UK How twisted pair cable works
Twisted pair cable is cable made from pairs of insulated copper conductors twisted around each other along their length. An Ethernet cable holds four pairs under one jacket, a telephone cable one or two, and a fieldbus cable usually one. Alexander Graham Bell patented twisted-pair telephone wiring in 1881, and the principle has carried voice and data ever since.
Each pair carries a differential signal: one wire carries the signal and the other carries its mirror image. The receiver measures the difference between the two. Interference from motors, power cables or radio arrives on both wires at once, and because the wires keep swapping position along each twist, both pick up almost the same noise. Subtracting one wire from the other cancels it. This is called common-mode rejection.
The twist also reduces crosstalk between neighbouring pairs. Each pair in a cable uses a slightly different twist rate, so their fields do not line up. Tighter, more precisely controlled twists are one of the main differences between cable categories.
UTP vs STP cable
UTP cable (unshielded twisted pair) relies on the twist alone to reject noise. It is lighter, cheaper, more flexible and easier to terminate, which is why it is used for most office and building networks.
STP cable (shielded twisted pair) adds a foil or braided shield around each pair, around all pairs or both. The shield blocks interference before it reaches the pairs and limits emissions from the cable. It needs shielded connectors and a bonded earth at the equipment to work, and a poorly earthed shield can do more harm than good.
The ISO/IEC 11801 standard names each construction with a code in the form overall shield / pair shield followed by TP.
| Code | Overall shield | Pair shield | Common name | Typical use |
|---|---|---|---|---|
| U/UTP | None | None | UTP | Offices, building networks |
| F/UTP | Foil | None | FTP | Light industrial, near power cables |
| U/FTP | None | Foil per pair | Pair-shielded | 10 Gb/s runs with low alien crosstalk |
| SF/UTP | Braid and foil | None | SFTP | Factory floors, cable trays with drives |
| S/FTP | Braid | Foil per pair | Fully shielded | High-noise sites, vehicles, rail |
Categories and colour codes
Four-pair data cable is graded into categories under the TIA-568 standard, and into matching classes under ISO/IEC 11801. The category sets the frequency the cable is tested to and the crosstalk it must meet: Cat5e is tested to 100 MHz and carries gigabit Ethernet, Cat6 to 250 MHz and Cat6A to 500 MHz for 10 Gb/s over the full 100 m. Each step uses tighter twists and better pair separation.
The four pairs are colour-coded so that both ends can be wired the same way. Each pair has a solid-coloured wire (blue, orange, green or brown) twisted with a white wire striped in the same colour. The order in which those colours land on the RJ45 pins is set by the T568A or T568B wiring scheme. Keeping each pair together on the correct pins matters more than the colours themselves, because splitting a pair across two signals destroys the noise cancelling that the twist provides.
Solid vs stranded conductors
Twisted pair is made with either solid or stranded copper conductors, and the two are not interchangeable.
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Solid core. Each conductor is a single wire. It has lower loss and is used for fixed installation runs inside walls and trays, terminated on punch-down jacks and patch panels.
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Stranded core. Each conductor is a bundle of fine wires. It is more flexible and survives repeated bending, so it is used for patch leads, moving machinery and cables inside vehicles.
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High-flex and drag-chain grades. Stranded cables with special jackets rated for millions of bending cycles suit robots, gantries and cable chains.
Where twisted pair is used
Ethernet is the best-known use, but many industrial networks run over twisted pair with a different impedance and signalling scheme.
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Ethernet. Four-pair 100 ohm cable carries Ethernet to 100 m, and also delivers Power over Ethernet to cameras and access points from industrial PoE switches.
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RS-485 and RS-422. A single 120 ohm shielded pair carries serial data up to around 1,200 m between controllers, meters and drives.
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CAN bus. A twisted pair terminated at each end with 120 ohm resistors links controllers in vehicles and machines. Our guide to how CAN bus networks work covers the topology.
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Single Pair Ethernet. Standards such as 100BASE-T1 and 1000BASE-T1 run Ethernet over one pair in vehicles and machines, and PCIe expansion cards add T1 ports to an industrial PC. Our guide to automotive-grade Single Pair Ethernet at the edge explains where it fits.
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Telephone and alarm circuits. Voice lines, door entry and alarm loops have used twisted pair for over a century.
Quick answers on twisted pair
Twisted pair cable: common questions
What does UTP stand for?
Unshielded twisted pair. It is twisted pair cable with no foil or braid shield, and the most common type of Ethernet cable.
Why do communication circuits use twisted pair?
To cancel interference cheaply. Twisting makes both wires pick up the same noise, which a differential receiver removes, so data runs further without expensive shielding.
When should I use STP instead of UTP?
Near strong interference or where emissions matter. Factories with drives and welders, vehicles, rail and outdoor runs usually justify shielded cable, provided the shield is properly earthed.
Is twisted pair the same as Ethernet cable?
Ethernet cable is one kind of twisted pair. It uses four 100 ohm pairs and RJ45 or M12 connectors, while RS-485, CAN and telephone cable use other pair counts and impedances.
Twisted pair vs coaxial cable: which is better?
Each suits different signals. Twisted pair carries networks and fieldbuses, and is cheaper to install. Coax carries RF and analogue video better over long runs.
We fit the network, serial and CAN interfaces each installation needs as standard, with the termination, isolation and connectors the cabling calls for, and test every port before the system ships. Send us your cable types and device list, and we will specify the interfaces to match.