Communications Cable

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Communications cable

A cable that transmits information signals between geographically separated points. The heart of a communications cable is the transmission medium, which may be optical fibers, coaxial conductors, or twisted wire pairs. A mechanical structure protects the heart of the cable against handling forces and the external environment. The structure of a cable depends on the application.

Optical communications cables are used in both terrestrial and undersea systems. Optical communications cables for terrestrial use may be installed aerially, by direct burial, or in protective ducts. The terrestrial cable requires only enough longitudinal strength to support its own weight over relatively short pole-to-pole spans, or to allow installers to pull the cable into ducts or lay it in a trench. For the undersea cable, the high-strength steel strand allows it to be laid and recovered in ocean depths up to 4.5 mi (7315 m). See Optical communications

Optical communications cables are often used to carry input and output data to computers, or to carry such data from one computer to another. Then they are generally referred to as optical data links or local-area networks. The links are generally short enough that intermediate regeneration of the signals is not needed. See Fiber-optic circuit, Local-area networks

Signals in these cables are carried by light pulses which are guided down the optical fiber. In most applications, two fibers make up a complete two-way signal channel. The guiding effect of the fiber confines light to the core of the glass fiber and prevents interference between signals being carried on different fibers. The guiding effect also delivers the strongest possible signal to the far end of the cable. Exceptionally pure silica glass in the fiber minimizes light loss for signals passing longitudinally through the glass fiber.

Optical cable systems are usually digital. Thus, information is coded into a train of off-or-on light pulses. These are detected by a photodetector at the far end of a cable span and converted into electronic pulses which are amplified, retimed, recognized in a decision circuit, and finally used to drive an optical transmitter. In the transmitter, a laser converts the electric signals back into a train of light pulses which are strong enough to traverse another cable span. By placing many spans in tandem, optical cable systems can carry signals faithfully for thousands of miles.

Rather than undersea regenerators, current optical-fiber cable systems use erbium-doped fiber amplifiers (EDFAs) to boost the optical signal on long spans. Conversion from optical to electronic modes and back again is then not needed in the undersea repeaters.

Coaxial communication systems evolved before optical systems. Most of these systems are analog in nature. Signals are represented by the amplitude of a wave representing the signal to be transmitted. In a multichannel system, each voice, data, or picture signal occupies its unique portion of a broadband signal which is carried on a shared coaxial conductor or “pipe.” In the transmitting terminal, various signals are combined in the frequency-division transmitting multiplex equipment. At the receiving end of a link, signals are separated in the receiving demultiplex equipment. This combining and separation operates much as broadcast radio and television do, and the principles are identical. See Coaxial cable, Electrical communications, Telephone service, Telephone systems construction

Communications Cable


a cable used to transmit information by currents of various frequencies. Telegrams, photographs, telephone conversations, sound and television broadcasts, statistical data for computer centers, and signals of telemechanical systems are transmitted through communications cables.

The history of communications cables began almost 150 years ago, soon after the invention of the electric telegraph by the Russian scientist P. L. Shilling in 1832. The current-carrying conductors or telegraph cables were insulated at first with guttapercha and later with cotton fabric impregnated with an insulating compound; the conductors were twisted together, forming a core. The core was pulled into steel or lead tubing to provide protection against moisture. The use of cores enclosed in a continuous lead sheath began in the late 1870’s. Telegraph cables were operated in a single-wire system; the ground was the second conductor.

The manufacture of symmetrical cables for urban telephone exchanges began after the invention of the telephone in 1876. The telephone cables differed from telegraph cables in that they had twisted pair lay-up. To improve signal transmission characteristics, cotton insulation was gradually replaced by dry air-and-paper insulation. The first underground cable conduit systems appeared in cities in 1882. The conduits consisted of concrete-covered steel pipes in which lead-sheathed cables were laid. In the 19th century the number of circuits (pairs) in a telephone cable did not exceed 200. However, the growth of telephone traffic in the cities was accompanied by an increase in the number of circuits per cable. In 1901 a 400-pair cable was manufactured. In 1910 the number of pairs per cable reached 900; in 1932, 2, 400; and in 1961, 3, 600.

The construction of long-distance telephone lines began in the early 20th century. At that time inventions made by the American engineer M. Pupin and the introduction of electron-tube line repeaters made feasible an increase in the distance of signal transmission by cable. Beginning in 1930, multiplexing and high frequencies were used to increase the traffic rate of communications cables. Coaxial cables, which were capable of transmitting television broadcasts, appeared in the 1930’s and 1940’s. Until World War II (1939–), paper was the primary insulation material for communications cables. After the war polymeric materials, such as polyethylene and polystyrene, became predominant. As a rule, the current-carrying conductors of symmetrical cables are single copper wires with a diameter of 0.3–.6 mm. The insulated conductors of symmetrical cables are twisted into pairs (one circuit) or into quads (two circuits). The number of pairs in symmetrical low-frequency cables ranges from one to 3, 600 (in experimental cables, up to 4, 800); in coaxial cables, from two to 20 (each pair can transmit up to 3, 600 telephone conversations).

Communications cables are made with six types of sheathing. Metal sheaths are made of lead, smooth or corrugated aluminum, or corrugated steel. Plastic sheaths are made of polyethylene or polyvinyl chloride; metal-plastic sheaths are made of alumopolyethylene. Adjacent sections of communications cables are joined in cable junction boxes; cables are connected to communications equipment through cable distribution boxes.

Communications cables are classified according to design (symmetrical and coaxial), the range of transmission frequencies f(low-frequency, where f < 10 kHz, or high-frequency, where f > 10 kHz), sphere of use (for long-distance lines or for local urban and rural exchanges, for radio broadcasting, and for communications in mines), and the properties of the cable run (underground cables, cables laid in trenches or conduit systems; overhead, or suspension, cables; and underwater cables, which are divided into two groups—cables laid on the bottom of rivers, canals, and lakes and cables laid in seas or oceans at a great depth by a cable ship for long-distance, overseas, or intercontinental communications lines).


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