Wireless Communications.

« 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 severalfrequency 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-talkiesbroadcast 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 40different 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 orham radio operators.

Shortwave radio broadcasts can travel long distances because of the concentration of ionized, or electrically charged, particles in the layer of theatmosphere known as the ionosphere.

The ionoshere reflects radio signals, so that signals transmitted upward are reflected back to the surface of Earth.

This skippingof waves against the ionosphere can greatly increase the range of the transmitter.

These broadcasts can travel thousands of kilometers.

Under certain conditions andon special “clear channel” frequencies, listeners of AM radio can receive a signal from several time zones away.

Shortwave radio listeners sometimes can receive signalsfrom 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 concentrationof ions necessary for reflecting radio waves only at the higher frequencies of the shortwave band.

At night, the ionosphere has the concentration necessary forreflecting lower frequencies within the lower parts of the shortwave band.

If there is an inadequate concentration of ions, the radio waves simply continue through theionosphere into space. D Cellular Radio Telephones 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 tothe rest of the public telephone system used by nonmobile callers.

An early form of radio telephone communicated with a single powerful antenna within a givengeographic 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 offrequencies that could be used, because radio telephone frequencies would often overlap and cause interference.

As a result, only a limited number of simultaneouscalls could be handled, because only a small block of channels could be generated over the available radio spectrum allocated for the service.

Modern cellular telephonesuse 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 smallerarea, 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 regulartelephones 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 centralmobile 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 amobile cell phone user (in a moving vehicle, for instance) moves from one cell area to another. E Satellite Communications Satellite communications services connect users directly to the telephone network from almost anywhere in the world.

Special telephones are available to consumersthat communicate directly with communications satellites orbiting Earth.

The satellites transmit these signals to ground stations that are connected to the telephonesystem.

These satellite services, while more expensive than cellular or other wireless services, give users access to the telephone network in areas of the world whereno wired or cellular telephone service exists.

Satellite phones are also able to deliver video images through videophones that use tiny cameras and transmit their imagesvia 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 tothe Internet.

To connect users with the Internet, Wi-Fi devices use low-power transmitters and receivers equipped with special computer chips containing radiomodems.

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 digitalbitstreams, the same format used by computers.

Wi-Fi-equipped computers, cell phones, and PDAs provide mobile, wireless access to e-mail and Internet sites.

Theradio 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 aWi-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 orhotel.

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 avast 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).

TheWiMax technology also expands the capabilities of broadband connections by enabling users to remain connected to Internet hot spots even when traveling in anautomobile 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 radiofrequency but sends its radio signals in short pulses across the entire radio spectrum.

This technology reduces interference and enables UWB to send larger amounts ofdata 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 beconnected 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 astandoff 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 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. »