A circuit that carries information signals in both directions over the same physical link or path. A two-wire circuit commonly is provisioned through the use of a single twisted pair copper wire, within which two wires are required to complete the electrical circuit, with the current in one wire opposite to the current in the other, and with both wires carrying the information signal. The most common example of a two-wire circuit is an electrically-based unshielded twisted pair (UTP) local loop access circuit between a telephone company central office (CO) switching center and a residential or small business premises, where the loop ultimately terminates in an individual single-line or multiline telephone set, data terminal, or key service unit (KSU) associated with a key telephone system (KTS).Two-wire circuits generally span a relatively short distance, and are analog, narrowband, and single channel in nature.Two-wire UTP local loops, for example, typically are designed to be less than 18,000 feet (6 kilometers) in length, analog voice grade, and single channel. However, the same loop will support ISDN BRI (Basic Rate Interface) service, a digital transmission technology. See Figure T-6. See also BRI, channel, four-wire circuit, ISDN, local loop, narrowband, and voice grade.