Twisted pair

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25-pair color code chart 25 pair color code chart.svg
25-pair color code chart

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 from the pair and crosstalk between neighboring pairs and improves rejection of external electromagnetic interference. It was invented by Alexander Graham Bell. [1]

Electronic circuit electrical circuit with active components such as transistors, valves and integrated circuits; electrical network that contains active electronic components, generally nonlinear and require complex design and analysis tools

An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. To be referred to as electronic, rather than electrical, generally at least one active component must be present. The combination of components and wires allows various simple and complex operations to be performed: signals can be amplified, computations can be performed, and data can be moved from one place to another.

Electromagnetic compatibility

Electromagnetic compatibility (EMC) is the ability of electrical equipment and systems to function acceptably in their electromagnetic environment, by limiting the unintentional generation, propagation and reception of electromagnetic energy which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage in operational equipment. The goal of EMC is the correct operation of different equipment in a common electromagnetic environment. It is also the name given to the associated branch of electrical engineering.

Single-ended signaling is the simplest and most commonly used method of transmitting electrical signals over wires. One wire carries a varying voltage that represents the signal, while the other wire is connected to a reference voltage, usually ground.

Contents

Explanation

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.

In telecommunications and professional audio, a balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. The chief advantage of the balanced line format is good rejection of external noise when fed to a differential amplifier. Common forms of balanced line are twin-lead, used for radio frequency signals and twisted pair, used for lower frequencies. They are to be contrasted to unbalanced lines, such as coaxial cable, which is designed to have its return conductor connected to ground, or circuits whose return conductor actually is ground. Balanced and unbalanced circuits can be interconnected using a transformer called a balun.

A balanced circuit is circuitry for use with a balanced line or the balanced line itself. Balanced lines are a common method of transmitting many types of electrical communication signals between two points on two wires. In a balanced line the two signal lines are of a matched impedance to help ensure that interference induced in the line is common-mode and can be removed at the receiving end by circuitry with good common-mode rejection. To maintain the balance, circuit blocks which interface to the line, or are connected in the line, must also be balanced.

Electromagnetic interference Disturbance generated in an electrical circuit due to external fields or sources of radio-frequency electromagnetic radiation

Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The disturbance may degrade the performance of the circuit or even stop it from functioning. In the case of a data path, these effects can range from an increase in error rate to a total loss of the data. Both man-made and natural sources generate changing electrical currents and voltages that can cause EMI: ignition systems, cellular network of mobile phones, lightning, solar flares, and auroras. EMI frequently affects AM radios. It can also affect mobile phones, FM radios, and televisions, as well as observations for radio astronomy and atmospheric science.

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]

Metre SI unit of length

The metre or meter is the base unit of length in the International System of Units (SI). The SI unit symbol is m. The metre is defined as the length of the path travelled by light in a vacuum in 1/299 792 458 of a second.

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.

Telephone Telecommunications device

A telephone, or phone, is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be heard directly. A telephone converts sound, typically and most efficiently the human voice, into electronic signals that are transmitted via cables and other communication channels to another telephone which reproduces the sound to the receiving user.

History

Wire transposition on top of pole Wire Transposition.svg
Wire transposition on top of pole

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.

Single-wire earth return technology to supply energy by single wire where earth will serve as return conductor

Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line which supplies single-phase electric power from an electrical grid to remote areas at low cost. Its distinguishing feature is that the earth is used as the return path for the current, to avoid the need for a second wire to act as a return path.

Tram Vehicle used for tramway traffic

A tram is a rail vehicle that runs on tramway tracks along public urban streets; some include segments of segregated right-of-way. The lines or networks operated by tramcars are called tramways. Historically the term electric street railways was also used in the United States. In the United States, the term tram has sometimes been used for rubber-tired trackless trains, which are unrelated to other kinds of trams.

A lawsuit is a proceeding by a party or parties against another in the civil court of law. The archaic term "suit in law" is found in only a small number of laws still in effect today. The term "lawsuit" is used in reference to a civil action brought in a court of law in which a plaintiff, a party who claims to have incurred loss as a result of a defendant's actions, demands a legal or equitable remedy. The defendant is required to respond to the plaintiff's complaint. If the plaintiff is successful, judgment is in the plaintiff's favor, and a variety of court orders may be issued to enforce a right, award damages, or impose a temporary or permanent injunction to prevent an act or compel an act. A declaratory judgment may be issued to prevent future legal disputes.

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. [3] 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

Cross-section of cable with four unshielded twisted pairs UTP-cable.svg
Cross-section of cable with four unshielded twisted pairs

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), [4] 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.

Unshielded twisted pair cable with different twist rates UTP cable.jpg
Unshielded twisted pair cable with different twist rates

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.

