Adaptive Multi-Rate (AMR) codec consists of a family of codecs (source and channel codecs with different trade-off bit-rates) operating in the GSM FR and HR channels modes
The AMR system exploits the channel performance and robustness added by the coding rates by adapting the speech and channel coding rates according to the quality of the radio channel
AMR adapts its error protection level (select its optimum channel mode and codec mode) to the local radio channel and traffic load conditions to deliver the best possible combination of speech quality and system capacity
Codec mode adaptation for AMR is based on received channel quality estimation in both MS and BTS, followed by a decision on the most appropriate speech and channel codec mode to apply at a given time
The basic AMR codec mode sets for MS and BTS are provided by BSC via layer 3 signaling
MS shall support all speech codec modes, although only a set of up to 4 speech codec modes is used during a call
GSM FR/EFR channel gross bit-rate is 22.8 kbit/s in GSM FR/EFR: 13 kbit/s speech coding and 9.8 kbit/channel coding (HR channel gross bit rate 11.4 kbit/s)
For AMR case, different codecs use different bit rate to encode speech (source coding). The rest of the gross bit-rate is used for channel protection
