Controlling Codec Selection Between IP Phones
The codec between phones is controlled by the Region setting, which is applied to the Device Pool and in turn the physical IP phone device. Regions are the logical and physical building blocks of the network. Each WAN site, particularly where bandwidth is at a premium, should be represented as a separate Region. Codecs are applied on Region-to-Region relations; so, for example, Remote-A to HQ. Note that codec selection must only be selected in one direction - CallManager automatically applies the converse rule.
The concept of regions is closely integrated with Locations (which control the maximum bandwidth allowed for voice calls to a site). Once a Region is created, it must be applied to the appropriate Device Pool.
To change the codec used between 2 Regions, browse to System > Regions and select the Region (or create a new region). In the "Modify Relationship to other Regions", select the Region for which the relationship should be changed. Select the new codec, and click "Save". The change will take effect immediately, or at least for the next call; existing calls will continue to use the previously defined codec. There is no need to reset phones for a codec change. Note that this is different from changing the Region of a Device Pool; changing the Region of a Device Pool *does* require a reset of all devices which reference it.
Verification of codec change can be achieved by making a call between 2 IP phones, and then web browsing to the IP address of one of the devices (the IP addresses can be obtained from Device > Phones). There are 2 screens in the Phone Web GUI that indicate codec selection:
- Stream 1 shows statistics about the currently active call including, importantly, Tx and Rx codec
- Console logs /FS/cache/log3 provides verbose call detail information. This includes the codec selection
The output of both can be seen in the example below.
Interestingly, it is not possible to restrict or prefer g711a law versus g711mu law. When one IP phone communicates with another IP phone and the Region to Region codec is G.711, mu law will always be selected.
Observe Remote to HQ Codec Selection (G.729)
Browse to the IP phone web page to verify active call codec
Browse to the Console Log in the IP Phone Web GUI
Verify Codec Selection
