Thursday, January 12, 2012

Wireless Communications


Wireless Communications

Wireless Communications, various telecommunications systems that use radio waves to carry signals and messages across distances. Wireless communications systems include cellular telephones, pagers, radio telegraphs, satellite telephones, laptop computers, personal digital assistants (PDAs), shortwave radios, and two-way radios. They are used primarily to transmit private communications. Commercial radio and television are also wireless telecommunications systems, but radio and television are mainly public broadcast services rather than private communications systems (see Radio and Television Broadcasting). This article focuses on wireless communications systems that are used primarily for private communications.
Wireless communications allow people greater flexibility while communicating, because they do not need to remain at a fixed location, such as a home or office, but instead can communicate with other people while traveling in a car or walking along a street. Wireless technologies make communications services more readily available than traditional wire-based services (such as ordinary telephones), which require the installation of wires in fixed locations. Wireless communications devices are useful in places where communications services are only temporarily needed, such as at outdoor festivals or large sporting events. These technologies are also useful for communicating in remote locations, such as mountains, jungles, or deserts, where wire-based telephone service might not exist. Police, fire, and other emergency departments use wireless devices, such as two-way radio, to communicate information between vehicles that are already responding to emergency calls. Construction and utility workers frequently use handheld radios for short-range communication and coordination. Many businesspeople use wireless devices, such as cellular radio telephones, also known as cell phones, to stay in contact with colleagues and clients while traveling. Increasingly, people are using wireless devices for a variety of everyday purposes, such as scheduling appointments, arranging meeting places, shopping for food, or agreeing on home video selections while in a video store.
All wireless communications devices use radio waves to transmit and receive signals. These devices operate on different radio frequencies so that signals from one device will not overlap and interfere with nearby transmissions from other devices. The number of companies offering wireless communications services has grown steadily in recent years. For example, in 1988 about 500 companies offered cell phone services. By 2001 that number had grown to more than 2,500 companies serving about 120 million subscribers. Currently, telecommunications companies throughout the world are activating more wireless service subscriptions than they are conventional wire-based service subscriptions. Wireless communication is becoming increasingly popular because of the convenience and mobility it affords; the expanded availability of radio frequencies for transmitting, which makes it possible to handle a larger volume of calls; and improvements in technology, which have added other services such as Internet access and improved the clarity of voice transmissions.
II
PRINCIPLES OF WIRELESS COMMUNICATIONS
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.

Wireless communications begin with a message that is converted into an electronic signal by a device called a transmitter. There are two types of transmitters: analog and digital. An analog transmitter sends electronic signals as modulated radio waves. The analog transmitter modulates the radio wave to carry the electronic signal and then sends the modified radio signal through space. A digital transmitter encodes electronic signals by converting messages into a binary code, the series of zeros and ones that are the basis of all computer programming. The encoded electronic signal is then sent as a radio wave. Devices known as receivers decode or demodulate the radio waves and reproduce the original message over a speaker.
Wireless communications provide more flexibility than wire-based means of communication. However, there are some drawbacks. Wireless communications are limited by the range of the transmitter (how far a signal can be sent), and since radio waves travel through the atmosphere they can be disturbed by electrical interferences (such as lightning) that cause static.
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.

Wireless communications systems involve either one-way transmissions, in which a person merely receives notice of a message, or two-way transmissions, such as a telephone conversation between two people. An example of a device that only receives one-way transmission is a pager, which is a high-frequency radio receiver. When a person dials a pager number, the pager company sends a radio signal to the desired pager. The encoded signal triggers the pager’s circuitry and notifies the customer carrying the pager of the incoming call with a tone or a vibration, and often the telephone number of the caller. Advanced pagers can display short messages from the caller, or provide news updates or sports scores.
Two-way transmissions require both a transmitter and a receiver for sending and receiving signals. A device that functions as both a transmitter and a receiver is called a transceiver. Cellular radio telephones and two-way radios use transceivers, so that back-and-forth communication between two people can be maintained. Early transceivers were very large, but they have decreased in size due to advances in technology. Fixed-base transceivers, such as those used at police stations, can fit on a desktop, and hand-held transceivers have shrunk in size as well. Several current models of handheld transceivers weigh less than 0.2 kg (0.5 lb). Some pagers also use transceivers to provide limited response options. These brief return-communication opportunities allow paging users to acknowledge reception of a page and to respond using a limited menu of options.
III
MODES OF WIRELESS COMMUNICATION
Wireless communications systems have grown and changed as technology has improved. Several different systems are used today, all of which operate on different radio frequencies. New technologies are being developed to provide greater service and reliability.
A
Sea and Air Transceivers
Radio Telegraph Operators
Navy radio telegraph operators on shore type messages to ships at sea in this photo. Radio telegraph transmitters send typed messages by radio waves to receiving stations located within broadcasting range of the transmitter. Early radio telegraphs transmitted keystrokes into electric pulses, which the receiving telegraph would translate into letters.

