When an MS is powered off it is detached from the network. When the subscriber switches power on, the MS scans the GSM frequencies for special channels called control channels. When it finds a control channel, the MS measures the signal strength it receives on that channel and records it. When all control channels have been measured, the MS tunes to the strongest one.
When the MS has just been powered on, the MS must register with the network which will then update the MS's status to idle. If the location of the MS is noticed to be different from the currently stored location then a location update will also take place.
As the MS moves through the network, it continues to scan the control channels to ensure that it is tuned to the strongest possible channel. If the MS finds one which is stronger, then the MS retunes to this new control channel2. If the new control
channel belongs to a new LA, the MS will also inform the network of its new location.
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