Adaptive Multi Rate (AMR) is a speech and channel codec for both half rate and full rate. By adapting the codec rate to the radio conditions the speech quality is enhanced. At low C/I, a large amount of channel coding is applied and less speech coding. When the C/I increases the speech coding is increased and the channel coding is decreased.
Both the BTS (uplink ) and the MS (downlink) continuously measures the radio quality (C/I) and based on these measurements the codec rate is changed. AMR requires support in all network nodes, i.e. MSC, BSC, BTS and MS and AMR is only supported in cells where all TRUs are AMR capable.