rate speech coder, a source controlled rate scheme including a voice activity detector (VAD) and a comfort noise generation system, and an error concealment mechanism to combat the effects of transmission errors and lost packets. The speech encoder takes its input as a 13-bit uniform Pulse Code Modulated (PCM) signal either from the audio part of the UE or on the network side, from the Public Switched Telephone Network (PSTN) via an 8-bit A-law or u-law to 13-bit uniform PCM conversion. The encoded speech at the output of the speech encoder is packetized and delivered to the network interface. In the receive direction, the inverse operations take place. The multi-rate speech coder is a single integrated speech codec with eight source rates ranging from 4.75 kbps to 12.2 kbps, and a low rate background noise encoding mode, as listed in Table 3-1. Here, the speech coder is capable of switching its bit-rate every 20 ms speech frame on command. Codec mode Source codec bit-rate AMR_12.20 12,20 kbps (GSM EFR) AMR_10.20 10,20 kbps AMR_7.95 7,95 kbps AMR_7.40 7,40 kbps (IS-641) AMR_6.70 6,70 kbps (PDC-EFR) AMR_5.90 5,90 kbps AMR_5.15 5,15 kbps AMR_4.75 4,75 kbps AMR_SID 1,80 kbps Table A.2: Source codec bit-rates for the AMR codec [3GP99a] In each 20 ms speech frame, 95, 103, 118, 134, 148, 159, 204 or 244 bits are produced, corresponding to a bit-rate of 4.75, 5.15, 5.90, 6.70, 7.40, 7.95, 10.2 or 12.2 kbps. The following table shows the bit allocation of the AMR codec modes for 12.2 kbps speech. A.6 AMR Parameter total per frame subframe subframe subframe subframe 2 LSP sets 12.2 kbps Pitch delay (GSM EFR) Pitch gain Algebraic code Codebook gain Table A.3: Bit allocation of the AMR coding algorithm for 20 ms frame The coder operates on speech frames of 20 ms corresponding to 160 samples at the sampling frequency of 8000 sample/s. It performs the mapping from input blocks of 160 speech samples in 13-bit uniform PCM format to encoded blocks of 95, 103, 118, 134, 148, 159, 204, and 244 bits and in reverse from encoded blo