telecommunications
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Communications over a distance, generally by electronic means. Long-distance voice communication was pioneered in 1876 by Scottish scientist Alexander Graham Bell when he invented the telephone. The telegraph, radio, and television followed. Today it is possible to communicate internationally by telephone cable or by satellite or microwave link, with over 100,000 simultaneous conversations and several television channels being carried by the latest satellites.
History
The first mechanical telecommunications systems were semaphore and the heliograph (using flashes of sunlight), invented in the mid-19th century, but the forerunner of the present telecommunications age was the electric telegraph. The earliest practicable telegraph instrument was invented by William Cooke and Charles Wheatstone in Britain in 1837 and used by railway companies. In the USA, Samuel Morse invented a signalling code, Morse code, which is still used, and a recording telegraph, first used commercially between England and France in 1851.Following German physicist Heinrich Hertz's discovery of electromagnetic waves, Italian inventor Guglielmo Marconi pioneered a ‘wireless’ telegraph, ancestor of the radio. He established wireless communication between England and France in 1899 and across the Atlantic in 1901.
The modern telegraph uses teleprinters to send coded messages along telecommunications lines. Telegraphs are keyboard-operated machines that transmit a five-unit Baudot code (see baud). The receiving teleprinter automatically prints the received message. The modern version of the telegraph is e-mail in which text messages are sent electronically from computer to computer via network connections such as the Internet.
Communications satellites
The chief method of relaying long-distance calls on land is microwave radio transmission. The drawback to long-distance voice communication via microwave radio transmission is that the transmissions follow a straight line from tower to tower, so that over the sea the system becomes impracticable. A solution was put forward in 1945 by the science fiction writer Arthur C Clarke, when he proposed a system of communications satellites in an orbit 35,900 km/22,300 mi above the Equator, where they would circle the Earth in exactly 24 hours, and thus appear fixed in the sky. Such a system is now in operation internationally, by Intelsat. The satellites are called geostationary satellites (syncoms). The first to be successfully launched, by Delta rocket from Cape Canaveral, was Syncom 2 in July 1963. Many such satellites are now in use, concentrated over heavy traffic areas such as the Atlantic, Indian, and Pacific oceans. Telegraphy, telephony, and television transmissions are carried simultaneously by high-frequency radio waves. They are beamed to the satellites from large dish antennae or Earth stations, which connect with international networks.Integrated Services Digital Network (ISDN) makes videophones and high-quality fax possible; the world's first large-scale centre of ISDN began operating in Japan in 1988. ISDN is a system that transmits voice and image data on a single transmission line by changing them into digital signals.
Fibre-optic cables consisting of fine glass fibres present an alternative to the usual copper cables for telephone lines. The telecommunications signals are transmitted along the fibres in digital form as pulses of laser light.
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