The
mobile phone is a low powered
radio transceiver that provides voice
telephone service to mobile users.
Cellular telephones operate as portable telephones; whereas normal telephone requires a cord to connect to a
jack in order to access the extensive
wireline networks operated by local telephone companies, a cellular telephone is not restricted by a
cord.
The mobile telephone works by transmitting radio waves to cellular towers. These towers vary in the area they cover, receiving nearby signals as short as 1.5 to 2.4km to distances as long as 48 to 56km. The area covered by a tower is referred to as a cell; the towers within these cells are networked to a central switching station usually by wires, fibreoptic cables or microwave. Central switching stations handling cellular calls in a given area are picked up by towers and relayed to the rest of the telephone network. Since cells overlap, as a caller moves from one cell into another the towers "hand off" calls so communication is uninterrupted.
A connection between a telephone and a base unit occupies one of 832 channels currently allocated to cellular communications. Each channel corresponds to radio waves of 2 frequencies; a lower frequency (range 825 - 850 MHz) that's emitted by the telephone and a higher frequency (870 - 895 MHz) that's emitted by the base unit.
Network management functions, performed at a central facility known as a Mobile Telephone Switching Office (MTSO), include the ability to measure and compare the transmission quality between a single handset and multiple towers. This function is important so that the MTSO can select the last transmission link between mobile telephones and towers. This optimal link is used to pass transmissions from one tower to another as the mobile telephone moves etween cells. All cell towers in a given area connect with the MTSO, which in turn has links to the wireline local exchange carrier that handles normal telephone calls. The link between MTSO and the wireline local telephone company is essential for connecting wireless and wireline calls. The vast majority of calls handled by a cellular radio network begin on the wireline network or end there.
A new generation of cellular radio technology, called Personal Communications Services (PCS) operates much like earlier celluar services, but at higher frequencies (around 1900 MHz). So PCS also uses completely digital transmissions rather than the analog transmissions that many current cellular telephone use. Digital transmissions convert sound into digital form, which can be transmitted faster and more efficiently than analog signals.
The high frequency waves ensure reliable cellular links to and from towers over short distances. However high frequency waves cannot effectively travel as far as low frequency signals. For cellular networks this limited range is advantageous, because ther are a limited number of radio frequencies available to cell phone companies. It also allows cellular network providers to accomodate a large number of users.
*this was an essay for an exam just gone...