U interface

The U interface or U reference point is a Basic Rate Interface (BRI) in the local loop of an Integrated Services Digital Network (ISDN). It is characterized by the use of a 2-wire transmission system that connects the network termination type 1 (NT1) on the customer's premises and the line termination (LT) in the carrier's local exchange. It is not as distance sensitive as a service using an S interface or T interface.

In America, the NT1 is customer premises equipment (CPE) which is purchased and maintained by the user, which makes the U interface a User–network interface (UNI). The American variant is specified by the American National Standards Institute (ANSI) in T1.601. In Europe, the NT1 belongs to the network operator, so the user doesn't have direct access to the U interface. The European variant is specified by the European Telecommunications Standards Institute (ETSI) in recommendation ETR 080. The ITU-T has issued recommendations G.960 and G.961 with world-wide scope, encompassing both the European and American variants of the U interface.

Like all other ISDN basic rate interfaces, the U interface carries two B (bearer) channels at 64 kbit/s and one D (data) channel at 16 kbit/s for a combined bitrate of 144 kbit/s (2B+D).

While in a four-wire interface such as the ISDN S and T-interfaces one wire pair is available for each direction of transmission, a two-wire interface needs to implement both directions on a single wire pair. To that end, ITU-T recommendation G.961 specifies two duplex transmission technologies for the ISDN U interface, either of which shall be used: Echo cancellation (ECH) and Time Compression Multiplex (TCM).

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