Just for info,
In case of AMR,To be able to perform the codec mode adaptation the receiving side performs quality measurements on the incoming link. This measurement results in a Quality Indicator. The Quality Indicator produced on the MS side is converted to a Mode Request that is sent to the BTS using inband signalling. There, a codec mode control entity selects the codec mode to be used on the downlink, based on the Mode Request sent by the MS.
The codec mode control entity also selects the codec mode to be used on the uplink, based on the Quality Indicator produced in the BTS. The selected uplink mode is sent inband to the MS. The presently applied codec mode is also sent in band both uplink and downlink. This mode indication is sent every second speech frame and the Mode Request (uplink)/Mode Command (downlink) is sent in the speech frames in between.
In band messages:
UL: Mode Request (2 bits per speech frame, giving a suggestion of a DL codec mode)
UL: Mode Indication (2 bits per speech frame, giving the present UL codec mode)
DL: Mode Command (2 bits per speech frame, telling the MS which UL codec mode to use)
DL: Mode Indication (2 bits per speech frame, giving the present DL codec mode)