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ISDN 2B1Q signal format | ISDN 2B1Q signal on U-interface

This page describes ISDN 2B1Q signal format or structure. ISDN 2B1Q signal is used on U-interface. 2B1Q is the short form of 2 Binary 1 Quaternary code.

As we know ISDN uses standard twisted pair bandwidth which is limited to 4KHz. Hence special techniques are employed to increase the speed of data transmission to conserve the bandwidth available.

Hence ISDN uses multi-level digital signal known as 2B1Q on U-interface. 2B1Q is different compare to normal binary signals having only two levels. It is called line coding technique. Here data is converted into format such that they can be transmitted without even applying any form of modulation to the signal.


Binary Input 2B1Q voltage output
00 -3V
01 -1V
10 +3V
11 +1V

Table above represent 2B1Q mapping between binary data and voltage levels. As mention, signal uses four voltage levels (-3V, -1V, +1V, +3V) to map two binary bits. For example, 11 is mapped to +1V, 10 is mapped to +3V, 01 is mapped to -1V and 00 is mapped to -3V.

ISDN 2B1Q Signal Waveform


ISDN 2B1Q signal format

As ISDN 2B1Q line coding uses 2 bits for mapping the binary sequence is first grouped into pair of two bits each before mapping is applied. The same technique is used in QPSK modulation where in 1 symbol represents 2 bits.

Here in ISDN 2B1Q, each level is mapped to 1 symbol (i.e. 2 bits). Hence 2 bits are mapped per baud in this line coding technique.

Figure-1 above depicts ISDN 2B1Q signal waveform for one such binary pattern.
Here Baud rate = Symbol rate = 1/t.

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