Cable shielding

F/UTP cable FTP cable3.jpg
F/UTP cable
S/FTP cable S-FTP CAT 7.jpg
S/FTP cable
U/FTP, F/UTP and F/FTP are used in Cat 6a cables 10G BASE-T x 3 cables.svg
U/FTP, F/UTP and F/FTP are used in Cat 6a cables

Twisted pair cables often incorporate shielding 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.

Such shielding can be applied to individual pairs or quads; or to a collection of pairs. Individual pairs are foil shielded, while an overall cable (of multiple pairs) may use any of braided screen or foil or braiding with foil.

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. [5] [6]

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. [7]

Because the shielding is made of metal, it may also serve as a path to ground; usually a screen shielded, twisted pair cable has an integrally incorporated, grounding wire, called a drain wire, which makes electrical contact with the shield or screen. The purpose of the drain wire is for easy connection of the screen to terminals, which are usually designed for connection of round wires.

Common shield construction types include:

Individual shield (U/FTP)
Individual shielding with aluminum foil for each twisted pair or quad. Common names: pair in metal foil, shielded twisted pair, screened twisted pair. This type of shielding protects cable from external EMI entering or exiting the cable and also protects neighboring pairs from crosstalk.
Overall shield (F/UTP, S/UTP, and SF/UTP)
Overall foil, braided shield or braiding with foil across all of the pairs within the 100 ohm twisted pair cable. Common names: foiled twisted pair, shielded twisted pair, screened twisted pair. This type of shielding helps prevent EMI from entering or exiting the cable.
Individual and overall shield (F/FTP, S/FTP, and SF/FTP)
Individual shielding using foil between the twisted pair sets, and also an outer foil or braided shielding. Common names: fully shielded twisted pair, screened foiled twisted pair, shielded foiled twisted pair, screened shielded twisted pair, shielded screened twisted pair. This type of shielding helps prevent EMI from entering or exiting the cable and also protects neighboring pairs from crosstalk.

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. [5] [8]

Common industry nomenclature for cable construction types
Industry abbreviations ISO/IEC 11801 designationCable shieldingPair shielding
UTP, TPU/UTPNoneNone
STP, ScTP, PiMFU/FTPNoneFoil
FTP, STP, ScTPF/UTPFoilNone
STP, ScTPS/UTPBraidingNone
SFTP, S-FTP, STPSF/UTPBraiding and FoilNone
FFTP, STPF/FTPFoilFoil
SSTP, SFTP, STP, STP PiMFS/FTPBraidingFoil
SSTP, SFTP, STPSF/FTPBraiding and FoilFoil

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:

U = unshielded
F = foil shielding
S = braided shielding (outer layer only)
TP = twisted pair
TQ = twisted pair, individual shielding in quads

Types

Analog telephone

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. [9]

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. [10] The cable termination in termination boxes were sealed with molten wax or epoxy resin to prevent the ingress of moisture which would seriously degrade the insulating properties of the paper insulation. [11] 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.

Building infrastructure

NameTypical constructionBandwidthApplicationsNotes
Level 1 0.4 MHzTelephone and modem linesNot described in EIA/TIA recommendations. Unsuitable for modern systems. [12]
Level 2 4 MHzOlder terminal systems, e.g. IBM 3270 Not described in EIA/TIA recommendations. Unsuitable for modern systems. [12]
Cat 3 UTP [13] 16 MHz [13] 10BASE-T / 100BASE-T4 [13] Described in EIA/TIA-568. Unsuitable for speeds above 16 Mbit/s. Now mainly for telephone cables [13]
Cat 4 UTP [13] 20 MHz [13] 16 Mbit/s [13] Token Ring Not commonly used [13]
Cat 5 UTP [13] 100 MHz [13] 100BASE-TX / 1000BASE-T [13] Common for current LANs. Superseded by Cat5e, but most Cat5 cables meet Cat5e standards. [13] Limited to 100m between equipment.
Cat 5e UTP, [13] STP [14] 100 MHz [13] 1000BASE-T / 2.5GBASE-T [13] Enhanced Cat5. Common for current LANs. Same construction as Cat5, but with better testing standards. [13] Limited to 100m between equipment.
Cat 6 UTP, [13] STP [15] 250 MHz [13] 5GBASE-T / 10GBASE-T 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/FTP500 MHz 5GBASE-T / 10GBASE-T Improved standards, tested to 500 MHz. 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/FTP600 MHz 5GBASE-T / 10GBASE-T or POTS/CATV/1000BASE-T over single cableFully shielded cable. ISO/IEC 11801 2nd Ed. (2002). It is not recognized by the EIA/TIA.
Cat 7A S/FTP, F/FTP1000 MHz 5GBASE-T / 10GBASE-T or POTS/CATV/1000BASE-T over single cableUses 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/FTP2000 MHz 25GBASE-T / 40GBASE-T ANSI/TIA-568-C.2-1, ISO/IEC 11801-1:2017
Cat 8.2 S/FTP, F/FTP2000 MHz 25GBASE-T / 40GBASE-T ISO/IEC 11801-1:2017

Loaded

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. [16] In this context a line without load coils is referred to as an unloaded line.

