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Daily Dose of Curious

035

Daily Dose of Curious

HOW IT WORKS

Why are the pairs in an Ethernet cable twisted?

05 OCT 2026ENGINEERING

This is why a patch lead run alongside mains cable can still work, and why a cheap unshielded cable in a clean environment often outperforms a shielded one that has been terminated badly.

So that interference hits both wires of a pair equally, and the receiver can subtract it away. The twist is not tidiness. It is the entire reason the cable works at all.

What actually happens

  1. The signal is a difference

    Each pair carries the same signal twice, one inverted. The receiver does not read either wire on its own; it reads the gap between them.

  2. Interference arrives

    A motor, a fluorescent fitting or the pair next door induces a voltage. Because the two wires are twisted together, it lands on both almost identically.

  3. The receiver subtracts

    Taking one wire from the other cancels anything common to both. The interference disappears and the wanted signal doubles.

  4. Each pair twists differently

    The four pairs use different twist rates so they do not stay aligned with each other along the cable, which is what keeps them from bleeding into one another.

The clever bit

Untwisting more than a couple of centimetres at a connector undoes it locally. That is why a socket punched down neatly works and a hurried one drops packets under load while testing fine on a quiet afternoon.

The words

Common mode
A voltage that appears on both wires of a pair at once — usually noise.
Crosstalk
Signal from one pair leaking into another inside the same cable.

Where this comes from

  • ANSI/TIA-568 — balanced twisted-pair cabling and termination
  • Differential signalling and common-mode rejection

This one is not an opinion. It is how the thing works, and these are what that rests on.

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