timated number of bit errors in a number of bursts to which corresponds a value from 0 to 7 (best to worst) of the RXQUAL. After the channel decoder has decoded a 456 bits block, it is coded again using the convolutional polynom in the channel coder and the resulting 456 bits are compared with the 456 input bits. The number of bits that differs between these two 456 bits blocks corresponds to the number of bit errors in the block. The number of bit errors is accumulated in a BER sum for each SACCH multiframe and the result is classified from 0 to 7 according to the BER-RXQUAL conversion Table 2.19. RXQUAL is still considered as a basic measurement. It simply reflects the average BER over a period of 0.5 S. However, listener speech quality evaluation is a complex mechanism that is influenced by several factors. Some of these factors that RXQUAL does not consider are: Time distribution of BER. For a given BER, if the rate fluctuates a lot, the perceived quality is less than if the BER is constant over the time. When entire frames are lost, speech quality is negatively impacted. Handovers generate some frame losses. It is not evident in RXQUAL measurements since, during handovers, BER measurements are skipped. Overall quality depends closely on the type of codec used. In conclusion, RXQUAL does not capture many phenomena that affect the listener's perception of speech quality. That is why other metrics are defined. FER (frame erasure rate) range goes from 0 (being the best performance) to 100%. This represents the percentage of blocks with an incorrect CRC (cyclic redundancy check). Since the BER is calculated before the decoding with no gain from frequency hopping, the FER is then used in this case. Being even more stable than the BER, the FER also depends on codec type. The smaller the speech codec bit rate, the more sensitive it becomes to frame erasures. The MOS (mean opinion score) is a subjective KPI in the way that it scores human beings' perception of different calls. Scores go from 1 to 5 (bad, poor, fair, 