Bonded

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. [17] This may not be necessary and it could have detrimental effects in applications which require flexible cable. [18]

Twisted ribbon cable

Twisted ribbon cable used for SCSI connections Scsi intern hd68 lvd term.jpg
Twisted ribbon cable used for SCSI connections

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 to enable connectors and PCB headers to be terminated using the usual ribbon cable IDC techniques. [19]

Solid-core vs. stranded cable

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. [13] 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). [13]

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. Stranded cable is used for fly leads at patch panel 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 a method known as insulation-displacement which pierces the sides of the insulation and "bites" in to the copper conductor to form a connection.

Advantages

Disadvantages

See also

Related Research Articles

Transmission medium material substance that can propagate energy waves

A transmission medium is something that can mediate the propagation of signals for the purposes of telecommunication.

Electrical cable two or more wires running side by side and bonded, twisted, or braided together to form a single assembly

An electrical cable is an assembly of one or more wires running side by side or bundled, which is used to carry electric current.

Coaxial cable A type of electrical cable with an inner conductor surrounded by concentric insulating layer and conducting shield

Coaxial cable, or coax is a type of electrical cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables also have an insulating outer sheath or jacket. The term coaxial comes from the inner conductor and the outer shield sharing a geometric axis. Coaxial cable was invented by English physicist, engineer, and mathematician Oliver Heaviside, who patented the design in 1880.

Category 5 cable twisted pair cable for carrying signals

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 1000BASE-T. Cat 5 is also used to carry other signals such as telephony and video.

Balanced audio is a method of interconnecting audio equipment using balanced lines. 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.

Balun

A balun is an electrical device that converts between a balanced signal and an unbalanced signal. A balun can take many forms and may include devices that also transform impedances but need not do so. Transformer baluns can also be used to connect lines of differing impedance. 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 ignoring, common mode signals.

Audio multicore cable

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 signaling method for electrically transmitting information using two complementary signals

Differential signaling 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 or traces on a circuit board. The receiving circuit responds to the electrical difference between the two signals, rather than the difference between a single wire and ground. The opposite technique is called single-ended signalling. Differential pairs are usually found on printed circuit boards, in twisted-pair and ribbon cables, and in connectors.

Shielded cable electrical conductors enclosed by a conductive layer

A shielded cable or screened cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper, a non-braided spiral winding of copper tape, or a layer of conducting polymer. Usually this shield is covered with a jacket.

Category 6 cable standardized cable for Gigabit Ethernet and other network physical layers

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.

Feed line Transmission line in radio antennas

In a radio antenna, the feed line (feedline), or feeder, is the cable or other transmission line that connects the antenna with the radio transmitter or receiver. In a transmitting antenna, it feeds the radio frequency (RF) current from the transmitter to the antenna, where it is radiated as radio waves. In a receiving antenna it transfers the tiny RF voltage induced in the antenna by the radio wave to the receiver. In order to carry RF current efficiently, feed lines are made of specialized types of cable called transmission line. The most widely used types of feed line are coaxial cable, twin-lead, ladder line, and at microwave frequencies, waveguide.

Unbalanced line

In electrical engineering, an unbalanced line is a transmission line, often coaxial cable, whose conductors have unequal impedances with respect to ground; as opposed to a balanced line. Microstrip and single-wire lines are also unbalanced lines.

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 covers both balanced copper cabling and optical fibre cabling.

Twinaxial cabling communications protocol

Twinaxial cabling, or "Twinax", is a type of cable similar to coaxial cable, but with two inner conductors instead of one. Due to cost efficiency it is becoming common in modern (2013) very-short-range high-speed differential signaling applications.


In copper twisted pair wire networks, copper cable certification is achieved through a thorough series of tests in accordance with Telecommunications Industry Association (TIA) or International Organization for Standardization (ISO) standards. These tests are done using a certification-testing tool, which provide pass or fail information. While certification can be performed by the owner of the network, certification is primarily done by datacom contractors. It is this certification that allows the contractors to warranty their work.

ANSI/TIA-568 is a set of telecommunications standards from the Telecommunications Industry Association (TIA). The standards address commercial building cabling for telecommunications products and services.

Copper conductor

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.

Star quad cable

Star-quad cable is a four conductor cable that has a special quadrupole geometry that 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.

References

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  5. 1 2 Anitech Systems MP 4000 Manual
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  10. Telephony, vol. 153, p. 118, Telephone Publishing Corporation 1957.
  11. Paper Maker and British Paper Trade Journal, vol. 83-84, p. 294, November 1, 1932 OCLC   10634178
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  13. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 "Comparison between CAT5, CAT5e, CAT6, CAT7 Cables".
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