Twisted pair cabling is a type of communications cable in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted balanced pair, a twisted pair reduces electromagnetic radiation from the pair and crosstalk between neighboring pairs and improves rejection of external electromagnetic interference. It was invented by Alexander Graham Bell. For additional noise immunity, twisted-pair cabling may be shielded. Cable with shielding is known as shielded twisted pair (STP) and without as unshielded twisted pair (UTP).
Description A twisted pair can be used as a balanced line, which as part of a balanced circuit can greatly reduce the effect of noise currents induced on the line by coupling of electric or magnetic fields. The idea is that the currents induced in each of the two wires are very nearly equal. The twisting ensures that the two wires are on average the same distance from the interfering source and are affected equally. The noise thus produces a common-mode signal which can be canceled at the receiver by detecting the difference signal only, the latter being the wanted signal. Common-mode rejection starts to fail on untwisted wires when the noise source is close to the signal wires; the closer wire will couple with the noise more strongly and the receiver will be unable to eliminate it. This problem is especially apparent in telecommunication cables where pairs in the same cable lie next to each other for many miles. Twisting the pairs counters this effect as on each half twist the wire nearest to the noise source is exchanged. Provided the interfering source remains uniform, or nearly so, over the distance of a single twist, the induced noise will remain common mode. The twist rate (also called pitch of the twist, usually defined in twists per meter) makes up part of the specification for a given type of cable. When nearby pairs have equal twist rates, the same conductors of the different pairs may repeatedly lie next to each other, partially undoing the benefits of twisting. For this reason, it is commonly specified that, at least for cables containing small numbers of pairs, the twist rates must differ. In contrast to shielded or foiled twisted pair (typically S/FTP or F/UTP cable shielding), UTP cable is not surrounded by any shielding. UTP is the primary wire type for telephone usage and is very common for computer networking.
History
The earliest telephones used telegraph lines, which were single-wire earth return circuits. In the 1880s electric trams were installed in many cities, which induced noise into these circuits. In some countries, the tram companies were held responsible for disruption to existing telegraph lines and had to pay for remedial work. For new installations, however, it was necessary to protect against existing trams from the outset. Interference on telephone lines is even more disruptive than it is on telegraph lines. Telephone companies converted to balanced circuits, which had the incidental benefit of reducing attenuation, hence increasing range. As electrical power distribution became more commonplace, this measure proved inadequate. Two wires, strung on either side of cross bars on utility poles, shared the route with electrical power lines. Within a few years, the growing use of electricity again brought an increase of interference, so engineers devised a method called wire transposition, to cancel out the interference. In wire transposition, the wires exchange position every several poles. In this way, the two wires would receive similar EMI from power lines. This represented an early implementation of twisting, with a twist rate of about four twists per kilometer, or six per mile. Such open-wire balanced lines with periodic transpositions still survive today in some rural areas. Twisted-pair cabling was invented by Alexander Graham Bell in 1881. By 1900, the entire American telephone network was either twisted pair or open wire with transposition to guard against interference. Today, most of the twisted pairs in the world are outdoor landlines, owned and maintained by telephone companies, used for voice service.
Unshielded twisted pair
Unshielded twisted pair (UTP) cables are found in many Ethernet networks and telephone systems. For indoor telephone applications, UTP is often grouped into sets of 25 pairs according to a standard 25-pair color code originally developed by AT&T. A typical subset of these colors (white/blue, blue/white, white/orange, orange/white) shows up in most UTP cables. The cables are typically made with copper wires measured at 22 or 24 American Wire Gauge (AWG), with the colored insulation typically made from an insulator such as polyethylene or FEP and the total package covered in a polyethylene jacket. For urban outdoor telephone cables containing hundreds or thousands of pairs, the cable is divided into small but identical bundles. Each bundle consists of twisted pairs that have different twist rates, as pairs having the same twist rate within the cable can still experience some degree of crosstalk. The bundles are in turn twisted together to make up the cable.
UTP is also the most common cable used in computer networking. Modern Ethernet, the most common data networking standard, can use UTP cables, with increasing data rates requiring higher specification variants of the UTP cable. Twisted-pair cabling is often used in data networks for short and medium-length connections because of its relatively lower costs compared to optical fiber and coaxial cable. As UTP cable bandwidth has improved to match the baseband of television signals, UTP is now used in some video applications, primarily in security cameras. As UTP is a balanced transmission line, a balun is needed to connect to unbalanced equipment, for example any using BNC connectors and designed for coaxial cable.
Cable shielding
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