Twisted-pair cable

Twisted-pair cable carries signals over pairs of insulated copper wires, each pair twisted around itself. The twist means electromagnetic interference reaches both wires of a pair almost equally, so the receiver, which reads the difference between them, can cancel much of it; twisting also reduces crosstalk between neighbouring pairs. Ethernet cable commonly holds four pairs and ends in an RJ-45 connector. Unshielded twisted pair (UTP) relies on the twist alone, while shielded twisted pair (STP) adds a foil or braided screen for noisy places. Grades called categories (for example Cat 5e, Cat 6 and Cat 6A) are defined in the ANSI/TIA-568 standards, and a twisted-pair Ethernet segment is commonly limited to 100 metres.

Like other copper transmission media, twisted pair radiates emanations, can be tapped, and loses signal over distance. It is cheaper than fibre-optic cable but more exposed. Shielding reduces emanations but does not replace physical protection of cable runs. Cable in air-handling spaces commonly needs a plenum-rated jacket, which gives off less toxic smoke. Coaxial cable is the older copper alternative.

Exam relevance: a scenario may describe a cable run that picks up interference near heavy machinery, or ask which medium best resists eavesdropping. Candidates are expected to see twisted pair as the common, cheaper and more exposed choice, with STP or fibre as the answers to interference.