What Is A Twisted Pair Cable?
May 11, 2024
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Introduction
What is a twisted pair cable? This is a frequently asked question by many people. What we often call "twisted pair cable" is a type of cable that use a twisted pair structure. The twisted pair cable is considered the ideal choice for local area network (LAN) wiring. Originally, the Ethernet standard relied on coaxial cables similar to those used in cable television. At that time, the transmission speed of coaxial cables was considered impeccable.Over time, however, coaxial cables started showing performance bottlenecks and inherent drawbacks, including high costs, complicated maintenance, and stiff shielding layers that made installation challenging. Eventually, twisted pair cables replaced coaxial cables.
A twisted pair cable is made up of two insulated wires twisted together in a specific direction to form a set of cables. But how much do you really know about twisted pair cables? This article offers an in-depth overview of the classification of twisted pair cables, their performance parameters, transmission speeds, twist pitches, conductor cores, test data, markings, and fire resistance ratings. By the end of this article, you'll have a thorough understanding of twisted pair cables.
When computers are networked together, the initial challenge encountered is the communication lines and channel transmission issues. Currently, computer communications are classified into two types: wired and wireless. Wired communication uses cables, fiber optic cables, or telephone wires as transmission conductors, whereas wireless communication employs satellites, microwaves, or infrared rays as transmission mediums.
The selection of network communication lines must take into account network performance, cost, use regulations, ease of installation, scalability, and other factors. The cables used in network wiring systems are generally divided into twisted pair cable, coaxial cable, bulk cables, and fiber optic cables. There are many types and models of cables supplied on the market, and engineering technicians should select them based on the actual project needs, primarily considering their function, model, type, and main performance.
A Twisted pair cable (TP) is the most commonly used transmission medium in integrated wiring projects. They consist of two copper conductors, each with an insulating protective layer. The two insulated copper conductors are twisted together at a certain density, which can reduce the degree of signal interference, as the electromagnetic wave radiated by each conductor during transmission will be canceled out by the wave emitted from the other conductor. Typically, twisted pairs are made by intertwining two insulated copper conductors of gauge sizes 22, 24, or 26. When one or more pairs of twisted wires are placed within an insulating sheath, this becomes a twisted pair cable. In a twisted pair cable (also known as a twisted pair), different pairs have different twist lengths, usually between 38.1 to 140mm, twisted in a counterclockwise direction. The twist length of adjacent pairs should be more than 12.7mm. Generally, the tighter the twisting, the stronger the interference resistance. Compared with other transmission media, twisted pairs have certain limitations in terms of transmission distance, channel width, and data transmission speed, but their cost is relatively low.
Currently, twisted pairs are divided into unshielded twisted pairs (UTP) and shielded twisted pairs (STP), with shielded twisted pair cables wrapped in an aluminum foil layer on the outside, which results in a relatively higher price.
Although twisted pairs are mainly used for transmitting analog voice information, they are also suitable for digital signal transmission, especially for short-distance information transmission. During transmission, the signal attenuates significantly, and this can cause waveform distortion.
The bandwidth of local area networks using twisted pairs depends on the quality of the conductors used, the length of the conductors, and the transmission technology. As twisted pairs radiate signals while transmitting information, they can be easily eavesdropped on, so additional costs are incurred to shield them to reduce radiation (though it cannot be completely eliminated). This is what we refer to as shielded twisted pair cables. Shielded twisted pair cables are comparatively more expensive and more difficult to install than unshielded twisted pair cables.
Twisted pair cables have the following advantages:
Small diameter, saving space;
Light weight, easy to bend and install;
Minimizes or eliminates crosstalk;
Flame retardant;
Offers flexibility and independence, suitable for structured integrated wiring.
1. Classification of Twisted Pairs
Category 1: Used for telephone voice communications, not for computer network data communications.
Category 2: With a transmission frequency of 1MHz, it is used for voice transmission and data transmission with a maximum transfer rate of 4Mbps, often seen in older token ring networks using the 4Mbps token passing protocol.
Category 3: Used for voice transmission and data transmission with a maximum transfer rate of 16Mbps, mainly used for 10BASE-T networks.
Category 4: This type of cable has a transmission frequency of 20MHz, used for voice transmission and data transmission with a maximum transfer rate of 20Mbps, mainly used for token-based local area networks and 10BASE-T/100BASE-T networks.
