GSM mobile phones are a means of connecting the user to the telephone network. The phone number is associated with the Subscriber Information Module (SIM) card, a small “smartcard” that also stores the user’s address book.
In a call, speech is converted to a stream of data and passed to and from the base station, at the same time as both the handset and the base are monitoring what’s going on to help decide whether there is a better cell to transfer to.
When not in a call, the mobile keeps tabs on what base stations it can reach, and the network keeps track of where the mobile is.
All this is done in a way that uses as little battery power, radio transmission and computing overhead as possible.
Base stations provide coverage in a series of cells, arranged rather like the patches in a patchwork quilt. Each cell can be from 50 metres across to tens of miles across.
When you switch a mobile phone on, it searches for a suitable network, and chooses the best to log onto. It reports if it moves to another area, and logs on again every so often, but otherwise it just waits for a call.
As the mobile moves around, it transfers from cell to cell, with the network switching any current call, so that the changes are seamless.
The network has to keep track of where all its mobiles are, wherever in the world they happen to be, and be able to send an incoming call to wherever the mobile is.
Mobile phones use low-power transmitters running at frequencies similar to those used by TV transmitters, but at vastly lower power. [TV transmitter 2,000,000 watts: mobile phone average maximum 0.25 watts (GSM900) or 0.125 watts (GSM1800), but is normal for the power used to be one hundredth of this maximum or less.]
Mobile phone base stations normally have an output power of about 50 watts (yes, less than a dim light bulb), though this is beamed so that it goes out in a flat circle, over the heads of bystanders.