The first wireless communications devices were radio telegraphs. A telegraph is a device that sends simple electrical pulses along copper wires or through the air as radio waves. The pulses were caused by contact between two metal surfaces, and receivers interpreted these electrical pulses as tones or beeps. A code of long and short signals was developed to represent the letters of the alphabet (see Morse Code, International), and in this way coded messages could be sent between telegraphs. Radio telegraphs used radio waves rather than wire telegraph lines to send and receive messages. Radio telegraphs sent telegraph signals over long distances and were ideal for ship-to-shore communication. Bulky radio telegraphs were installed on ships as early as 1899 and were widely used by 1905.
Maritime Radio
Ships must carry communications equipment on board for safety and navigation. Military vessels have entire rooms devoted to radio equipment, but a small ship’s radio system can fit on a desktop.

Maritime and aviation telecommunications systems now use high-frequency radios and satellites capable of transmitting speech and text, rather than wireless telegraphy, to send messages. Aircraft pilots use radios to communicate with air traffic controllers at airports and also to communicate with other pilots. Navigation beacons are equipped with transmitters that send automated signals to help ships and aircraft in distress determine their positions. While high-frequency radio can transmit signals over long distances, the quality of these signals can be diminished by bad weather or by electrical interference in the atmosphere, which is often caused by radiation from the Sun.
B
Handheld Radio Transceivers
Police, fire, and other emergency organizations, as well as the military, have used two-way wireless radio communication since the 1930s. Early vehicle-based radios were large, heavy units. After the invention of the transistor in 1948, radios shrank in size to small handheld radio transceivers. Public two-way radios with several frequency options are widely available as well. Usually limited in range to a few miles, these units are great aids for such mobile professionals as construction workers, film crews, event planners, and security personnel. Simpler two-way radios, called walkie-talkies, have been popular children’s toys for years. Most walkie-talkies broadcast on channel 14 of the citizens band (CB), a range of frequencies grouped into channels and allocated for public use. CB radios can transmit and receive on 40 different channels. An unlicensed radio service, the Family Radio Service, allows individuals to use high-frequency wireless devices with a range of up to 3.2 km (2 mi).
C
Shortwave Transceivers
Long-range broadcast services and frequencies, in what is known as the shortwave radio band (with frequencies of 3 to 30 megahertz), are available for amateur or ham radio operators. Shortwave radio broadcasts can travel long distances because of the concentration of ionized, or electrically charged, particles in the layer of the atmosphere known as the ionosphere. The ionoshere reflects radio signals, so that signals transmitted upward are reflected back to the surface of Earth. This skipping of waves against the ionosphere can greatly increase the range of the transmitter. These broadcasts can travel thousands of kilometers. Under certain conditions and on special “clear channel” frequencies, listeners of AM radio can receive a signal from several time zones away. Shortwave radio listeners sometimes can receive signals from the other side of the world. The degree of reflectivity of the ionosphere depends on the time of day. During daylight hours, the ionosphere has the concentration of ions necessary for reflecting radio waves only at the higher frequencies of the shortwave band. At night, the ionosphere has the concentration necessary for reflecting lower frequencies within the lower parts of the shortwave band. If there is an inadequate concentration of ions, the radio waves simply continue through the ionosphere into space.
D
Cellular Radio Telephones
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 radio telephones, or cell phones, combine their portable radio capability with the wired, or wire-based, telephone network to provide mobile users with access to the rest of the public telephone system used by nonmobile callers. An early form of radio telephone communicated with a single powerful antenna within a given geographic or metropolitan area. This large antenna was wired to the telephone system. With only one antenna for a large metro area, this limited the number of frequencies that could be used, because radio telephone frequencies would often overlap and cause interference. As a result, only a limited number of simultaneous calls could be handled, because only a small block of channels could be generated over the available radio spectrum allocated for the service. Modern cellular telephones use a network of several short-range antennas known as towers that connect to the telephone system. Because the antennas have a shorter range and cover a smaller area, often as short as 1.5 to 2.4 km (1.0 to 1.5 mi), frequencies can be reused a short distance away without overlapping and causing interference.
Cell phone towers pick up requests from cell phones for a dial tone and also deliver inbound calls to the appropriate cell phone or deliver calls to people using regular telephones on the wire-based system. To do any of these things, the cell phone must have a singular identity that can be recognized by computers housed in a central mobile telephone switching office (MTSO). When a cell phone is turned on, it connects by radio waves to the nearest cell tower (tower receiving the strongest signal). The cell towers are spaced so their receiving ranges slightly overlap. This continuous contact makes it possible for the MTSO to transfer a call from tower to tower as a mobile cell phone user (in a moving vehicle, for instance) moves from one cell area to another.
E
Satellite Communications
Communications Satellite
The Syncom 4 communications satellite was launched from the space shuttle Discovery. Modern communications satellites receive, amplify, and retransmit information back to earth, providing television, telefax, telephone, radio, and digital data links around the world. Syncom 4 follows a geosynchronous orbit—that is, it orbits at the same speed as the earth spins, keeping the satellite in a fixed position above earth. This type of orbit enables uninterrupted communication links between ground stations.