Category 5: This type of cable has increased wrap density and is sheathed in a high-quality insulation material, with a transmission rate of 100MHz, used for voice transmission and data transmission with a maximum transfer rate of 100Mbps, mainly used for 100BASE-T and 10BASE-T networks. This is the most commonly used Ethernet cable.
Category 5e: This category includes cables that have less attenuation and crosstalk, as well as a higher attenuation to crosstalk ratio (ACR) and signal-to-noise ratio (Structural Return Loss), and reduced delay errors, thus significantly improving performance. Category 5e is mainly used for Gigabit Ethernet (1000Mbps).
Category 6: This category of cables has a transmission frequency ranging from 1 to 250MHz. Category 6 cabling systems should have a considerable margin of the Power Sum Attenuation to Crosstalk Ratio (PS-ACR) at 200MHz, providing twice the bandwidth of Category 5e. The transmission performance of Category 6 cabling greatly exceeds the Category 5e standards, making it most suitable for applications with transmission rates higher than 1Gbps. One important difference between Category 6 and Category 5e is the improved performance in terms of crosstalk and return loss, which is extremely important for the next generation of full-duplex high-speed network applications. The basic link model has been omitted in Category 6 standards, and the cabling standards adopt a star topology with required cabling distances: the permanent link length must not exceed 90m, and the channel length must not exceed 100m. Category 6 cables are divided into 6E and 6A, with 6E having a transmission frequency of 200MHz and 6A having a transmission frequency of 250MHz.
Category 7: This category is mainly designed to cater to the application and development of 10 Gigabit Ethernet technology but is no longer an unshielded twisted pair; instead, it is a shielded twisted pair. Therefore, its transmission frequency can reach at least 600MHz, which is more than twice that of Category 6 and Category 6a cables. Category 7 cables are divided into 7F and 7A, with 7F having a transmission frequency of 600MHz and 7A having a transmission frequency of 620MHz.
Category 8: International standards have fundamentally recognized Category 8 wiring. Category 8 cables are divided into 8.1 and 8.2, where 8.1 must be compatible with Category 6, and 8.2 must be compatible with Category 7. The types of four-pair twisted pair cables used in computer network integrated cabling are shown in Figure 1.
Figure 1: Types of Twisted Pair Cables Used in Computer Network Engineering
The physical structure of four-pair unshielded twisted pairs for Categories 3, 5, and 5e is shown in Figure 2.
Figure 2: Physical Structure of Four-Pair Unshielded Twisted Pairs for Categories 3, 5, and 5e
The composition of wire colors for four pairs of twisted pairs is shown in Table 1.
Table 1: Wire Color Composition for Four Pairs of Twisted Pairs
|
Pair
|
Color Code
|
|---|---|
|
1
|
White/Blue//Blue
|
|
2
|
White/Orange//Orange
|
|
3
|
White/Green//Green
|
|
4
|
White/Brown//Brown
|
2. Parameters of Twisted Pairs Cables
For twisted pairs (whether it's Category 3, 5, 6, 7, 8, shielded, or unshielded), users are concerned with parameters such as attenuation, near-end crosstalk, DC resistance, characteristic impedance, distributed capacitance, etc.
(1) Attenuation
Attenuation is a measure of signal loss along a link. Since attenuation varies with frequency, it should be measured across the entire frequency range applicable.
(2) Near-End Crosstalk
Near-end crosstalk loss measures the signal coupling from one pair of wires to another in a UTP link. For UTP links, this is a crucial performance indicator and also one of the hardest to measure accurately, especially as the difficulty increases with signal frequency. Crosstalk is classified into near-end crosstalk (NEXT) and far-end crosstalk (FEXT). Testers mainly measure NEXT, and due to line losses, the effect of FEXT is minimal. FEXT is disregarded in Category 3 and 5 systems. NEXT does not represent the crosstalk value generated at the near-end; it only represents the crosstalk value measured at the near-end. This value decreases with cable length; the longer the cable, the smaller the measured value. Additionally, the signal at the transmitter end will also attenuate, reducing crosstalk to other pairs. Experiments have shown that NEXT values measured within 40 meters are more accurate. If the other end of the link is an information socket farther than 40m, it will create a certain degree of crosstalk that the tester may not be able to detect. For this reason, it is best to measure NEXT at both endpoints. Current testers are equipped with corresponding devices that enable the measurement of NEXT values at both ends of the link from a single side.
