Why distance ruins a signal

A long wire is not a perfect conductor. Every metre adds resistance, which weakens the signal, and the wire plus its return path form a capacitor, which stores charge. Together they work like a resistor and a capacitor in series, and that pair rounds off the sharp edges of your square wave.

Capacitance is the reason length hurts more than resistance alone would suggest. The cable has to be charged and discharged before its voltage can change, so a fast edge arrives late and rounded. Round a bit enough and it merges with the next one, which is why receivers start guessing. That merging is called intersymbol interference.

# Why a long cable smears a clean square wave. # Resistance is spread along the wire. # Capacitance between the signal wire and the return path stores charge. # Together they behave like a resistor and a capacitor in series: # short cable -> the edge arrives fast, the square stays square # long cable -> the stored charge has to be filled first, # so edges round off and arriving bits merge (intersymbol interference) # More resistance in the way = slower rise and fall = fewer bits per second.

Two wires running side by side also talk to each other. A changing signal in one induces a small voltage in its neighbour — crosstalk. Twisting the pair, or shielding it, keeps that under control; running them in parallel over metres does not.