A 2-wire ISDN circuit - essentially today's standard one pair telephone company local loop made of twisted-wire. The U interface is the most common ISDN interface and extends from the central office.

A twisted pair subscriber loop that connects the NT1 reference point to the ISDN network, as defined in the 1.411 Recommendation. This interface provides Basic Rate Access with an operating frequency of 160 kbps and an information rate of 144 kbps. Under U.S. regulations, this also marks the line of demarcation between customer-owned equipment and the public network.

The U-Interface carries ISDN formatted signals over a single pair or wires between a subscriber's location and a telephone company's central office.