This European Telecommunication Standard (ETS) gives a description of the general baseband operation of half rate speech traffic channels in the transmitter and in the receiver of GSM Mobile Stations (MS)s and Base Station Systems (BSS)s during Discontinuous Transmission (DTX). For clarity, the description is structured according to the block diag rams in figures 1 and 4. Except in the case described below, this stru cture of distributing the various functions between system entities is not mandatory for implementation, as long as the operation on the air interface and on the speech decoder output remains the same. In the case of BSSs where the speech transcoder is located remotely in the Base Station Controller (BSC), the implementation of the interfac es between the DTX Handlers and the Radio Sub System (RSS) as describe d in this ETS together with all their flags is mandatory, being a part of the A-bis interface as described in GSM 08.61 (ETS 300979) [10]. The DTX functions described in this ETS are mandatory for implementati on in all GSM MSs. The receiver requirements are mandatory for impleme ntation in all GSM BSSs, the transmitter requirements only for those w here downlink DTX will be used.