Tables of attenuation and NEXT test values are shown in Tables 2 and 3.
|
Frequency (MHz)
|
Maximum Attenuation 20°C
|
|||||||||
|
Channel(100m)
|
Link (90m)
|
|||||||||
|
|
Cat. 3
|
Cat.4
|
Cat.5
|
Cat5E
|
Cat.6
|
Cat.3
|
Cat.4
|
Cat.5
|
Cat.5E
|
Cat.6
|
|
1
|
4.2
|
2.6
|
2.5
|
2.5
|
2.1
|
3.2
|
2.2
|
2.1
|
2.1
|
1.9
|
|
4
|
7.3
|
4.8
|
4.5
|
4.5
|
4.0
|
6.1
|
4.3
|
4.0
|
4.0
|
3.5
|
|
8
|
10.2
|
6.7
|
63
|
6.3
|
5.7
|
8.8
|
6.0
|
5.7
|
5.7
|
5.0
|
|
10
|
11.5
|
7.5
|
7.0
|
7.0
|
6.3
|
10.0
|
6.8
|
6.3
|
6.3
|
5.6
|
|
16
|
14.9
|
9.9
|
9.2
|
9.2
|
8.0
|
13.2
|
8.8
|
8.2
|
8.2
|
7.1
|
|
20
|
|
11.0
|
10.3
|
10.3
|
9.0
|
|
9.9
|
9.2
|
9.2
|
7.9
|
|
25
|
|
|
11.4
|
11.4
|
10.1
|
|
|
10.3
|
10.3
|
8.9
|
|
31.25
|
|
|
12.8
|
12.8
|
11.4
|
|
|
11.5
|
11.5
|
10.0
|
|
62.5
|
|
|
18.5
|
18.5
|
16.5
|
|
|
16.7
|
16.7
|
14.4
|
|
100
|
|
|
24.0
|
24.0
|
21.3
|
|
|
21.6
|
21.6
|
18.5
|
|
200
|
|
|
|
|
31.5
|
|
|
|
|
27.1
|
|
250
|
|
|
|
|
36.0
|
|
|
|
|
30.7
|
Table 2: Attenuation Limits for Various Connections at Maximum Length per Frequency
|
Frequency(MHz)
|
Minumum NEXT/20°C
|
|||||||||
|
Channel(100m)
|
Link(90m)
|
|||||||||
|
|
Cat. 3
|
Cat.4
|
Cat.5
|
Cat5E
|
Cat.6
|
Cat.3
|
Cat.4
|
Cat.5
|
Cat.5E
|
Cat.6
|
|
1
|
39.1
|
53.3
|
60.0
|
60.0
|
65.0
|
40.1
|
54.7
|
60.0
|
60.0
|
65.0
|
|
4
|
29.3
|
43.3
|
50.6
|
53.6
|
63.0
|
30.7
|
45.1
|
51.8
|
54.8
|
64.1
|
|
8
|
24.3
|
38.2
|
45.6
|
48.6
|
58.2
|
25.9
|
40.2
|
47.1
|
50.0
|
59.4
|
|
10
|
22.7
|
36.6
|
44.0
|
47.0
|
56.6
|
24.3
|
38.6
|
45.5
|
48.5
|
57.8
|
|
16
|
19.3
|
33.1
|
40.6
|
43.6
|
53.2
|
21.0
|
35.3
|
42.3
|
45.2
|
54.6
|
|
20
|
|
31.4
|
39.0
|
42.0
|
51.6
|
|
33.7
|
40.7
|
43.7
|
53.1
|
|
25.0
|
|
|
37.4
|
40.4
|
52.0
|
|
|
39.1
|
42.1
|
51.5
|
|
31.25
|
|
|
35.7
|
38.7
|
48.4
|
|
|
37.6
|
40.6
|
50.0
|
|
62.5
|
|
|
30.6
|
33.6
|
43.4
|
|
|
32.7
|
35.7
|
45.1
|
|
100.0
|
|
|
27.1
|
30.1
|
39.8
|
|
|
29.3
|
32.3
|
41.8
|
|
200
|
|
|
|
|
34.8
|
|
|
|
|
36.9
|
|
250
|
|
|
|
|
33.1
|
|
|
|
|
35.3
|
Table 3: NEXT Test Limits at Specific Frequencies
(3) DC Resistance
DC loop resistance consumes part of the signal and converts it into heat. It refers to the sum of the resistance of a pair of wires, which per ISO/IEC 11801 specifications must not exceed 19.2Ω. The difference between pairs should not be too large (less than 0.1Ω), or it indicates poor contact and the connection points must be checked.
