 signal. Based on these measurement results reported to the BSS the needed power level can be calculated for both the MS and BTS transmissions. The measurements will ensure that the transmission power is kept at a certain level that the quality of the connection does not fall below required level. 2.7.2 Discontinuous Transmission Discontinuous transmission (DTX) is another method used to decrease co-channel interference and therefore improve spectrum efficiency. When co-channel interference decreases the quality of those particular cells improves. DTX works for both downlink and uplink. In the uplink direction, discontinuous transmission also saves the battery lifetime. The discontinuous transmission feature is based on recognition of silent moments. During any tele- phone conversation silent moments occur, because in an average conversation one person normally speaks for half of the time. The mobile is able to separate the silent and the conversation moments using voice activity detection (VAD). During silent moments DTX minimizes the transmission. 2.7.3 Frequency Hopping Frequency hopping is a feature of the GSM system used to decrease the simultaneous usage of the same frequencies and in this way averages the interference level. Compared to normal frequency re-use, which is static, frequency hopping provides a benefit by allowing the dynamic frequency to change. In practice, by using the frequency hopping algorithm the carrier frequency changes either cyclically or randomly. By frequency hopping the interference is averaged more effectively, which increases the overall perceived quality. Frequency diversity and interference averaging can be gained using frequency hopping. The GSM channel structure was referred to in Section 2.7.1 (Power Control) and has been specified in Section 2.6.1 (Capacity Planning in GSM Networks). As discribed, the carrier frequencies are divided timewise into eight timeslots, according to time division multiple access (TDMA). One timeslot is allocated to serve one call, i.e. one subscr