A mobile phone base station is a radio transmitter and receiver that provides a radio link between mobile phones in the area around the base station and the telephone system.
Each service provider (e.g. O2, Vodaphone, T-mobile, Orange) has lots of base stations. Base stations next to each other use different channels so that they do not interfere with each other. The range of the base station is not very far so that the same channels can be reused at a base station some distance away. This means the whole country can have mobile phone access with a limited range of frequencies by reusing the same frequencies of channel many times.
Example of cells with frequency reuse
885.1-890.1 / 930.1-935.1 MHz
894.7-902.1 / 939.7 - 947.1 MHz
909.9-914.9 / 954.9 -959.9 MHz
(N.B. In a real system each cell would use many 200kHz bandwidth channels that were not adjacent to each other to avoid cross channel interference)
The location of all mobile phone base stations can be found at Sitefinder from the government communications agency, OFCOM.
The location of O2 mobile phone base stations can be found at webmap
The frequencies used by each mobile phone operator in Europe is available in the GSM Frequency Allocation list.
There is lots of detailed information about the GSM system on the web.
Click here for a worksheet to test your understanding some of this material.