twisted-pair cabling

Look inside an Ethernet cable and you will find not loose strands but pairs of copper wires twisted gently around each other along the whole length. That twist is not decoration — it is the whole point. Twisted-pair cabling carries a signal on two wires as the difference between them, and twisting the pair makes outside interference hit both wires almost equally, so it cancels out when the receiver looks only at the difference.

Here is the mechanism. The transmitter sends a signal as a voltage difference across the two wires of a pair (this is called balanced or differential signalling). Any electrical noise from a nearby motor or power line tends to induce the same unwanted voltage on both wires, since they run side by side and the twist keeps swapping their positions. The receiver subtracts one wire from the other; the signal (which is opposite on the two wires) survives, while the common noise (equal on both) cancels to near zero. Tighter, more uniform twisting rejects more noise and supports higher data rates, which is why cable categories climb: Cat 5e for 1 Gbps, Cat 6 and Cat 6a for 10 Gbps over shorter runs.

Twisted-pair is the workhorse of local-area networking and telephone lines because it is cheap, flexible, and easy to terminate, but it is the most attenuation-prone of the guided media, which is why Ethernet over copper is capped at about 100 metres per run. Unshielded twisted pair (UTP) relies on the twist alone; shielded twisted pair (STP) adds a metal foil for noisier environments. Beyond that 100-metre reach, or where electrical noise is severe, optical fiber takes over.