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 neighbouring pairs and improves rejection of external electromagnetic interference. It was invented by Alexander Graham Bell. [1]
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).
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 cancelled 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 metre) 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. [2]
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.
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. [lower-alpha 1] 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 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. [4] By 1900, the entire American telephone 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 and maintained by telephone companies, used for voice service.
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 colours (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) (0.644 or 0.511 mm²), [5] with the coloured 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 fibre 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. [6] 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 may incorporate shielding in an attempt to prevent electromagnetic interference. Shielding provides an electrically conductive barrier to attenuate electromagnetic waves external to the shield. The shield also provides a conduction path by which induced currents can be circulated and returned to the source via ground reference connection. Such shielding can be applied to individual pairs or to a collection of pairs. Shielding may be foil or braided wire.
When shielding is applied to a collection of pairs, it is usually referred to as screening, but usage among vendors and authors in applying such words as screening, shielding, and STP (shielded twisted pair) can be subject to variability. [7] [8]
ISO/IEC 11801:2002 (Annex E) attempts to internationally standardize the various shielding designations for twisted pair (TP) cables using an explicit two-part abbreviation in the form of x/xTP, where the first x indicates the shielding for the overall cable and the second x indicates the shielding for individual pairs or quads, where each x can be:
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. [9]
Because the shielding is conductive, it may also serve as a path to ground. A foil-shielded, twisted pair cable may have an integrally incorporated grounding wire called a drain wire which makes electrical contact with the shield. The purpose of the drain wire is for easy connection to terminals which are usually designed for connection of round wires.
Common shield construction types include:
An early example of shielded twisted-pair was IBM STP-A, which is 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. [7] [11]
Industry abbreviations | ISO/IEC 11801 designation [upper-alpha 1] | Cable shielding | Pair shielding | Illustration |
---|---|---|---|---|
UTP, TP | U/UTP | None | None | ![]() |
FTP, STP, ScTP | F/UTP | Foil | None | ![]() |
STP, ScTP | S/UTP | Braiding | None | ![]() |
SFTP, S-FTP, STP | SF/UTP | Braiding and Foil | None | ![]() |
STP, ScTP, PiMF | U/FTP | None | Foil | ![]() |
FFTP, STP | F/FTP | Foil | Foil | ![]() |
SSTP, SFTP, STP, STP PiMF | S/FTP | Braiding | Foil | ![]() |
SSTP, SFTP, STP | SF/FTP | Braiding and Foil | Foil | ![]() |
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 colours on the cores making it difficult to identify the pair it was twisted with without stripping back a large amount of sheath. To solve this problem. CW1308 has narrow rings of the paired color printed over the base colour. Both cables are a similar standard to category 3 cable. [12] [13] Cables with categories 3 through 7 have 4 twisted pairs. [14]
Prior to the common use of polyethylene and other plastics for insulation, telephone twisted pair cable was insulated with waxed paper or cotton with a wax coating applied to the copper. 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. [15] The cable termination in termination boxes were sealed with molten wax or a resin to prevent the ingress of moisture which would seriously degrade the insulating properties of the paper insulation. [16] However, such seals made future maintenance and changes more difficult. These cables are no longer made but are still occasionally encountered in old buildings and in various external areas, commonly rural villages.
