Telephone II :



IV TRANSOCEANIC TELEPHONY
Overseas radio-telephone service was introduced commercially in 1927, but the problem of amplification prevented the laying of telephone cables until 1956, when the world’s first transoceanic submarine telephone cable, extending between Newfoundland and Scotland, was placed in service.



V CARRIER-CURRENT TELEPHONY
Through the use of frequencies above the voice range, extending from about 4,000 to several million cycles per second, or hertz, as many as 13,200 telephone messages can be carried simultaneously over a single conducting medium. Carrier-current telephony techniques are also being used to send telephone messages over the normal distribution lines without interfering with regular service. With the growth in size and complexity of systems, solid-state amplifiers, called repeaters, are used to amplify the messages at regular intervals.



VI COAXIAL CABLE
Developed in 1936, the coaxial cable uses cable conductors to carry a large number of circuits. The modern coaxial cable consists of copper tubes 0.95 cm (0.375 in) in diameter. Each has a thin copper wire held exactly in the centre of the tube by plastic disc insulators about 2.5 cm (1 in) apart. The tube and the wire have the same centre; that is, they are coaxial. The copper tubes shield the transmitted signal from electrical interference and prevent energy losses by radiation. A cable, consisting of up to 22 coaxial tubes arranged in tight rings sheathed in polyethylene and lead, can carry 132,000 messages simultaneously.



VII OPTICAL FIBRES
Coaxial cables are increasingly being replaced by optical glass fibres. Messages are digitally coded into pulses of light and transmitted over great distances by these slender fibres. A fibre cable may contain up to 50 fibre pairs, each pair carrying up to 4,000 voice circuits. The basis of the new fibre optics technology, the laser, exploits the visible region of the electromagnetic spectrum, where frequencies are thousands of times higher than in radio and thus able to carry much larger volumes of information. The light-emitting diode (LED), a simpler device, is adequate for most transmission purposes.
One fibre-optic cable, TAT 8, carries more than twice the number of transatlantic circuits that were available in the 1980s. Used in a system that stretches from New Jersey to Britain and France, it can transmit up to 50,000 conversations at once. Such cables also provide channels for high-speed transmission of computer data that are more secure than those offered by communications satellites. Another major advance in telecommunications, TAT 9, which is an even higher capacity fibre cable, came into operation in 1992 and can carry 75,000 calls simultaneously... .

To be continued .