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Twisted pair
From Wikipedia, the free encyclopedia
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Twisted pair cabling is a form of wiring in which two conductors are wound together for the purposes of canceling out electromagnetic interference (EMI) from external sources, electromagnetic radiation from the UTP cable, and crosstalk between neighboring pairs.
Twisting wires decreases interference because the loop area between the wires (which determines the magnetic coupling into the signal) is reduced. In balanced pair operation, the two wires typically carry equal and opposite signals (differential mode) which are combined by addition at the destination. The common-mode noise from the two wires (mostly) cancel each other in this addition because the two wires have similar amounts of EMI that are 180 degrees out of phase. This results in the same effect as subtraction. Differential mode also reduces electromagnetic radiation from the cable, along with the attenuation that it causes.
The twist rate (also called pitch of the twist, usually defined in twists per metre) makes up part of the specification for a given type of cable. Where pairs are not twisted, one member of the pair may be closer to the source than the other, and thus exposed to slightly different induced EMF.
Where twist rates are equal, the same conductors of different pairs may repeatedly lie next to each other, partially undoing the benefits of differential mode. 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 FTP (foiled twisted pair) and STP (shielded twisted pair) cabling, UTP (unshielded twisted pair) cable is not surrounded by any shielding. It is the primary wire type for telephone usage and is very common for computer networking, especially as patch cables or temporary network connections due to the high flexibility of the cables.
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Twisted pair cables were first used in telephone systems by Bell in 1881 and by 1900 the entire American network was twisted pair, or else open wire with similar arrangements to guard against interference. Most of the billions of conductor feet (millions of kilometres) of twisted pairs in the world are outdoors, owned by telephone companies, used for voice service, and only handled or even seen by telephone workers. The majority of data or Internet connections use those wires.
UTP cables are not shielded. This lack of shielding results in a high degree of flexibility as well as rugged durability. 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 AD1L colors (white/blue, blue/white, white/orange, orange/white) shows up in most UTP cables.
For urban outdoor telephone cables containing hundreds or thousands of pairs, different twist rates for each pair are impractical. For this design, the cable is divided into smaller but identical bundles, with each bundle consisting of twisted pairs that have different twist rates. The bundles are in turn twisted together to make up the cable. Because they reside in different bundles, twisted pairs having the same twist rate are shielded by physical separation. Still, pairs having the same twist rate within the cable will have greater crosstalk than pairs of different twist rate. Thus to minimize crosstalk within a large cable, careful pair selection is important.
Twisted pair cabling is often used in data networks for short and medium length connections because of its relatively lower costs compared to fiber and coaxial cabling.
Unshielded twisted pair (UTP) cabling, because of its 100-year history of use by telephone systems, both indoors and out, is also the most common cable used in computer networking. It is a variant of twisted pair cabling. UTP cables are often called ethernet cables after Ethernet, the most common data networking standard that utilizes UTP cables, although not the most reliable.
Soon after the invention of the telephone, open wire lines were used for transmission. Two wires, strung on either side of cross bars on poles, share the route with electrical power lines. At first, interference from power lines limited the practical distance for telephone signals. Discovering the cause, engineers devised a method, called wire transposition, to cancel out the interference, where once every several poles, the wires crossed over each other. In this way, the two wires would receive similar EMI from power lines. Today, such open wire lines with periodic transpositions can still be found in rural areas. This represented an early implementation of twisting with a twist rate of about 4 twists per kilometre.
Twisted pair cables are often shielded in attempt to prevent electromagnetic interference. Because the shielding is made of metal, it may also serve as a ground. However, usually a shielded or a screened twisted pair cable has a special grounding wire added called a drain wire. This shielding can be applied to individual pairs, or to the collection of pairs. When shielding is applied to the collection of pairs, this is referred to as screening. The shielding must be grounded for the shielding to work.
STP cabling includes metal shielding over each individual pair of copper wires. This type of shielding protects cable from external EMI (electromagnetic interferences). e.g. the 150 ohm shielded twisted pair cables defined by the IBM Cabling System specifications and used with token ring networks.
S/STP cabling, also known as Screened Fully shielded Twisted Pair (S/FTP),[1] is both individually shielded (like STP cabling) and also has an outer metal shielding covering the entire group of shielded copper pairs (like S/UTP). This type of cabling offers the best protection from interference from external sources.
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