The U interface or U reference point is a Basic Rate Interface (BRI) in the local loop of an Integrated Services Digital Network (ISDN), connecting the network terminator (NT1/2) on the customer's premises to the line termination (LT) in the carrier's local exchange, in other words providing the connection from subscriber to central office.
Unlike the ISDN S/T interfaces, the U interface was not originally electrically defined by the ITU ISDN specifications, but left up to network operators to implement, although the ITU has issued recommendations G.960 and G.961 to formalize the standards adopted in the US and EU. It is not as distance sensitive as the S interface or T interface, and can operate at distances up to 18,000 feet. Typically the U interface does not connect to terminal equipment (which typically has an S/T interface) but to an NT1 or NT2 (network terminator type 1 or 2.)
An NT1 is a discrete device that converts the U interface to an S/T interface, which is then connected to terminal equipment (TE) having an S/T interface. However, some TE devices integrate an NT1, and therefore have a direct U interface suitable for connection directly to the loop.
An NT2 is a more sophisticated local switching device such as a PBX, that may convert the signal to a different format or hand it off as S/T to terminal equipment.
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 ANSI T1.601.
Echo cancellation (ECH)
When a transmitter applies a signal to the wire-pair, parts of the signal will be reflected as a result of imperfect balance of the hybrid and because of impedance discontinuities on the line.
Appendix III of G.961 specifies a line system based on the Time Compression Multiplex (TCM) duplex method and an alternate mark inversion (AMI) line code.