The mobile service provider divides the area covered by it into six parts. Each part is called a cell. It has an antenna and six control centers , called base stations. Each base station is controlled by Mobile Switching Center ( MSC ) , where call connection , call information recording , billing system etc. are controlled in a computerized manner. Cell sizes usually range from 1 mile to 12 miles , depending on the density of the population. Cells are usually hexagonal in appearance. However, it can be square , round or irregular in shape. Radio frequencies are used in cellular networks. Each cell has a different frequency allocation , which has an equivalent radio base station. A frequency class is used in a cell in such a way that the adjacent cells never have the same frequency. Mobile phones can be connected to multiple base stations and used anywhere at any distance. In case of transit, the subscriber or recipient can switch to one or more base stations by any user or both of them keeping the call uninterrupted. Calls can be received or sent from a mobile phone to a public telephone network or to another mobile phone.