(4) Characteristic Impedance
Different from loop DC resistance, characteristic impedance includes resistance as well as inductive and capacitive reactances at frequencies from 1 to 100MHz. It is related to the distance between pairs of wires and the electrical properties of insulation. Various cables have different characteristic impedances. For twisted-pair cables, there are typically 100Ω, 120Ω, and 150Ω types (120Ω cables are neither used nor produced domestically).
(5) Attenuation to Crosstalk Ratio (ACR)
In certain frequency ranges, the ratio of crosstalk to attenuation is another important parameter that reflects cable performance. ACR is sometimes expressed as a Signal-to-Noise Ratio (SNR), calculated by the difference between the worst-case attenuation and NEXT values. A larger ACR value indicates a stronger ability to resist interference, and the system requires a minimum of more than 10dB.
(6) Cable Characteristics
The quality of a communication channel is described by its cable characteristics (signal-noice ratio, SNR). SNR is a measure of data signal strength in consideration of interfering signals. Low SNR can lead to the inability of the receiver to distinguish between data and noise signals upon receipt, ultimately causing data errors. Therefore, to limit data errors within a certain range, a minimum acceptable SNR must be defined.
3. Twisted Pair Transmission Speeds
The Electronic Industries Alliance (EIA) has defined different quality types of twisted-pair cables.
Computer network integrated cabling uses Category 3, 4, 5, 5e (5E), and 6 twisted pairs, which are defined as:
Category 3: Specifies the cable currently designated in ANSI and EIA/TIA 568 standards. This cable's maximum transmission characteristic specification is up to 16MHz, used for voice transmission and data transmission with a maximum rate of 10Mbps.
Category 4: This type of cable's maximum transmission characteristics specification is up to 20MHz, used for voice transmission and data transmission with a maximum rate of 16Mbps.
Category 5: This type of cable has an increased wrapping density, and the sheath is made of high-quality insulation material, with maximum transmission characteristics up to 100MHz, used for voice transmission and data transmission with a maximum rate of 100Mbps.
Category 5e: This type, based on Category 5 twisted pairs, has added extra parameters (ps NEXT, ps ACR) and some performance improvements, but the transmission rate is still l00Mbps.
Category 6: Physically different from Category 5e, with the pairs separated from each other, this type has a transmission rate of 250Mbps, and its standard was passed on June 5, 2002.
4. Twisted Pair's twist pitch
Within a twisted-pair cable, different pairs have different twist pitch. Generally, the twist pitch cycle of four pairs of twisted wires is within 38.1mm, twisted in a counterclockwise direction, with a twist pitch of one pair being within 12.7mm.
5. Twisted Pair Cable Conductor Core
The American Wire Gauge (AWG) is a standard for measuring the diameter of copper wires and DC resistance. The gauge number ranges from 0000 to 28, and their diameter, DC resistance, and weight relationships are shown in Table 4.
