One requirement for line transmission is that there should be no DC build-up on the line, so the accumulated DC build-up is monitored and the codewords are chosen accordingly. Of the 16 binary information words, some are always mapped to a DC-component free (ternary) code word, while others can be mapped to either one of two code words, one with a positive and the other with a negative DC-component. In the latter case, the transmitter chooses whether to send the code-word with negative or positive DC-component based on the accumulated DC-offset.