User interface - the final reference point in the ISDN network. Describes the full-duplex data signal on the local loop.

The two-wire interface that connects to the NT1 on a user's premises. In North America it can be integrated into the customer premises equipment. In other countries, it is typically supplied by the local carrier.

In ISDN, a two-wire loop of twisted pair cables.

A two-wire interface presented to the customer by the telco in the US market. The customer is responsible for converting this signal to the four wire S/T interface to make a connection.

A 2-wire ISDN circuit, essentially today's standard 1-pair telephone company local loop made of twisted-wire. The U interface is the most common ISDN interface and extends from the central office.

User reference point interface. A single-pair connection between the local ISDN provider and the customer premises equipment.

U interface: For basic-rate access in an Integrated Services Digital Network (ISDN) environment, a user-to-network interface reference point that is characterized by the use of a 2-wire-loop transmission system that (a) conveys information between the 4-wire user-to-network interface, i.e., the S/T reference point, and the local exchange, (b) is located in the servicing central office, and (c) is not as distance sensitive as a service using a T interface.