Communications cable (redirected from transmission line)

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