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.
[1] 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.
[5] 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.
[7][2] In Europe, the NT1 belongs to the network operator, so the user doesn't have direct access to the U interface.
[8][2] 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.
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).
[9] The Time Compression Multiplex (TCM) duplex method, also referred to as "burst mode", solves the echo problem indirectly.
4B3T converts each group of 4 data bits into 3 "ternary" line signal states (3 symbols).
[9] In the latter case, the transmitter chooses whether to send the code-word with negative or positive DC-component based on the accumulated DC-offset.
2B1Q means that two bits are combined to form a single Quaternary line state (symbol).
A 1.5 ms frame carrying 216 scrambled bits of 2B+D data is mapped to 108 quaternary symbols.
[9] The CL channel is used for communication between LT and NT1, a 12-bit cyclic redundancy check (CRC), and various other physical layer functions.
[9] The TCM / AMI ISDN line system, also referred to as TCM-ISDN, is used by Nippon Telegraph and Telephone in its "INS-Net 64" service.