GSM defines five classes of mobile stations, according to their
peak transmitter power, rated at 20, 8, 5, 2, and
0.8 W. To minimize co-channel interference and to conserve power,
both the mobiles and the base transceiver station (BTS) operate
at the lowest power level at which an acceptable signal
quality can be maintained. Power levels can be stepped up
or down in steps of 2 dB from the peak
power for the class down to a minimum of 13
dBm (20 mW).
Signal quality is measured by mobile and
base station. The mobile station passes results of its measurements
to the BSC, which is responsible for power control.