & advertise G.722 Codec enterprise parameter is enabled, CUCM will advertise G.722 for this call. If the enterprise service parameter is disabled, CUCM will advertise G.711. CUCM will never advertise G729 for this call, although it has bit rate of 8kbps which is lower than configured max audio bit rate, even if it requested by the other party (in this case xcoder will be deployed or call will fail).
In Summary CUCM codec selection is based on Best Quality which fits within the configure Max Audio Bit Rate.
Note: Within G711 you can choose between alaw or ulaw by changing the service parameters G.711 A-Law Codec Enabled & G.711 Mu-Law Enabled. You can also change this on SIP trunk if MTP is selected by changing the parameter MTP Preferred Originating Codec.
New parameter has been introduced which is Audio Codec Preference Lists. The Maximum Audio Bit rate is still applied for calls within a region and between regions, but rather that using the highest audio quality codec; the codec selection is now made based on the codec order in the Audio Codec Preference List and the codecs that the endpoints support.
To configure this feature:
Create new Audio Codec Preference List by navigating to Cisco Unified CM Administration > System > Region Information > Audio Codec Preference List.
Note: Codecs cannot be removed from the Audio Codec Preference List. You can change the order only
Assign the list to your inter-region/intra-region configuration
During call setup, CUCM will check the list of supported codecs by endpoint and audio codec preference list. Accordingly, CUCM will advertise the overlapping codecs in its call setup which fulfill max audio bit rate requirement.
Note: When configuring ICT trunk between CUCM clusters, it is very important that both clusters should have mirrored Audio Codec Preference Lists and Inter-Region/Intra-Region configuration.
In deployments where calls can pass through more than one CUCM cluster, for example SME deployments; the inter-region Audio Codec Preference List of the intermediary CUCM cluster can over-ride the preferred codec selection between the calling and called device. To ensure that the endpoints’ codec preferences are honored as calls pass through SME, enable the SIP Profile feature “Accept Audio Codec Preferences in Received Offer” on all SME SIP Trunks
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