January 21, 2010
Cellular Radio Telephone, also called cellular telephone or cell phone, low-powered, lightweight radio transceiver (combination transmitter-receiver) that provides voice telephone and other services to mobile users. Cellular telephones primarily operate like portable or cordless telephones. However, unlike conventional wire-based cordless phones, cellular telephones are completely portable and do not require proximity to a jack to access the wire-based networks operated by local telephone companies. A new generation of services for cell phones includes videoconferencing and Internet access with the ability to send e-mail. Enhanced cell phones known as smartphones can have many additional features and capabilities that make them similar to handheld personal computers and personal media devices. Cellular telephones have become very popular with professionals and consumers as a way to communicate while away from their regular, wire-based phones—for example, while commuting, at work, shopping, or traveling, as well as when in remote locations lacking regular phone service. As cellular radio service proliferates and achieves greater market penetration, some users have begun to use cell phones as an alternative to conventional wire-based services. Worldwide, cell phone use has expanded greatly since the beginning of the 21st century, growing to more than 4 billion registered subscribers in 2008.
Cellular telephones work by transmitting radio signals to cellular towers. These towers vary in their capability to receive cellular telephone signals. Some towers can receive signals from distances of only 1.5 to 2.4 km (1.0 to 1.5 mi), while others can receive signals from distances as far as 48 to 56 km (30 to 35 mi). The area a tower can cover is referred to as a cell. However, more than one tower may exist in a given cell area. The cells overlap so that the system can handle increased telephone traffic volume. The towers within these cells are networked to a central switching station, usually by wire, fiber-optic cable, or microwave. The central switching station handling cellular calls in a given area is directly connected to the wire-based telephone system. Cellular calls are picked up by the towers and relayed to another cell telephone user or to a user of the conventional wire-based telephone network. Since the cells overlap, as a mobile caller moves from one cell into another, the towers “hand off” the call so communication is uninterrupted.
Cellular phone networks exist in nearly every metropolitan area throughout the world, and cellular coverage is expanding in rural areas. Due to the convenience and mobility of cellular telephones, users typically pay a higher fee than they would for normal telephone use. A newer generation of cellular radio technology, called Personal Communications Services (PCS), operates much like earlier cellular services, but at higher frequencies, the number of times a radio wave oscillates or completes a cycle, which is measured in a unit known as a hertz (Hz). (The higher frequencies of PCS operate at around 1900 megahertz [MHz] in the United States.) PCS also utilizes completely digital transmissions, rather than both the analog and digital transmissions that many cellular telephones use. Digital transmissions convert sound into digital form, which can be transmitted more efficiently than analog signals. Digital technologies can also generate more channel capacity over the same amount of the radio spectrum. Telephone companies have also developed so-called 3G (for “third generation”) cellular networks that provide wide-area Internet access at broadband-like speeds.