Line coding and intersymbol interference in local network transmission systems

The intersymbol interference experienced in digital transmission over local network cables is characterized by a simple empirical model of the cable pulse response. Using the model, it is shown that balanced linear binary codes, especially WAL 2, have sufficient self-equalization to give an open eye over the range of cable lengths for which crosstalk is not a problem. The multiple response codes (1, −1) and (1, 0, − 1) are shown not to confer any significant advantage over binary transmission in the absence of any other equalization. However, they do enable the length of a decision feedback equalizer used to reduce the intersymbol interference to be significantly shortened. This effect is quantified using an approximate formula for the bit error rate resulting from both the residual intersymbol interference and additive gaussian noise ; the validity of the formula is confirmed by simulations.