2B1Q code
The 2B1Q code ( 2 binary 1 quaternary ) is a line code in communications technology .
The 2B1Q code forms a quaternary (four-valued) value from two binary values (2 bits) and is used especially with ISDN for the basic connection on the subscriber line.
Using the code reduces the required step speed for the basic connection from 160 to 80 k baud (with the same data transmission rate of 160 kbit / s) and thus the required frequency spectrum : it is shifted to lower frequencies, which results in lower attenuation . This enables a longer transmission range to be achieved.
| Binary value | 00 | 01 | 10 | 11 |
| Quaternary value | −3 | −1 | +3 | +1 |
| tension | −2.5 V | −0.833 V | +2.5V | +0.833 V |
The assignment of the voltages to the bit codes was chosen so that there are as few bit errors as possible in the event of transmission errors (voltage fluctuations). A Gray code is used. For example, if a 10 is to be transmitted (voltage +2.5 V) and errors occur, so that only a lower voltage arrives at the receiver (e.g. in the range of +0.833 V), the receiver reads an 11, i.e. only reads in One bit error. If the bit sequences and the voltage had been coded according to their size, the receiver would have read 01, and thus 2 wrong bits.
With HDSL , at higher data transmission rates, because of the higher line attenuation , higher voltages are used than indicated in the table (order of magnitude up to 20 volts).