BILL BACHMANN / SCIENCE PHOTO LIBRARY BILL BACHMANN / SCIENCE PHOTO LIBRARY
Mobile telephone. Businessman speaking on a cellular or mobile telephone outdoors. These telephones use low-power radio signals to transmit calls to a base station in the user's local area, or cell. A wide range of frequencies can be used in each cell because the signals are too weak to interfere with those in other cells. Base stations relay calls by cable to switching centres, from where calls can be transferred to other base stations, to the conventional telephone exchange, or to satellite stations for international calls.
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