A, B and C, where bits in class A are most sensitive and bits in class C least sensitive. A frame is only declared damaged if there are bit errors found in the most sensitive bits, i.e. class A bits. On the other hand, it is acceptable to have some bit errors in other classes, i.e. class B and C bits. Moreover, a damaged frame is still useful for error concealment at the decoder since some of the less sensitive bits can still be used. This approach can improve the speech quality compared to discarding the damaged frame. The AMR codec offers eight different modes, each mode offering a different bit rate (see Table 4.8). For dimensioning purposes it is assumed that AMR mode 7 corresponds to the GSM EFR mode. This mode provides the best voice quality and consequently the highest bit rate. For example, the number of bits in the mode 7 frame is 244 bits, or 31 bytes (30.5 to be exact). A frame is produced every 20 ms, producing a bit rate of 12.2kbit/s. AMR specifies silence suppression, which is aimed at reducing the effective bandwidth to about 7.5 kbps for each connection. This is based on the following assumptions: The voice/silence ratio is 60/40. The SID (silence detection) is sent every 8 frames (0.16 s) The SID frame is 39 bits long. In one second, with 60 % activity, 1000 ms X 20 = 30 speech samples are generated. There SIDs are generated. Thus the resulting bandwidth (in one direction) is 30 samples X 31 bytes + 3 samples X 3GPP Release 4 Circuit Core Network Table 4.8 Modes of the AMR codec Index Class A bits Speech bits total AMR SID AMR 12.2 AMR 10.2 AMR 7.95 AMR 7.4 AMR 6.7 AMR 5.9 AMR 5.15 AMR 4.75 5 bytes = 7560 bit/s. For comparison, G.711 A-law and u-law produce a constant bit rate of 64 kbps and no silence suppression is used. G.711 Codec G.711 is the international standard for encoding telephone audio on a 64 kbps channel. It is a pulse code modulation (PCM) scheme operating at an 8 kHz sample rate, with 8 bits per sample, fully meeting the ITU-T recommendations. Assuming that G.711 coding is used 