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However some countries still have an analogue service based on
AMPS or TACS. These provide national coverage with roaming facil-
ity and hand-off from cell to cell as a mobile travels across the
network, calling for more complex signalling than local trunking.
The control system must know at all times the location of all
operational mobiles. It does this by continuously monitoring signal
strength and instructing the mobile to change channel as it crosses
cell boundaries.
To permit the maximum re-use of frequencies mobiles are
instructed by the system controller to reduce transmitted power to the
minimum necessary for acceptable communications in either eight or
five steps, depending on the mobile classification, to 0.01 W. Mobiles
are classified by