Wireless technology provides the ability to communicate between two or more entities over distances without the use of wires or cables of any sort. This includes communications using radio frequency (RF) as well as infrared (IR) waves .
The birth of wireless technology started with the discovery of electromagnetic waves by Heinrich Hertz (1857–1894). Guglielmo Marconi (1874–1937) established the very first commercial RF communications, the wireless telegraph, in the late 1890s—more than fifty years after the first commercial wired telegraph service that was demonstrated in 1832 by Samuel F. B. Morse (1791–1872). Marconi was also the first to transmit radio signals to a mobile receiver on ships in the early 1900s. Wireless technology has always been preceded by wired technology and is usually more expensive, but it has provided the additional advantage of mobility, allowing the user to receive and transmit information while on the move.
Another major thrust of wireless technology has been in the area of broadcast communications like radio, television, and direct broadcast satellite. A single wireless transmitter can send signals to several hundreds of thousands of receivers as long as they all receive the same information. Today, wireless technology encompasses such diverse communication devices as garage-door openers, baby monitors, walkie-talkies, and cellular telephones, as well as transmission systems such as point-to-point microwave links, wireless Internet service, and satellite communications.
Wireless technology involves transmitting electromagnetic signals over the air. Interference and obstacles that block RF signals are common problems with wireless technology. Wireless technology allows users to communicate simultaneously over the same medium without their signals interfering with one another. This is made possible because of two physical phenomena—the weakening of electromagnetic signals with distance, and the electromagnetic spectrum . While listening to a radio station as one drives along a highway, one can observe how an RF or IR signal rapidly loses its strength as it travels away from the transmitter. Thus, two people can transmit at the same time if they are sufficiently far apart. If there are no obstacles, signals fall as the square of the distance. This is called freespace loss.
RF and IR signals can also be generated at different frequencies that do not interfere with each other. The range of frequencies is from a few cycles per second—called hertz (Hz) in honor of the scientist who discovered electromagnetic waves—to trillions of hertz, and is called the electromagnetic spectrum. Visible light is included on this spectrum. The 3 kilohertz (kHz) to 300 gigahertz (GHz) frequency range is the RF spectrum. The IR spectrum corresponds to frequencies beyond 300 GHz. There are strict government regulations on the usage of chunks of the RF spectrum (called frequency bands ) in all nations of the world. In the United States, the Federal Communications Commission (FCC) decides who uses what frequency bands and for what purpose. They also set limitations on transmit power and on how much interference can be caused between frequency bands.
Frequency bands are either licensed or unlicensed. Licensed bands are owned by certain companies or facilities for specific purposes and cannot be used by anyone else. Unlicensed bands are free and anyone can use them, subject to certain etiquettes. Licensed bands are usually free from interference and more reliable, since there is control over who can transmit in them.
Numerous applications of wireless technology continue to appear on the horizon. Wireless keyboards and mice for computers, wireless speakers and headphones, and wireless smart sensors are now available on the market. In addition to point-to-point microwave links and broadcast radio, wireless appliances and devices are becoming increasingly common. Wireless technology is often employed to provide communications in places where it is difficult to run cables, for mobile communications, as extensions to wired communications, and for emergency deployment. Bluetooth is a new cable replacement wireless technology that can connect almost any appliance that can be networked to any other appliance—a digital camera to a laptop, or a coffee machine to the Internet, for example. Bluetooth applications include cordless telephones, laptops, and other devices.
Wireless technology can also be classified based on voice or data applications or based on mobility—fixed, stationary, portable, and mobile. Cordless and cellular telephones are common examples of voice applications. Cordless telephones operate in unlicensed bands, and cell phones in licensed bands, at frequencies around 1,000 megahertz (MHz) . Satellites have been used for a long time to provide voice communications. Pagers are examples of data applications.
Today, it is also possible to access the Internet using wireless technology. Cellular digital packet data (CDPD) service is available for accessing the Internet in the same licensed frequency bands as cell phones. It is possible to buy a CDPD modem for handheld computers and palmtops and to browse the web and send e-mail without connecting via a cable to the Internet.Wireless local area networks (WLANs) in unlicensed bands are also very popular, both in companies and for residential networking, for shared access to the Internet.
Modern fixed wireless technology applications include wireless local loops (WLLs) that provide local telephone service using rooftop antennas, and local multipoint distribution service (LMDS), a digital wireless transmission at 28 GHz that can provide several megabits per second of data for access to the Internet. Stationary wireless technology includes desktop computers that connect to the Internet using WLANs. Cordless phones, laptops, and palmtop computers with wireless connectivity fall into the portable category, while cell phones are the most common example of mobile wireless technology.
Wireless access to the Internet is expected to exceed wired access in the next few years, and the prospects for the future are exciting.
see also Network Topologies; Networks; World Wide Web.
Carr, Joseph J. Microwave and Wireless Communications Technology. Boston: Butterworth-Heinemann, 1997.
Gralla, Preston. How Wireless Works. Indianapolis, IN: Que, 2002.