Making calls or transferring data by mobile phone requires a mobile phone network. These networks consist of radio cells, each with their own base station (= transceivers which both transmit and receive signals) that provides them with a signal. The size of the cells and the number of base stations depends on the number of subscribers and the required data transfer rate. As the number of subscribers and the data transfer rate rise, a more tightly-knit mobile phone network, i.e. more base stations, will be required.
Mobile phones do not communicate directly with each other. They use base station antennas that transform the radio signal into a wire transmission signal and wire transmission signals into radio signals. A connection is established when a radio signal, in the form of electromagnetic waves, is sent from the mobile phone to the next base station. The call is then transferred to a central computer, either via radio relay or cable. The central computer serves as the switching centre and knows the location of all mobile phones registered in it. Depending on the number dialled, a connection is then established with the fixed network or with the corresponding radio cell, in which the called party is located at that moment. The base station of this radio cell then transmits the radio signal to the mobile phone of the receiving party.
To keep the central computer up to date with the location of all mobile phones, devices send out a short status signal at certain longer intervals whenever they are turned on. Mobile phones also receive the radio signals of the surrounding base stations and automatically choose the strongest one. If the signal of another base station becomes stronger, for instance, while the caller is travelling in a car, the new base station will take over the communication with the mobile phone.