This European Telecommunication Standard (ETS) gives the detailed requ irements for the correct operation of the background acoustic noise ev aluation, noise parameter encoding/decoding and comfort noise generati on 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 this ETS are mandatory for implementatio n in all GSM MSs capable of supporting the half rate speech traffic ch annel. The receiver requirements are mandatory for implementation in all GSM BSSs capable of supporting the half rate speech traffic channel, the t ransmitter requirements are only for those where downlink DTX will be used. The requirements described in this ETS are mandatory for implementatio n in all GSM MSs capable of supporting the half rate speech traffic ch annel. The receiver requirements are mandatory for implementation in all GSM BSSs capable of supporting the half rate speech traffic channel, the t ransmitter requirements are only for those where downlink DTX will be used.