|
Wire Gauge (AWG)
|
direct current (DC) of Cable
|
DC Resistance (Ω/km)
|
Weight (kg/km)
|
|
|
28
|
0.320
|
0.0126
|
214
|
0.716
|
|
27
|
0.361
|
0.0142
|
169
|
0.908
|
|
26
|
0.404
|
0.0159
|
135
|
1.14
|
|
25
|
0.455
|
0.0179
|
106
|
1.44
|
|
24
|
0.511
|
0.0201
|
84.2
|
1.82
|
|
23
|
0.574
|
0.0226
|
66.6
|
2.32
|
|
22
|
0.643
|
0.0253
|
53.2
|
2.89
|
|
21
|
0.724
|
0.0285
|
41.9
|
3.66
|
|
20
|
0.813
|
0.0320
|
33.3
|
4.61
|
|
19
|
0.912
|
0.0359
|
26.4
|
5.80
|
|
18
|
1.020
|
0.0403
|
21.0
|
732
|
|
17
|
1.144
|
0.045
|
16.3
|
9.24
|
|
16
|
1.296
|
0.051
|
13.4
|
11.65
|
|
15
|
1.449
|
0.057
|
10.4
|
14.69
|
|
14
|
1.627
|
0.064
|
8.1
|
18.09
|
|
13
|
1.830
|
0.072
|
6.5
|
23.39
|
|
12
|
2.059
|
0.081
|
5.2
|
29.50
|
|
11
|
2.313
|
0.091
|
4.2
|
37.10
|
|
10
|
2.593
|
0.102
|
3.3
|
46.79
|
|
9
|
2.898
|
0.114
|
2.6
|
59
|
|
8
|
3.254
|
0.128
|
2.0
|
74.5
|
|
7
|
3.660
|
0.144
|
1.6
|
93.87
|
|
6
|
4.118
|
0.162
|
1.3
|
118.46
|
|
5
|
4.626
|
0.182
|
1.0
|
49.00
|
|
4
|
5.186
|
0.204
|
0.8
|
187.74
|
|
3
|
5.821
|
0.229
|
0.7
|
236.91
|
|
2
|
6.558
|
0.258
|
0.5
|
299.49
|
|
1
|
7.346
|
0.289
|
0.4
|
376.97
|
|
0
|
8.261
|
0.325
|
0.3
|
475.31
|
|
00
|
9.278
|
0.365
|
0.26
|
600.47
|
|
000
|
10.422
|
0.410
|
0.2
|
756.92
|
|
0000
|
11.693
|
0.460
|
0.16
|
955.09
|
6. Twisted Pair Cable Testing Data
100Ω 4-pair unshielded twisted pair (UTP) cables are categorized into Cat 3, Cat 4, Cat 5, and Cat 6. They are bound by the following parameters: attenuation, distributed capacitance, DC resistance, DC resistance unbalance, characteristic impedance, return loss, and near-end crosstalk (NEXT). Their standard testing data are illustrated in Tables 5 and 6.
|
Category
|
Attenuation (dB)
|
Distributed Capacitance (at 1kHz)
|
DC Resistance Correction Value at 20°C
|
DC Resistance Deviation Correction Value at 20°C
|
|---|---|---|---|---|
|
Cat 3
|
W 2.320√(f) + 0.238(f)
|
W 33Opf/100m
|
W 9.38Ω/100m
|
5%
|
|
Cat 4
|
W 2.050√(f) + 0.1(f)
|
W 33Opf/100m
|
Same as above
|
5%
|
|
Cat 5
|
W 1.9267√(f) + 0.75(f)
|
W 33Opf/100m
|
Same as above
|
5%
|
Table 5 Standard Testing Data for Twisted Pair Cables
|
Category
|
Impedance Characteristics from 1MHz to Highest Reference Frequency
|
Return Loss for Lengths >100m
|
Near-End Crosstalk Attenuation for Lengths >100m
|
|---|---|---|---|
|
Cat 3
|
100Ω ±15%
|
12dB
|
43dB
|
|
Cat 4
|
Same as above
|
12dB
|
58dB
|
|
Cat 5
|
Same as above
|
23dB
|
64dB
|
Table 6 Standard Testing Data for Twisted Pair Cables
7. Types of Twisted Pair Cables in Low-Voltage Systems
In low-voltage systems, twisted pair cables are divided into two main categories: shielded twisted pair (STP) and unshielded twisted pair (UTP). Within these categories, they further divide into 100 Ω cables, twinaxial cables, large pair count cables, and 150 Ω shielded cables. Several specific models exist, as shown in Figure 3.
Figure 3 Types of Twisted Pair Cables in Low-Voltage Systems
8. The Printing Text on the Exterior of a Twisted Pair Cable
When examining a twisted pair cable, it is important to note that there is text every two feet. Taking a cable from our company as an example, this text reads:
XXXX SYSTEMS CABLE E138034 0100
24 AWG (UL) CMR/MPR OR C (UL) PCC
FT4 VERIFIED ETL CAT5 044766 FT 9907
Where:
XXXX: Represents the name of the company.
0100: Indicates 100 Ω.
24: Indicates the wire gauge is 24 (wire gauges come in 22, 24, 26).
AWG: Stands for American Wire Gauge, a standard wire gauge system in the United States.
UL: Indicates certification and is a certification mark.
FT4: Indicates 4 pairs.
CAT5: Indicates Category 5 cable.
044766: Indicates the current number of feet of cable.
9907: Represents the year and month of production.
9. Fire Resistance Levels of Cables
The insulation materials in communication cables contain chemicals used as fire retardants. Cables based on PVC (Plenum, