Four-wire circuit

In <a href="/facts/Telecommunications/vxoj1cxR">telecommunications</a>, a four-wire circuit is a two-way <a href="/facts/Telecommunication_circuit/DYF1dSRU">circuit</a> using two paths so arranged that the respective <a href="/facts/Signal_(information_theory)/41YqUJ8c">signals</a> are transmitted in one direction only by one path and in the other direction by the other path. The four-wire circuit gets its name from the fact that is uses four conductors to create two complete electrical circuits, one for each direction. The two separate circuits (channels) allow full-<a href="/facts/Duplex_(telecommunications)/LCULeVxr">duplex</a> operation with low <a href="/facts/Crosstalk/IT2LIwth">crosstalk</a>.
In telephony a four-wire circuit was historically used to transport and switch <a href="/facts/Baseband/yC0jJwbL">baseband</a> audio signals in the phone company <a href="/facts/Telephone_exchange/PaRpGm8I">telephone exchange</a> before the advent of <a href="/facts/Digital_modulation/aAXPjINv">digital modulation</a> and the <a href="/facts/Electronic_switching_system/rqHUmBML">electronic switching system</a> eliminated baseband audio from the telco plant except for the <a href="/facts/Local_loop/rSRfLPaq">local loop</a>. The local loop is a <a href="/facts/Two-wire_circuit/j0hbjBtD">two-wire circuit</a> for one reason only: to save copper. Using half the number of copper wire conductors per circuit means that the infrastructure cost for wiring each circuit is halved. Although a lower quality circuit, the local loop allows full duplex operation by using a <a href="/facts/Telephone_hybrid/4goz8rw8">telephone hybrid</a> to keep near and far voice levels equivalent.
As the <a href="/facts/Public_switched_telephone_network/eVrCTDU7">public switched telephone network</a> expanded in size and scope, using many individual wires inside the telco plant became so impractical and labor-intensive that in-office and inter-office signal wiring progressed to high bandwidth coaxial cable (still a popular interconnection method in the 21st century, used with the Lucent 5ESS <a href="/facts/Class-5_telephone_switch/EH7MCeG8">Class-5 telephone switch</a> to present day), <a href="/facts/Microwave_radio_relay/bz9H9Ete">microwave radio relay</a> and ultimately <a href="/facts/Fiber-optic_communication/dc6S0Hg5">fiber-optic communication</a> for high-speed trunk circuits. At the end of the 20th century, four-wire circuits saw renewed growth for corporate local loop service for use in <a href="/facts/Dedicated_line/EUcgDjdM">dedicated line</a> service for computer modems to interconnect company <a href="/facts/Computer_networks/3w5RM99p">computer networks</a> and to connect networks to an <a href="/facts/Internet_service_provider/dUacAxef">Internet service provider</a> for Internet connectivity before commodity <a href="/facts/DSL/3pY5osov">DSL</a> and <a href="/facts/Cable_modem/YsXWhXhK">cable modem</a> connectivity was widely available.

<ul><li>A History of engineering and science in the Bell System: transmission technology (1925-1975)</li></ul>

Four-wire circuit open-in-new

Four-wire circuit