Basic Rate Interface (BRI, 2B+D, 2B1D) is an Integrated Services Digital Network (ISDN) configuration intended primarily for use in subscriber lines similar to those that have long been used for plain old telephone service. The BRI configuration provides 2 bearer channels (B channels) at 64 kbit/s each and 1 data channel (D channel) at 16 kbit/s. The B channels are used for voice or user data, and the D channel is used for any combination of data, control/signalling, and X.25 packet networking. The 2 B channels can be aggregated by channel bonding providing a total data rate of 128 kbit/s. The BRI ISDN service is commonly installed for residential or small business service (ISDN PABX) in many countries.
In contrast to the BRI, the Primary Rate Interface (PRI) configuration provides more B channels and operates at a higher bit rate.
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The BRI is split in two sections: a) in-house cabling (S/T reference point or S-bus) from the ISDN terminal up to the NT and b) transmission from the NT to the central office (U reference point).
2B1Q coding is the standard used in North America. 2B1Q means that two bits are combined to form a single Quaternary line state (symbol). 2B1Q combines two bits at a time to be represented by one of four signal levels on the line. The signal rate, therefore, is 80 kilobaud and the line rate 160 kbit/s. It operates with a maximum frequency range (bandwidth) of 40 kHz.
2B1Q coding is defined in ANSI T1.601 and ETR 080, Annex A. It can operate at distances up to about 18,000 feet (5.5 km) with loss up to 42dB. An internal termination impedance of 135 ohms is presented to the line at each end of the U-interface. Echo cancellation techniques allow full-duplex operation on the line.
2B1Q transmission can be simply described as an amplitude modulation scheme for DC pulses.
4B3T is a standard used in Europe and elsewhere in the world. 4B3T is a "block code" that uses Return-to-Zero states on the line. 4B3T converts each group of 4 data bits into 3 "ternary" line signal states (3 symbols). The symbol rate is 3/4ths of the data rate (120 kbaud).
4B3T is defined in ETR 080, Annex B and other national standards, like Germany's 1TR220. 4B3T can be transmitted reliably at up to 4.2 km over 0.4 mm cable or up to 8.2 km over 0.6 mm cable. An internal termination impedance of 150ohms is presented to the line at each end of the U-interface. Echo cancellation techniques allow full-duplex operation on the line.
In 4B3T coding, there are three states presented to line: a positive pulse (+), a negative pulse (-), or a zero-state (no pulse: 0). An analogy here is that operation is similar to B8ZS or HDB3 in T1/E1 systems, except that there is an actual gain in the information rate by coding 16 possible binary states to one of 27 ternary states.
There are many ways to perform this conversion, but BRI standards define a code known as MMS43 (Modified Monitoring State 43).
One requirement for line transmission is that there should be no DC build-up on the line, so there are some code substitutions based upon the transmission of the previous bits.