Twisted Pair Cable
Cut open the blue cable plugged into a computer in your lab and you will find eight thin wires inside, in four coloured pairs, each pair wound round itself like a rope. That winding is not decoration. It is the whole reason the cable works.
1What a twisted pair cable is
- Each wire has its own plastic insulation, so the two never touch.
- Each pair works as one communication link.
- A network (Ethernet) cable has four pairs: orange, green, blue and brown.
2Why the wires are twisted
Electrical equipment nearby — a power cable, a motor, even the next pair in the same cable — creates electrical noise. A copper wire picks it up, and noise mixed into a signal can turn a 1 into a 0.
The receiver does not read each wire on its own. It reads the difference between the two wires of a pair. If both wires pick up exactly the same noise, the difference cancels it out. Twisting is how both wires get the same noise: at every half twist, they swap places, so neither stays nearer to the source of noise.
Switch the noise on and watch the signal that arrives at the far end. Drag sideways to turn.
The cancelling is worked out, not drawn by hand: noise falls off with distance, the receiver reads the difference between the two wires, and a twist swaps which wire is nearer. Straightening a cable is only imagined here — real network cables always come twisted.
Straighten the wires and one of them stays nearer the power cable for the whole length, so all the noise reaches the receiver. Twisted, only about 2.4% is left in the lab. NCERT puts it simply: the twist minimises the effect of electrical interference.
Think of roasting corn on a fire
3UTP and STP
| UTP — Unshielded Twisted Pair | STP — Shielded Twisted Pair | |
|---|---|---|
| Shield | None: just the twisted pairs in a plastic jacket | A metal foil (or braid) shield around the pairs |
| Protection from noise | Good, from the twist alone | Better: twist and shield together |
| Cost | Cheaper | More expensive |
| Where used | Most school and office LANs, telephone lines | Places with lots of electrical noise, like factories |
4Uses, advantages and disadvantages
Twisted pair is the most commonly used cable, in telephone lines and in LANs. The network cable with an RJ45 plug on each end, which you met in Network Devices, is a twisted pair cable, often called an Ethernet cable.
- The cheapest wired medium.
- Thin, flexible and easy to install.
- Easy to join with RJ45 connectors.
- Lower bandwidth than co-axial and fibre-optic cable.
- The signal weakens over distance — about 100 m for an Ethernet cable before a repeater is needed.
- More affected by interference than co-axial or fibre.
5From the board papers
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Ethernet cable — CBSE’s own marking scheme answer. An Ethernet cable is twisted pair: the word to spot is economically, and twisted pair is the cheapest.
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Twisted pair cable: cheap, and fast enough when speed can be compromised. Optical fibre is the fastest but costs the most.
6Check yourself
Why are the two wires of a twisted pair twisted around each other?
What is the difference between UTP and STP cable?