U интерфейсі: ISDN желісіндегі негізгі жылдамдық интерфейсі

Basic Rate 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), 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.

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. The AMI line code maps one input bit to one ternary symbol. Like with MMS43, the ternary symbol can either be a positive (+), zero (0), or negative ( ) voltage. A 0 bit is represented by a zero voltage, while a 1 bit is alternatingly represented by a positive and a negative voltage, resulting in a DC bias free signal. In a 2.5 ms interval, each side can send a 1.178 ms frame representing 360 bits of 2B+D data. To the 2B+D data, an 8 bit preamble, 8 bits from the CL channel, as well as a parity bit are added, yielding a frame size of 377 bits and a baud rate of 320 kilobaud. The CL channel is used for operations and maintenance, as well transmitting a 12 bit CRC covering 4 frames.

Appendix IV of G.961 specifies a line system based on echo cancellation and a substitutional 3B2T (SU32) line code, which maps three bits into 2 ternary symbols. As with MMS43 and AMI, the ternary symbol can either be a positive (+), zero (0), or negative ( ) voltage. The mapping from 23=8 to 32=9 symbols leaves one unused symbol. When two subsequent input (binary) information words are identical, the (ternary) code word is substituted by the unused code word. A 0.75 ms frame carrying 108 bits of 2B+D data is mapped to 72 ternary symbols. To this frame, a 6 symbol preamble, one CRC symbol, and 2 symbols from the CL channel are added, yielding a frame size of 81 ternary symbols and a symbol rate of 108 kilobaud. The CL channel is used for supervisory and maintenance functions between the LT and NT1. The 15 bit CRC covers 16 frames.