Communications Line

the aggregate of technical equipment and the physical medium used for the propagation of signals from a transmitter to a receiver. A communications line is a component of a communications (or transmission) channel, which sometimes consists of several communications lines (on various sections of a long-distance communications channel, cable lines or radio-relay lines may be used). More frequently the same communications line is used for transmission of signals belonging to several channels. Communications lines are classified as electric, acoustic, or optical, depending on the nature of the signals used in transmitting messages.

During the early stages of the development of electrical communications, a pair of wires served as the physical medium connecting a transmitter to a receiver (wire communications). Later, with the advent of wireless communications systems (radio communications), a communications line was defined as the aggregate of transmitting and receiving antennas and the medium through which the radio waves propagate. The main characteristic of such communications systems is the range of operating frequencies at which signals can be transmitted with acceptable attenuation. A communications line using steel wires is capable of transmitting signals at frequencies up to 25–30 kilohertz (kHz); using an overhead line with nonferrous conductors, up to 140–150 kHz; using a balanced cable, up to 500–550 kHz; using a coaxial cable, up to 12–15 MHz. Shortwave communications trunk lines operate in the frequency range from 3 to 30 MHz; waveguide communications lines operate at frequencies of hundreds of megahertz or tens of gigahertz.

The use of optical and acoustic communication lines is limited primarily by the strong absorption of optical and acoustic waves by the medium through which the waves propagate.