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RADIOTELEPHONE In addition to the wireline telephones described in The telephone instrument there exist a number of wireless instruments that are connected to the public switched telephone network PSTN At the present time these wireless telephones generally fall into one of three categories cordless telephones cellular radio systems or personal communication systems Eventually these systems will be expanded to include global satellite-based telephony Cordless telephones Cordless telephones are devices that take the place of a telephone instrument within a home or office and permit very limited mobility up to a hundred metres Because they are plugged directly into an existing telephone jack they essentially serve as a wireless extension to the existing home or office wiring Cordless transceivers communicate with the plugged-in base unit over a pair of frequencies in the 46- and 48-megahertz bands or over a single frequency in the 902-928-megahertz band Cellular radio Cellular telephones are transportable by vehicle or personally portable devices that may be used in motor vehicles or by pedestrians Communicating by radiowave in the 800-900-megahertz band they permit a significant degree of mobility within a defined serving region that may be hundreds of square kilometres in area In this section the concept of cellular radio and the development of cellular systems are discussed cellular telecommunication All cellular radio systems exhibit several fundamental characteristics as summarized in the following 1The geographic area served by a cellular radio system is broken up into smaller geographic areas or cells Uniform hexagons most frequently are employed to represent these cells on maps and diagrams in practice though radiowaves do not confine themselves to hexagonal areas so that the actual cells have irregular shapes 2All communication with a mobile or portable instrument within a given cell is made to the base station that serves the cell 3Because of the low transmitting power of battery-operated portable instruments specific sending and receiving frequencies assigned to a cell may be reused
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