Wednesday, October 19, 2016
Wifi Wireless Communications
Wifi 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,
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.
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.
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