Actually, during the communication process, the mobile subscriber talks only 40% of the time and there is not much useful information transmitted during rest of the time. If all the information is transmitted to the network, it will not only be a waste of the system resources but also add more interference to the system. In order to overcome this problem, the DTX technique is used in the GSM system, i.e. the transmission of radio signals is prohibited when there is no voice signal being transmitted. This is to reduce the interference level and increase the system efficiency. In addition, this mechanism can also save the battery of the mobile station and prolong the standby time of the mobile station. Note that, the DTX function is not used for data transmission.
There are two transmission modes for the GSM system: one is the normal mode. In this case, the noise obtains the same transmission quality as the voice; the other is the discontinuous transmission mode. In this case, the mobile station only transmits the voice signals. The noise at the receiving end is artificial.
The artificial noise is used to inform the hearer that communication connection is ok when none of the subscribers are speaking. And the noise is designed as a comfortable noise which will not make the hearer uncomfortable.
The comfortable noise transmission also meets the requirements of the system measurement. In DTX mode, only 260bit codes are transmitted per 480ms; while in normal mode, 260bit codes are transmitted per 20ms. In the DTX mode, these 260 bits will generate SID (Silence Descriptor) frames. These frames, like the voice frames, will be processed via channel coding, interleaving, encryption and modulation, and then be transmitted in 8 continuous bursts. In other time, there is no message transmitted.
The DTX mode is optional. However, the transmission quality will be reduced a bit in the DTX mode. Especially when both ends of the communication are mobile subscribers, the influence on the transmission quality will be more severe because the DTX will be used twice on the same path. In addition, to implement the DTX function, the system should be able to indicate when to start the discontinuous transmission and when to stop it; and when the DTX is active the coder should be able to detect whether the signal is a voice signal or a noise signal. Thus, the VAD technique has to be used. The VAD algorithm determines whether each output frame contains voice or background noise by comparing the measured signal energy with the threshold defined for it. The principle of the determination is that the noise energy should always be lower than the voice energy.