What is inside an Ethernet cable
Strip back the jacket of a LAN cable and you find four pairs of insulated copper wires. In each pair, one wire has a solid colour and its partner is white with a stripe of the same colour. The four colours are blue, orange, green and brown.
The colour code follows the TIA-568 standard, so an installer can terminate both ends of a link the same way. In a Cat6 cable you will often also see a plastic cross-shaped spline that keeps the pairs apart, and a thin ripcord used to open the jacket.
- Blue and white-blue
- Orange and white-orange
- Green and white-green
- Brown and white-brown
The problem: crosstalk and interference
Every signal travelling along a wire creates a small electromagnetic field around it. When two wires run side by side, the field of one induces unwanted voltage in the other. Noise picked up from a neighbouring pair is called crosstalk. Noise from outside the cable, such as power lines, motors or lighting ballasts, is called electromagnetic interference.
Either way, the receiver sees a distorted signal. At high data rates that means bit errors, retransmissions and, in the worst case, a link that falls back to a lower speed.
How twisting cancels the noise
The two wires of a pair do not carry two copies of the same signal. They carry equal but opposite signals: when one wire goes positive, the other goes negative by the same amount. This is called differential signalling. The receiver ignores the absolute voltage on each wire and reads only the difference between them.
Noise that couples into both wires equally shifts both voltages in the same direction, so the difference does not change and the noise disappears when the receiver subtracts one wire from the other. Twisting is what makes the coupling equal. With every twist the two wires swap places, so over the length of the cable each wire spends the same amount of time nearer to any source of noise.
Without the twist, one wire would always sit closer to the noise source than its partner, pick up more of it, and leave a difference the receiver cannot remove. Twisting also works in the other direction: the opposite fields of the two wires largely cancel each other, so a twisted pair radiates less noise into its neighbours.
Why each pair is twisted at a different rate
If all four pairs had the same twist length, the wires of neighbouring pairs would line up with each other along the whole cable and couple strongly. Cable makers therefore give each pair a slightly different twist rate, so no two pairs stay aligned for long.
Higher categories add more protection on top of the twist: tighter twists and closer manufacturing tolerances, a spline in most Cat6 designs, and foil or braid shielding in constructions such as F/UTP, U/FTP and S/FTP. Our Cat6A cable range shows these constructions side by side.
Balanced twisted pair, and why it matters on site
Put together, the two ideas give the cable its technical name: balanced twisted pair. Balanced means the two wires of a pair carry equal and opposite signals; twisted pair is how the cable keeps them equally exposed to noise.
The twist only works where it exists. When terminating a cable at a jack or patch panel, keep the pairs twisted as close to the termination as possible. For Category 5e and above, TIA-568 limits the untwisted length to 13 mm (0.5 in).
