In Integrated Services Digital Network (ISDN) Basic Rate Interface service, a U interface is the electrical interface for the single twisted pair wire connection from a local phone company (the central office) to a home or business. Unlike a regular 64 Kbps analog phone connection, however, the twisted-pair using ISDN carries two 64 Kbps channels (known as B, or bearer, channels) and an additional 16 Kbps channel (the D, for data or delta, channel) for control signals.
The U interface twisted-pair is usually connected at the home or business to a network terminator 1 (NT1) box, sometimes called a network terminating unit. (In the UK and some other countries, the NT1 is located at the central office.) The other side of the NT1 has plugs for four wires, which can be connected on a loop configuration known as an S-bus or S interface to up to eight devices (for example, two computers and six phones) or to a T interface. An NT1 can also be integrated into a modem or other device, in which case the ISDN connection can only serve that device.
The U-loop or U-V loop, as it is sometimes called, uses the 2B1Q line code protocol, meaning that two binary digits are used to represent one quadratude - that is, four possible variations of signal level (amplitude and polarity). Communication is full-duplex, meaning that data can be arriving at the same time you are sending data..
The U-V loop replaces the traditional local loop. The maximum distance for the ISDN loop is 6,500 meters (about 18,000 feet).
The following table summarizes the various ISDN electrical interfaces at different demarcation points or places in the traffic flow:
|Electrical interface||Between what two points|
|U interface||Central office and NT1|
|T interface||NT1 and NT2 devices (such as a PBX)|
|S interface||NT1 or NT2 and ISDN devices (such as a telephone or terminal adapter)|
|R interface||Terminal adapter and non-ISDN devices (such as a computer)|
|V interface||Within the ISDN node at the central office; separates line termination equipment from exchange termination equipment|