l GSM defines five power classes of user terminal. There are eight power classes in the full specification, but only five seem to apply to mobile telephones. These are shown in Table 7.1. These power levels are the burst power for the one active time slot SO the av- erage transmit power is one-eighth of these values. GSM power control is managed by the base station, the same as in AMPS. There are 15 power levels separated by 2 dB for a total range of 28 dB, the same as in AMPS. Each class of GSM mobile telephone can go down 15 steps from its own maximum power, the full range of 28 dB. The 2-dB changes can come every 60 ms, every 13 bursts at full rate. The GSM base station transmitters can bring their power all the way down to 13 dBm, 20 mW. 7.4.3 GSM handover GSM handovers occur when a call has poor signal quality or low signal path gain. They also occur for mobile telephones that are too far away from their serving base stations and for calls where the system determines that the call can be served somewhere else at a lower power level. After using one time slot to transmit and one to receive, the GSM mobile telephone makes use of the other six time slots to make power measurements of other cells. This information is sent on the SACCH to the serving BSC, where the handover determi- nation is made. We call this a mobile assisted handover procedure. There are four types of GSM handovers depending on how far up the chain the handover has to go. Type 1, intra-BTS handover. The call is changing from one face to another face in the same cell. The call keeps the same MSC, BSC, and BTS. Type 2, intra-BSS, inter-BTS handover. The call is changing from one cell to another cell covered by the same controller. The call keeps the same MSC and BSC but has a new BTS. TABLE 7.1 Eight GSM Power Classes Power Mobile telephone Base transceiver class maximum power, maximum power, 20.0 W 320 W 8.0 W 160 W 5.0 W 2.0 W 0.8 W Not applicable Not applicable Not applicable 2.5 W Downloaded from Digital Engineering Library @ McGraw-Hill (w