ntrol, it provides an opportunity to improve the efficiency of use of the spectrum. 2.4.9 Discontinuous Transmission Discontinuous transmission (DTX) is a feature that controls the power of the transmission when the mobile is in 'silent' mode. When the subscriber is not speaking on the mobile, a voice detector in the equipment detects this and sends a burst of transmission bits to BSS, indicating this inactivity. This function of the mobile is called voice-activity detection (VAD). On receiving this stream of bits indicating DTX, the BSS asks the mobile to reduce its power for that period of time, thereby reducing interference in the network and improving the efficiency of the network. 2.4.10 Frequency Hopping Before we go into the concept and process of frequency hopping, let us understand the frequency assignment criteria in the GSM network. In GSM 900, the frequency bands used are 890-915 MHz in the uplink direction and 935-960 MHz in the downlink direction, which means a bandwidth of 25 MHz in each direction. The whole or some fraction of this band is available to the network operator. The central frequencies start at 200 kHz from the 'edge' of the band and are spread evenly in it. There are 125 frequency slots in this band. The major interference problem is between the adjacent bands because of frequency overlapping at the borders of the individual channels. For this simple reason, the adjacent (and same-frequency) channels) are not used on the cells belonging to the same site. Frequency hopping is a technique by which the frequency of the signal is changed with every burst in such a way that there is minimum interference in the network, and at the same RADIO NETWORK DETAILED PLANNING time allocated channels are used effectively. This process in GSM is also known as slow frequency hopping (SFH). By using SFH, improvement takes place by virtue of frequency diversity and interference diversity. Frequency diversity: Since every burst has a different frequency, it will fade in a different way and time. Thus the 