06.513GPPGSM Enhanced full rate speech processing functions: General descriptionTS
During a normal phone 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 over the air interface is reduced, leading to better Radio Frequency (RF) spectrum efficiency.
The overall DTX mechanism is implemented in the DTX handlers (Transmit (TX) and Receive (RX)) described in GSM 06.81  and requires the following functions:
– a Voice Activity Detector (VAD) on the TX side, see GSM 06.82 ;
– evaluation of the background acoustic noise on the TX side, in order to transmit characteristic parameters to the RX side, see GSM 06.62 ;
– generation of comfort noise on the RX side during periods where the radio transmission is turned off, see GSM 06.62 .
The transmission of comfort noise information to the RX side is achieved by means of a Silence Descriptor (SID) frame. The SID frame is transmitted at the end of speech bursts and serves as an end of speech marker for the RX side. In order to update the comfort noise characteristics at the RX 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 (RSS).
The DTX handlers interwork with the RSS using flags. The RSS is in control of the actual transmitter keying on the TX side, and performs various pre‑processing functions on the RX side. This is described in GSM 06.81 .
The speech flag (SP) indicates whether information bits are speech or SID information. The SP flag is calculated from the VAD flag by the TX DTX handler. When SID information is transmitted (SP="0") the operation of the speech encoder is modified to reduce the remaining computation for that frame. This is described in GSM 06.62 .