Twisted pair cabling is a type of wiring 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, crosstalk between neighboring pairs and improves rejection of external electromagnetic interference. It was invented by Alexander Graham Bell.
In a balanced line, the two wires carry equal and opposite signals, and the destination detects the difference between the two. This is known as differential signaling. Noise sources introduce signals into the wires by coupling of electric or magnetic fields and tend to couple to both wires equally. The noise thus produces a common-mode signal which can be canceled at the receiver when the difference signal is taken.
Differential signaling starts to fail 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. Providing 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 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 shielded or foiled twisted pair (typically F/UTP or S/FTP cable shielding), UTP (unshielded twisted pair) cable is not surrounded by any shielding. UTP is the primary wire type for telephone usage and is very common for computer networking.
The earliest telephones used telegraph lines, or open-wire single-wire earth return circuits. In the 1880s electric trams were installed in many cities, which induced noise into these circuits. Lawsuits being unavailing, the 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 once 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 kilometre, 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 line network was either twisted pair or open wire with transposition to guard against interference. Today, most of the millions of kilometres of twisted pairs in the world are outdoor landlines, owned by telephone companies, used for voice service, and only handled or even seen by telephone workers.
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 Corporation. 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. The bundles are in turn twisted together to make up the cable. Pairs having the same twist rate within the cable can still experience some degree of crosstalk. Wire pairs are selected carefully to minimize crosstalk within a large cable.
UTP cable is also the most common cable used in computer networking. Modern Ethernet, the most common data networking standard, can use UTP cables. 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.
UTP is also finding increasing use in video applications, primarily in security cameras. Many cameras include a UTP output with screw terminals; UTP cable bandwidth has improved to match the baseband of television signals. 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.
Twisted pair cables are often shielded in an attempt to prevent electromagnetic interference. Shielding provides an electrically conductive barrier to attenuate electromagnetic waves external to the shield, and provides a conduction path by which induced currents can be circulated and returned to the source, via ground reference connection.
This shielding can be applied to individual pairs or quads, or to the collection of pairs. Individual pairs are foiled, while overall cable may use braided screen, foil, or braiding with foil.
When shielding is applied to the collection of pairs, this is usually referred to as screening, but different vendors and authors use "screening", "shielding", and "STP" (shielded twisted pair), inconsistently, in attempting to denote various shielded cable types.
ISO/IEC 11801:2002 (Annex E) attempts to internationally standardize the various designations for shielded cables by using combinations of three letters - U for unshielded, S for braided shielding (in outer layer only), and F for foil shielding - to explicitly indicate the type of screen for overall cable protection and for protecting individual pairs or quads, using a two-part abbreviation in the form of x/xTP.
Shielded Cat 5e, Cat 6/6A, and Cat 8/8.1 cables typically have F/UTP construction, while shielded Cat 7/7A and Cat 8.2 cables use S/FTP construction.
Because the shielding is made of metal, it may also serve as a ground. Usually a shielded twisted pair cable has a special grounding wire added called a drain wire which is electrically connected to the shield or screen. The drain wire simplifies connection to ground at the connectors.
Common shield construction types used include:
An early example of shielded twisted-pair is IBM STP-A, which was a two-pair 150 ohm S/FTP cable defined in 1985 by the IBM Cabling System specifications, and used with token ring or FDDI networks.
|Industry acronyms||ISO/IEC 11801 name||Cable shielding||Pair shielding|
|STP, ScTP, PiMF||U/FTP||None||Foil|
|FTP, STP, ScTP||F/UTP||Foil||None|
|SFTP, S-FTP, STP||SF/UTP||Braiding and Foil||None|
|SSTP, SFTP, STP PiMF||S/FTP||Braiding||Foil|
|SSTP, SFTP||SF/FTP||Braiding and Foil||Foil|
The code before the slash designates the shielding for the cable itself, while the code after the slash determines the shielding for the individual pairs:
Before digital communication and ethernet became widespread there was no international standard for telephone cable. Standards were set at a national level. For instance, in the UK the General Post Office specified CW1293 and CW1308 cables. CW1308 was a similar specification to the earlier CW1293 but with an improved color code. CW1293 used mostly solid colors on the cores making it difficult to identify the pair it was twisted with without stripping back a large amount of sheath. CW1308 has narrow rings of the paired color printed over the base color to solve this problem. Both cables are a similar standard to category 3 cable.
