The present document gives the detailed requirements for the correct operation of the background acoustic noise evaluation@ noise parameter encoding/decoding and comfort noise generation in GSM Mobile Stations (MS)s and Base Station Systems (BSS)s during Discontinuous Transmission (DTX) on half rate speech traffic channels. The requirements described in the present document are mandatory for implementation in all GSM MSs capable of supporting the half rate speech traffic channel. The receiver requirements are mandatory for implementation in all GSM BSSs capable of supporting the half rate speech traffic channel@ the transmitter requirements are only for those where downlink DTX will be used.