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Twisted Pair Cable

Tracy Pettigrue
Tracy Pettigrue
Sep 28, 2009
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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 subtraction at the destination. The common-mode noises from the two wires (mostly) cancel each other in this subtraction because the two wires have similar amounts of EMI 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.
 
Uses:
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.
 
Cable Shielding:
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.
 
 
Advantages
  • It is a thin, flexible cable that is easy to string between walls.
  • Most modern buildings come with CAT 5 UTP already wired into the wall outlets or at least run between the floors.[citation needed]
  • Because UTP is small, it does not quickly fill up wiring ducts.
  • UTP costs less per foot than any other type of LAN cable.
Disadvantages
Twisted pair’s susceptibility to the electromagnetic interference greatly depends on the pair twisting schemes (usually patented by the manufacturers) staying intact during the installation. As a result, twisted pair cables usually have stringent requirements for maximum pulling tension as well as minimum bend radius. This relative fragility of twisted pair cables makes the installation practices an important part of ensuring the cable’s performance.
 
Author's note: Twisted Pair Cable
Keywords: Twisted Pair Cable



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