Satellite communications services connect users directly to the telephone network from almost anywhere in the world. Special telephones are available to consumers that communicate directly with communications satellites orbiting Earth. The satellites transmit these signals to ground stations that are connected to the telephone system. These satellite services, while more expensive than cellular or other wireless services, give users access to the telephone network in areas of the world where no wired or cellular telephone service exists. Satellite phones are also able to deliver video images through videophones that use tiny cameras and transmit their images via the satellite phone.
F
Radio Modems
Wi-Fi, an abbreviation for wireless fidelity, is a wireless communication technology that can provide connections between portable computers and wired connections to the Internet. To connect users with the Internet, Wi-Fi devices use low-power transmitters and receivers equipped with special computer chips containing radio modems. The chips can be installed in laptop computers, personal digital assistants (PDAs), and cellular telephones.
Radio modems provide the same functions as modems that operate with conventional wire-based networks: They modulate and demodulate signals to mimic digital bitstreams, the same format used by computers. Wi-Fi-equipped computers, cell phones, and PDAs provide mobile, wireless access to e-mail and Internet sites. The radio modems must be in range of a Wi-Fi device containing a transmitter and receiver that is connected to a landline providing Internet access. Areas within range of a Wi-Fi transmitter and receiver are known as hot spots.
Current technical standards limit the range to distances of about 90 m (300 ft). Many transmitters, however, can be linked to cover a wider area, such as an airport or hotel. Current Wi-Fi standards enable data to be sent at high speeds ranging from 11 to 54 megabits per second. This is known as a broadband connection because a vast amount of data can be sent quickly. A new technology known as WiMax promises to extend the range of a transmitter and receiver to about 48 km (30 mi). The WiMax technology also expands the capabilities of broadband connections by enabling users to remain connected to Internet hot spots even when traveling in an automobile or train at speeds up to 250 km/h (155 mph).
G
Ultrawideband (UWB)
Wi-Fi may eventually give way to another radio technology known as ultrawideband (UWB), according to some experts. Unlike Wi-Fi, UWB does not use a single radio frequency but sends its radio signals in short pulses across the entire radio spectrum. This technology reduces interference and enables UWB to send larger amounts of data than Wi-Fi. UWB is expected to be used to connect all types of electronic equipment within a home without the use of wires. For example, stereo speakers could be connected to a high-definition television set, and the television could receive signals from a DVD player, and the DVD player could be connected to a personal computer, and all these connections could be done wirelessly.
A single standard for UWB technology was approved in March 2005 by the Federal Communications Commission (FCC). The single standard was expected to end a standoff between various industry groups and lead to faster implementation of UWB technology. Devices using UWB technology could reach the marketplace by 2006, according to some predictions.
IV
HISTORY
Guglielmo Marconi
Inventor of the radio-signaling system, Italian electrical engineer Guglielmo Marconi was the first to send wireless signals across the ocean. Prior to his invention, there was no way to communicate over long distances without telegraph wires to carry electric signals. His equipment played a vital role in rescuing survivors of sea disasters such as the sinking of the Titanic. He won the Nobel Prize in physics in 1909 for his work in wireless telegraphy.

The idea of wireless radio communications arose in the mid-1800s from the theories of two English physicists, Michael Faraday and James Clerk Maxwell. In 1888 German physicist Heinrich Hertz applied these theories to construct a spark-gap transmitter, a device that generated radio waves from an electric spark. In 1895 Italian electrical engineer Guglielmo Marconi extended the range of such transmissions and adapted the technology to send and receive wireless telegraph signals. In 1901 Marconi built the first transoceanic telegraph transmitter, which had a 3,400 km (2,100 mi) link from Poldhu, Cornwall, England, to St. John’s, Newfoundland.
Developments in vacuum tube technology in the early 1900s by English physicist and engineer Sir John Ambrose Fleming and American inventor Lee De Forest made it possible to modulate and amplify wireless signals to send voice transmissions. The range and clarity of voice transmissions increased as advancements in technology were made, and in 1915 the American Telephone & Telegraph Company transmitted a voice message by radio between the United States and France. By the 1930s small two-way radio transmitters were in common use among law enforcement and civil emergency authorities. Improvements in technology have made two-way communications systems smaller and lighter, with extended range and capabilities. See also Radio: History; Telecommunications: History; Telegraph.

No comments:

Post a Comment