Name | Typical construction | Bandwidth | Applications | Notes |
---|---|---|---|---|
Level 1 | 400 kHz | Telephone and modem lines | Not described in EIA/TIA recommendations. Unsuitable for modern systems. [17] | |
Level 2 | 4 MHz | Older terminal systems, e.g. IBM 3270 | Not described in EIA/TIA recommendations. Unsuitable for modern systems. [17] | |
Cat 3 | UTP [18] | 16 MHz [18] | 10BASE-T, 100BASE-T4 [18] | Described in EIA/TIA-568. Unsuitable for speeds above 16 Mbit/s. Now mainly for telephone cables. [18] |
Cat 4 | UTP [18] | 20 MHz [18] | 16 Mbit/s Token Ring [18] | Not commonly used [18] |
Cat 5 | UTP [18] | 100 MHz [18] | 100BASE-TX, 1000BASE-T [18] | Common for current LANs. Superseded by Cat 5e, but most Cat 5 cables meet Cat 5e standards. [18] Limited to 100 m between equipment. |
Cat 5e | UTP, [18] F/UTP, U/FTP [19] | 100 MHz [18] | 1000BASE-T, 2.5GBASE-T [18] | Enhanced Cat 5. Common for current LANs. Same construction as Cat 5, but with better testing standards. [18] Limited to 100m between equipment. |
Cat 6 | UTP, [18] F/UTP, U/FTP [20] | 250 MHz [18] | 5GBASE-T, 10GBASE-T | ISO/IEC 11801 2nd Ed. (2002), ANSI/TIA 568-B.2-1. Limited to 55 m distance at 10GBASE-T |
Cat 6A | UTP, F/UTP, U/FTP, S/FTP | 500 MHz | 5GBASE-T, 10GBASE-T | Improved standards, tested to 500 MHz. Full 100 m 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 [21] | ? | ISO/IEC 11801 2nd Ed. (2002). Only with GG45 or TERA connectors. It is not recognized by the EIA/TIA. |
Cat 7A | S/FTP, F/FTP | 1 GHz [21] | ? | ISO/IEC 11801 2nd Ed. Am. 2. (2008). Only with GG45 or TERA connectors. It is not recognized by the EIA/TIA. |
Cat 8.1 | F/UTP, U/FTP | 2 GHz [21] | 25GBASE-T, 40GBASE-T | ANSI/TIA-568-C.2-1, ISO/IEC 11801-1:2017 |
Cat 8.2 | S/FTP, F/FTP | 2 GHz | 25GBASE-T, 40GBASE-T | ISO/IEC 11801-1:2017 |
A loaded twisted pair has intentionally added inductance and was formerly common practice on telecommunication lines. The added inductors are known as load coils and reduce attenuation for voiceband frequencies but increase it on higher frequencies. Load coils reduce distortion in voiceband on very long lines. [22] In this context a line without load coils is referred to as an unloaded line.
A bonded twisted pair is a construction variant in which the two wires of each pair are bonded together for the length of the cable. Pioneered by Belden, it is intended to help assure configuration consistency during and after installation. One key benefit is that the noise immunity performance of the cable can be protected despite potentially rough handling. [23] The enhanced performance may be unnecessary and bonding reduces the flexibility of the cable and makes it prone to failure where it is flexed. [24]
A twisted ribbon cable is a variant of standard ribbon cable in which adjacent pairs of conductors are bonded and twisted together. The twisted pairs are then lightly bonded to each other in a ribbon format. Periodically along the ribbon, there are short sections with no twisting where connectors may be attached using the usual ribbon cable IDC techniques. [25]
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. [18] Stranded cable 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). [18]
Solid core cable is intended for permanently installed runs (permanent link). It is less flexible than stranded cable and is more prone to failure if repeatedly flexed due to work hardening. Stranded cable is used at patch panels and for connections from wall-ports to end devices (patch cord or drop cable), 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. These work via the insulation-displacement method, whereby the device pierces the sides of the insulation and "bites" into the copper conductor to form a connection. Punchdown blocks are used as patch panels or as break-out boxes, for twisted pair cable.
Twisted pair has the following useful attributes: [26]
Twisted pair has the following limitations:
In telecommunications and professional audio, a balanced line or balanced signal pair is an electrical circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths, to ground, and to other circuits. The primary advantage of the balanced line format is good rejection of common-mode noise and interference when fed to a differential device such as a transformer or differential amplifier.
A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulate sound, and a transmission medium for sounds may be air, but solids and liquids may also act as the transmission medium. Vacuum or air constitutes a good transmission medium for electromagnetic waves such as light and radio waves. While a material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission media they pass through, for instance, by absorption or reflection or refraction at the interfaces between media. Technical devices can therefore be employed to transmit or guide waves. Thus, an optical fiber or a copper cable is used as transmission media.
An electrical cable is an assembly of one or more wires running side by side or bundled, which is used as an electrical conductor to carry electric current.
Coaxial cable, or coax, is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ; many coaxial cables also have a protective outer sheath or jacket. The term coaxial refers to the inner conductor and the outer shield sharing a geometric axis.
