This European Telecommunication Standard (ETS) gives the detailed requirements for the correct operation of the background acoustic noise evaluation, noise parameter encoding/decoding and comfort noise generation in Mobile Stations (MSs) and Base Station Systems (BSSs) during Discontinuous Transmission (DTX) on enhanced full rate speech traffic channels. The requirements described in this ETS are mandatory for implementation in all GSM MSs capable of supporting the enhanced full rate speech traffic channel. The receiver requirements are mandatory for implementation in all GSM BSSs capable of supporting the enhanced full rate speech traffic channel, the transmitter requirements only for those where downlink DTX will be used.