A circuit that supports transmission in both directions over separate physical links or paths in support of full duplex (FDX), i.e., simultaneous two-way, transmission.The distinguishing characteristic of a four-wire circuit, as opposed to a two-wire circuit, is its ability to support multichannel communications and out-of-band signaling and control. A physical four-wire circuit, the traditional means of provisioning, is a circuit comprising four physical twisted-pair copper wires in a two-pair configuration, with one pair supporting transmission in the forward direction and the other pair supporting transmission in the reverse direction. A logical four-wire circuit comprises two-wires in a single-pair configuration. A four-wire circuit also can be provisioned as a radio circuit, which is wireless, of course. Four-wire circuits are used in bandwidth-intensive local loops, particularly multichannel loops, and backbone circuits. Specific examples include DDS, ISDN,T/E-carrier, and SDH/SONET. See Figure F-4. See also channel, circuit, DDS, E-carrier, FDX, ISDN, out-of-band signaling and control, SDH, SONET, T-carrier, twisted pair, and two-wire circuit.