Category 5 cable (Cat 5) is a twisted pair cable for computer networks. Since 2001, the variant commonly in use is the Category 5e specification (Cat 5e). The cable standard provides performance of up to 100 MHz and is suitable for most varieties of Ethernet over twisted pair up to 2.5GBASE-T but more commonly runs at 1000BASE-T speeds. Cat 5 is also used to carry other signals such as telephone and video.
Balanced audio is a method of interconnecting audio equipment using balanced interfaces. This type of connection is very important in sound recording and production because it allows the use of long cables while reducing susceptibility to external noise caused by electromagnetic interference. The balanced interface guarantees that induced noise appears as common-mode voltages at the receiver which can be rejected by a differential device.
A balun is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun can take many forms and may include devices that also transform impedances but need not do so. Sometimes, in the case of transformer baluns, they use magnetic coupling but need not do so. Common-mode chokes are also used as baluns and work by eliminating, rather than rejecting, common mode signals.
An audio multicore cable is a thick cable which usually contains 4–64 individual audio cables inside a common, sturdy outer jacket. Audio multicore cables are used to convey many audio signals between two locations, such as in audio recording, sound reinforcement, PA systems and broadcasting. Multicores often route many signals from microphones or musical instruments to a mixing console, and can also carry signals from a mixing console back to speakers.
Differential signalling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conductors can be wires in a twisted-pair or ribbon cable or traces on a printed circuit board.
A shielded cable or screened cable is an electrical cable that has a common conductive layer around its conductors for electromagnetic shielding. This shield is usually covered by an outermost layer of the cable. Common types of cable shielding can most broadly be categorized as foil type, contraspiralling wire strands or both. A longitudinal wire may be necessary with dielectric spiral foils to short out each turn.
Category 6 cable (Cat 6) is a standardized twisted pair cable for Ethernet and other network physical layers that is backward compatible with the Category 5/5e and Category 3 cable standards.
In telecommunications and electrical engineering in general, an unbalanced line is a pair of conductors intended to carry electrical signals, which have unequal impedances along their lengths and to ground and other circuits. Examples of unbalanced lines are coaxial cable or the historic earth return system invented for the telegraph, but rarely used today. Unbalanced lines are to be contrasted with balanced lines, such as twin-lead or twisted pair which use two identical conductors to maintain impedance balance throughout the line. Balanced and unbalanced lines can be interfaced using a device called a balun.
In electronics, crosstalk is a phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit or channel to another.
International standard ISO/IEC 11801Information technology — Generic cabling for customer premises specifies general-purpose telecommunication cabling systems that are suitable for a wide range of applications. It is published by ISO/IEC JTC 1/SC 25/WG 3 of the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). It covers both balanced copper cabling and optical fibre cabling.
In electrical engineering, a common-mode signal is the identical component of voltage present at both input terminals of an electrical device. In telecommunication, the common-mode signal on a transmission line is also known as longitudinal voltage.
Networking cable is a piece of networking hardware used to connect one network device to other network devices or to connect two or more computers to share devices such as printers or scanners. Different types of network cables, such as coaxial cable, optical fiber cable, and twisted pair cables, are used depending on the network's topology, protocol, and size. The devices can be separated by a few meters or nearly unlimited distances.
ANSI/TIA-568 is a technical standard for commercial building cabling for telecommunications products and services. The title of the standard is Commercial Building Telecommunications Cabling Standard and is published by the Telecommunications Industry Association (TIA), a body accredited by the American National Standards Institute (ANSI).
Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor.
Physical media refers to the physical materials that are used to store or transmit information in data communications. These physical media are generally physical objects made of materials such as copper or glass. They can be touched and felt, and have physical properties such as weight and color. For a number of years, copper and glass were the only media used in computer networking.
In electrical engineering, star-quad cable is a four-conductor electrical cable that has a special quadrupole geometry which provides magnetic immunity when used in a balanced line. Four conductors are used to carry the two legs of the balanced line. All four conductors must be an equal distance from a common point. The four conductors are arranged in a four-pointed star. Opposite points of the star are connected together at each end of the cable to form each leg of the balanced circuit.
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