06.013GPPFull Rate Speech Processing FunctionsTS
During a normal conversation, the participants alternate so that, on the average, each direction of transmission is occupied about 50 % of the time. Discontinuous transmission (DTX) is a mode of operation where the transmitters are switched on only for those frames which contain useful information. This may be done for the following two purposes:
1) in the MS, battery life will be prolonged or a smaller battery could be used for a given operational duration;
2) the average interference level on the "air" is reduced, leading to better spectrum efficiency.
The overall DTX mechanism is implemented in the DTX handlers (TX and RX) described in GSM 06.31  and requires the following functions which are described in separate technical specifications:
‑ a Voice Activity Detector on the transmit side;
‑ evaluation of the background acoustic noise on the transmit side, in order to transmit characteristic parameters to the receive side;
‑ generation on the receive side of a similar noise, called comfort noise, during periods where the radio transmission is cut.
The transmission of comfort noise information to the receive side is achieved by means of a special frame (Silence descriptor = SID). This frame is transmitted at the end of speech bursts and serves as an end of speech marker for the receive side. In order to update the comfort noise characteristics at the receive side, SID frames are transmitted at regular intervals also during speech pauses. This also serves the purpose of improving the measurement of the radio link quality by the radio subsystem.
For the overall DTX functionality, the DTX handlers interwork via various flags with the Radio Subsystem, which is in control of the actual transmitter keying on the TX side and which performs various pre‑processing functions on the RX side. This is also described in GSM 06.31 .
A common terminology used throughout the GSM 06‑series of technical specifications is also defined in the present document.