Thursday, January 12, 2012

Cellular Radio Telephone


Cellular Radio Telephone

Cellular Telephone
Portable cellular telephones have become an invaluable tool for people who need to stay in touch while on the move. Cellular telephone systems combine radio and television technology with computer systems. As a caller moves from one geographical cell (the name given to a specific part of the area being covered by the system) to another, computers in switching offices transfer calls among variously located antenna transmitters without interrupting service.

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. 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 traveling or when in remote locations lacking regular phone service. As cellular radio service proliferates and achieves greater market penetration, some users have begun to consider it an alternative to conventional wire-based services.
Cellular Radio Telephone
Students use a cellular radio telephone, also known as a cell phone. As cell phones have grown in popularity, they have also decreased in size.

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 current 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.
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MECHANICS OF CELLULAR RADIO TELEPHONES
Cell Phone Tower
Cell phone towers are part of the wireless communications network that makes cellular radio telephone calls possible. Antennae on each tower receive high-frequency radio waves from cell phones. Their range varies from distances as short as 1.5 to 2.4 km (1.0 to 1.5 mi) to distances as long as 48 to 56 km (30 to 35 mi). Cell phone towers are now commonly seen along roadways as cell phone communication has become increasingly popular.

Both cellular radio and PCS use high-frequency radio waves to transmit calls. High-frequency waves have short wavelengths that pass by a given point at a very high rate. High-frequency waves can provide better sound quality and more reliable short-distance transmission than lower-frequency waves (such as AM radio) as they are less susceptible to sound degradation caused by the noise generated by weather, such as lightning which causes static, and other noise generators such as motors. However, high-frequency signals cannot effectively travel as far as low-frequency signals can.
For cellular networks, the limited range of high-frequency waves is actually advantageous because it means the same frequencies can be reused at nearby locations. Cell phone calls connect with short-range antennas known as towers. If there were only one tower for a large area, more customers would be trying to use the same high-frequency waves, and these waves would tend to overlap and cause interference. But because cell phone networks establish many towers covering small areas, a smaller number of customers access a given tower, and frequencies can be reused when a cell phone call is handed off from one tower to another as a mobile cell phone user travels. This ability to reuse frequencies is helpful because there are a limited number of radio frequencies available to cell phone companies. It also allows cellular network providers to accommodate a larger number of users.
How Wireless Communications Work
Cellular radio telephones, also known as cell phones, communicate by sending radio signals to a cell tower. Each cell tower has a certain range within which it can receive the radio signals. The range of each tower overlaps with that of another tower so as a mobile cell phone user travels, communication is uninterrupted. To communicate with the user of a wired telephone, the cell phone radio signals are routed from the cell tower to a mobile switching center, which in turn routes the signals to the telephone company. The signals then travel over telephone lines to reach a wired telephone.

The transceiver inside a cellular phone is a much more complex device than a conventional phone used over the wire-based network. A cellular telephone has circuitry that creates a unique identity code that is used to locate and track the telephone. This identity code is necessary for coordinating calls to and from the telephone, and for billing such calls. Because a cellular telephone user may move quite a distance during the duration of a call, the cellular radio network must manage calls from different tower sites as the telephone moves out of the range of one tower and into the range of another tower.
Current cellular telephones offer such features as a memory database for storing frequently called numbers and a lock to deter theft. Most cell phones, whether old or new, also have a small liquid crystal screen to display the telephone number being called or the number from which an incoming call originated. Many newer cell phones can display a short text message, much like a pager displays this information. Some cellular phones can also access the Internet and display text from Web sites, such as stock quotes and news stories. Internet-capable cell phones can also send and receive e-mail. Because mobile telephones use radio waves to send and receive calls, the device must include a power source. Rechargeable batteries provide the usual source of power, but most cell phones can also be attached to the cigarette lighter in a vehicle or to some other external power device.
III
THE CELLULAR NETWORK
The cells in a cellular radio network refer to the coverage area of each tower that receives and transmits calls from mobile telephones. The cells are arranged in a honeycomb pattern, and they overlap so that the system can handle increases in anticipated telephone traffic volume. Network management functions, performed by computers 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 best transmission link between mobile telephones and towers. This optimal link is then used to pass transmissions from one tower to another as the mobile telephone moves between cells.
All cell towers in a given area connect with the MTSO, which in turn has links to the wire-based local exchange carrier that handles normal telephone calls. The link between the MTSO and the wire-based local telephone company is essential for connecting wireless and wire-based calls. The vast majority of calls handled by a cellular radio network either begin on the wire-based network or end there.
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HISTORY
The first commercial cellular telephones were tested in the late 1970s by Illinois Bell in Chicago, Illinois, and they were a great success. Cellular service carriers began nationwide operations in the mid-1980s operating in the 800- to 900-MHz frequency band. Before the availability of cellular radio service, mobile telephone service consisted of bulky mobile telephone radio units. These two-way radio units communicated with a single antenna in a city or area. The radio signals often interfered with those of other commercial radios, and due to the technology of the day the frequencies could not be reused as they can today. Therefore, limited numbers of channels were available for callers, and the service was unreliable and costly. Because of the consumer demand for cellular telecommunications, the Federal Communications Commission (FCC) in the mid-1990s authorized up to six additional mobile telephone service providers in each service area.
There were more than 120 million wireless subscribers in the United States in 2001, according to the Cellular Telecommunications and Internet Association, with the number of new users increasing significantly each year. Forecasters and regulators did not anticipate this growth, however. Network equipment and start-up costs were substantial, and the cost to consumers was high. Providers had limited their networks to a small group of high-volume business users. Eventually, the providers recognized that their businesses would be more profitable if they created innovative service packages and aggressively marketed their services to the general public. As a result, prices for cellular telephones and network access have dropped considerably.
Cellular radio telephone service has achieved great commercial success because users recognize that mobile telephone access can improve productivity and enhance safety. Delivery drivers, repair technicians, lawyers, and other professionals were early adopters of mobile telephone service. As more geographic areas are covered by cellular networks and as rates drop, new subscribers are buying cellular services for personal security, safety on the road, and general convenience. See also Telecommunications; Wireless Communications.



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