Low voltages near zero on any pin either + or - side are ignored, below a certain threshold of absolute voltage either way it's no-man's land. Signals are only

Low voltages near zero on any pin either + or - side are ignored, below a certain threshold of absolute voltage either way it's no-man's land. Signals are only considered real if they are boldly above the threshold, preferably close to +/- 12VDC either way to reliably qualify as a space or a mark, but no higher amplitude than +/- 25VDC. This helps to compensate for unavoidable differences in absolute ground potential between remote locations. The data signal is basically just square waves since it's ones & zeros, but hook a speaker up properly and that's the unforgotten audio sound of dial-up internet.