TELEPHONES

X MICROWAVE RELAY  

In this method of transmission, radio waves generally in the superhigh-frequency band, called microwaves, are relayed from station to station. Because the transmission of microwaves requires a clear line of sight between sending and receiving stations, the average distance between relay stations is about 40 km (25 mi). As many as 600 telephone conversations can be transmitted over one microwave relay channel.

XI. SATELLITE TELEPHONY  

In 1969 the first global telephone relay network was completed with a series of satellites in stationary orbits 35,880 km (22,300 mi) above the Earth. These satellites are powered by solar energy cells. Calls transmitted from an Earth antenna are amplified and retransmitted to distant ground stations. The integration of satellite and terrestrial facilities allows calls to be routed between continents as easily as between domestic points. Thanks in large part to digitization of transmissions, satellites of the global Intelsat series can relay up to 33,000 calls simultaneously as well as several television channels.

One satellite would not serve for a call from New York to Hong Kong, for example, but two would. Even considering the expense of a satellite such a path is cheaper to install and maintain per channel than the equivalent path using coaxial cables on the ocean floor. Consequently, as much use is made as possible of satellite links in long distance.

Satellites do have one serious shortcoming, however. Because of the satellite’s distance and the finite speed of radio waves, there is a noticeable lag in conversational responses. Because of this, many calls will only use a satellite for one direction of transmission (say from New York to San Francisco) and will use a ground microwave or coaxial link for the opposite direction. The participants in a call from New York to Hong Kong might be annoyed if carried over a two satellite link in both directions because they would find it difficult to interrupt—which is a normal occurrence in speech. They would also be bothered by the long time (over a second) it took the other party to respond after each had finished speaking.

A combination of microwave, coaxial cable, light fibre, and satellite paths now link the major cities of the world. The capacity of each type of system depends on its age and the territory covered (submarine cables are engineered very conservatively and have less capacity than land-based cables) but generally fall in the following sequence: simple digitization over a parallel pair yields tens of circuits per pair, coaxial yields hundreds of circuits per pair and thousands per cable, microwave and satellite yield thousands of circuits per link, and optical fibre has the potential for tens of thousands of circuits per fibre. The capacity of each type of system has significantly increased since its first introduction because of steady engineering improvement.

XII. TELEPHONE AND BROADCASTING

Long-distance telephone facilities can carry radio and television programmes over great distances to many scattered stations for simultaneous broadcasting. In some cases, the audio portion of television programmes may be transmitted by wire circuits either at audio frequencies or at the carrier frequencies used to transmit telephone conversations. Television images are transmitted by coaxial cable, microwaves, and satellite circuit.

XIII. VIDEO TELEPHONE  

A two-way video telephone was first demonstrated in 1930 by the American inventor Herbert Eugene Ives in New York. The video telephone can be linked with a computer for displaying reports, charts, and schedules over long distances. It also enables face-to-face meetings of callers in different cities and can serve as a link between conference centres in a network of major cities. Video telephones are now commercially available and can be used on domestic lines for face-to-face calls. Similar features are also now viable between suitably equipped personal computers.

XIV. CELLULAR MOBILE COMMUNICATION

Cellular, or mobile phones, originally used in cars, airliners, and passenger trains, but increasingly becoming ubiquitous, are basically low-power radio-telephones. Calls go through radio transmitters that are located within small geographical units called cells. Because each cell’s signals are too weak to interfere with those of other cells operating on the same frequencies, more channels can be used than would be possible with high-power radio frequency transmission. Narrow-band frequency modulation (FM) is the most common mode of transmission, and each message is assigned a carrier unique to the cell from which it is transmitted. Since the cellular phone was first tested in 1978, the cellular market in Britain alone had grown at a rapid rate to over 8.5 million users by 1997. In Japan it is as high as one mobile phone per ten people. However, while the number of cellular users has increased, many new subscribers are low users. In the United Kingdom cable operators are gaining some 500,000 extra subscribers per month.

XV. VOICE MAIL  

Voice mail allows incoming messages to be recorded for later playback when the call is not answered. In advanced forms of voice mail the user may record a message to be sent later in the day.

For residential service voice mail can either be purchased from the telephone company as an exchange-based service or it is available by purchasing an answering machine. This usually contains a regular telephone set along with a recording, playback, and automatic ring detection capability. If an incoming call is answered at any telephone on the line before a pre-set number of rings, the answering machine does nothing. However, after the pre-set number of rings, the answering machine goes off hook and plays a pre-recorded message stating that the owner cannot answer the phone now and inviting the caller to leave a message to be recorded.

The answering machine’s owner is alerted to the presence of a recorded message by a light or audible "beep" and can retrieve the message later. Most answering machines and all exchange-based services also allow the owner to retrieve recorded messages from a remote location by dialling a code after the machine has answered.

XVI. TECHNOLOGICAL TRENDS  

Replacement of transoceanic coaxial cables by fibre-optic cables has continued through the 1990s. Advances in integrated-circuit technology and semiconductors have made it possible to design and market telephones that not only produce high-fidelity speech quality, but also offer a host of features such as pre-stored numbers, call forwarding, call waiting, and caller identification. Cellular telephony has grown dramatically, and cellular phones are now offered as standard equipment in many cars.