Circuits have evolved from generally being built on physical connections between individual hardware cables, as in an analog phone switch, to virtual circuits established over packet switching networks.
Bi-directional circuits may support half-duplex operation, when only one end of the channel transmits at any one time, or they may support full-duplex operation where independent simultaneous transmission occurs in both directions.
Radio stations used them as studio transmitter links (STLs) or as remote pickup unit (RPU) for sound reproduction, sometimes as a backup to other means.
Later lines were digital, used in pair-gain applications, such as carrier systems, or in enterprise data networks.
This article incorporates public domain material from Federal Standard 1037C.