Understanding ISDN 2B1Q Signal Format

This document explains the ISDN 2B1Q signal format used on the U-interface.

2B1Q stands for “2 Binary 1 Quaternary.” ISDN leverages the standard twisted pair bandwidth, which is generally limited to 4 kHz. To maximize data transmission speed within this limited bandwidth, ISDN employs sophisticated techniques. One such technique is the use of a multi-level digital signal known as 2B1Q on the U-interface.

What is 2B1Q?

Unlike typical binary signals that have only two levels (e.g., high and low, or 0 and 1), 2B1Q is a line coding technique. It converts data into a format suitable for transmission without requiring any explicit modulation.

Here’s a table illustrating the 2B1Q mapping between binary data and voltage levels:

As shown, the signal utilizes four distinct voltage levels (-3V, -1V, +1V, +3V) to represent two binary bits. For instance:

• “11” is mapped to +1V.
• “10” is mapped to +3V.
• “01” is mapped to -1V.
• “00” is mapped to -3V.

ISDN 2B1Q Signal Waveform

Figure 1: ISDN 2B1Q Signal Waveform Example

The 2B1Q line coding technique groups the binary sequence into pairs of two bits before applying the mapping. This is similar to QPSK modulation, where one symbol represents two bits. In ISDN 2B1Q, each voltage level corresponds to one symbol (representing 2 bits).

Therefore, in this line coding method, 2 bits are mapped per baud. Figure 1 (above) illustrates an example of an ISDN 2B1Q signal waveform for a specific binary pattern.

In this context:

$Baud\ Rate = Symbol\ Rate = \frac{1}{t}$

Where ‘t’ represents the duration of each symbol.