Basic rate interface

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Basic rate interface (BRI, 2B+D, 2B1D) is an Integrated Services Digital Network configuration defined in the physical layer standard I.430 produced by the ITU. This configuration consists of two 64 kbit/s "bearer" channels (B channels) and one 16 kbit/s "data" channel (D channel). 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 two B channels can be bonded together giving a total data rate of 128 kbit/s. BRI is the kind of ISDN interface most likely to be found in residential service.

The I.430 protocol defines 48-bit packets comprising 16 bits from the B1 channel, 16 bits from B2 channel, 4 bits from the D channel, and 12 bits used for synchronization purposes. These packets are sent at a rate of 4 kHz, giving the data rates listed above for a maximum possible throughput of 144 kbit/s.

There are two modulation methods that can be used to deliver the U-Interface signals over an ordinary copper pair. The most popular is 2B1Q.

1 2B1Q line coding
2 4B3T line coding
3 Physical interfaces
4 References
5 See Also

[edit] 2B1Q line coding

2B1Q coding is the standard used in North America. 2B1Q means that two bits are combined to form a single Quaternary line state. 2B1Q combines two bits at a time to represent one of four amplitude levels on the line. The baud rate, therefore, is 80 kilobaud. It operates with a maximum frequency range 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 42 dB. 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 as indicated in the diagram below.

[edit] 4B3T line coding

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 combines 4 bits to represent one "ternary" line signal state. The baud 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 150 ohms 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 of the requirements 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. These are indicated in the lower half of the table above. The next state (S1 - S4) to be transmitted is indicated in the column labled Go. In the table above, the left-most bits and pulses are transmitted first.

The diagram below indicates a binary code of 1100 transmitted first (State S1). Then a squence of 1101 is transmitted (using State S4).

[edit] Physical interfaces

In ISDN technology there are three main types of interfaces for ISDN Basic Rate interface:

S/T Interface (S₀). Uses four wires with a separate pair of wires for the uplink and another pair for the downlink. The most used of the four interfaces
U_p Interface. Uses one wire pair using the half duplex "ping-pong" protocol. Used by many PABX systems that use ISDN technology. Depending of the possible cable range there are two varieties called U_pN and U_p0.
U_k0 Interface. one wire pair with echo cancellation. Used for the long last mile cable between the telephone exchange and the network terminator.