Discontinuous transmission is the ability of a mobile device or communications system to inhibit transmission when no activity or reduced activity is present on a communications channel. DTx is often used in mobile telephone systems to conserve battery life of portable mobile telephones.

The GSM system allows the mobile device to use DTx by intermittently stopping transmission during periods of low audio speech activity. Speech activity is determined by voice activity detection (VAD). When the VAD determines that there is no speech activity, it can temporarily shuts off the speech coder and inhibit the transmitter. To ensure the listener does not fell uncomfortable with complete silence periods, a background noise signal may be sent. This "comfort noise" is sent to minimize the change in background noise during inactive voice. During the silence period, the mobile device may continue to compress the background noise and create sent silence descriptor (SID) frames that are sent at a data rate of 500 bps. This small amount of data approximates the same background noise during the silence periods as occurs during normal speech periods. This provides for more uniform communication between the users.