Computer network cabling

Most wired local-area networks are built on twisted-pair copper cabling terminated in the eight-position RJ-45 modular connector. The cable contains four pairs of conductors, each pair twisted along its length so that interference is picked up almost equally on both wires and largely cancels at the receiver. This twisting is what allows ordinary copper to carry fast network signals reliably, and the number of twists and the quality of the cable are graded into categories that correspond to the data rates they can support.

Twisted-pair cable comes in several constructions. UTP (unshielded twisted pair) relies on the twisting alone and is the most common in offices and homes. STP (shielded twisted pair) and FTP (foiled twisted pair) add metal shielding around the pairs or the whole cable to give extra protection against interference in electrically noisy environments. The shielding must be properly grounded to be effective.

How the eight conductors map to the eight pins of the RJ-45 plug is fixed by the TIA/EIA 568A and 568B wiring schemes. These two standards differ only in the arrangement of certain pairs, and consistency matters: a cable wired the same way at both ends is a straight-through cable for connecting a computer to a switch, while one wired to 568A at one end and 568B at the other is a crossover cable, historically used to link two similar devices directly. Following these standards lets cabling, wall outlets and equipment from different makers work together.

Frequently asked questions

What connector is used for twisted-pair networks?

The eight-position RJ-45 modular connector, which terminates the four twisted pairs of the cable.

What is the difference between UTP, STP and FTP?

UTP is unshielded and relies on twisting alone; STP and FTP add braided or foil shielding around the pairs or the cable to better resist interference.

What is the difference between a straight-through and a crossover cable?

A straight-through cable is wired identically at both ends, while a crossover cable uses 568A at one end and 568B at the other to connect two similar devices directly.