Prior to the common use of polyethylene and other plastics for insulation, telephone twisted pair cable was insulated with waxed paper. The overall sheath of this type of cable was usually lead. This style of cable came into use in the late 19th century shortly after the invention of the telephone. The cable termination in termination boxes were sealed with molten wax to prevent the ingress of moisture which would seriously degrade the insulating properties of paper. However, such seals made future maintenance and changes more difficult. These cables are no longer made, but are still occasionally encountered in old buildings.
|Level 1||0.4 MHz||Telephone and modem lines||Not described in EIA/TIA recommendations. Unsuitable for modern systems.|
|Level 2||4 MHz||Older terminal systems, e.g. IBM 3270||Not described in EIA/TIA recommendations. Unsuitable for modern systems.|
|Cat 3||UTP||16 MHz||10BASE-T and 100BASE-T4 Ethernet||Described in EIA/TIA-568. Unsuitable for speeds above 16 Mbit/s. Now mainly for telephone cables|
|Cat 4||UTP||20 MHz||16 Mbit/s Token Ring||Not commonly used|
|Cat 5||UTP||100 MHz||100BASE-TX & 1000BASE-T Ethernet||Common for current LANs. Superseded by Cat5e, but most Cat5 cable meets Cat5e standards.|
|Cat 5e||UTP||100 MHz||100BASE-TX & 1000BASE-T Ethernet||Enhanced Cat5. Common for current LANs. Same construction as Cat5, but with better testing standards.|
|Cat 6||UTP||250 MHz||10GBASE-T Ethernet||ISO/IEC 11801 2nd Ed. (2002), ANSI/TIA 568-B.2-1. Most commonly installed cable in Finland according to the 2002 standard EN 50173-1. Limited to 55M distance at 10GBASE-T|
|Cat 6A||UTP, F/UTP, U/FTP||500 MHz||10GBASE-T Ethernet||Improved standards, tested to 500MHz. Full 100M distance at 10GBASE-T ISO/IEC 11801 2nd Ed. Am. 2. (2008), ANSI/TIA-568-C.1 (2009)|
|Cat 7||S/FTP, F/FTP||600 MHz||10GBASE-T Ethernet or POTS/CATV/1000BASE-T over single cable||Fully shielded cable. ISO/IEC 11801 2nd Ed. (2002). It is not recognized by the EIA/TIA.|
|Cat 7A||S/FTP, F/FTP||1000 MHz||10GBASE-T Ethernet or POTS/CATV/1000BASE-T over single cable||Uses all four pairs. ISO/IEC 11801 2nd Ed. Am. 2. (2008). It is not recognized by the EIA/TIA.|
|Cat 8/8.1||F/UTP, U/FTP||1600–2000 MHz||40GBASE-T Ethernet or POTS/CATV/1000BASE-T over single cable||In development (ANSI/TIA-568-C.2-1, ISO/IEC 11801 3rd Ed.)|
|Cat 8.2||S/FTP, F/FTP||1600–2000 MHz||40GBASE-T Ethernet or POTS/CATV/1000BASE-T over single cable||In development (ISO/IEC 11801 3rd Ed.)|
A solid-core cable uses one solid wire per conductor and in a four pair cable there would be a total of eight solid wires. Stranded conductor uses multiple wires wrapped around each other in each conductor and in a four pair with seven strands per conductor cable, there would be a total of 56 wires (2 per pair × 4 pairs × 7 strands).
Solid core cable is intended for permanently installed runs. It is less flexible than stranded cable and is more prone to failure if repeatedly flexed. Stranded cable is used for fly leads at patch panel and for connections from wall-ports to end devices, as it resists cracking of the conductors.
Connectors are designed differently for solid core than for stranded. Use of a connector with the wrong cable type can lead to unreliable cabling. Plugs designed for solid and stranded core are readily available, and some vendors even offer plugs designed for use with both types. The punch-down blocks on patch-panel and wall-port jacks are designed for use with solid core cable.
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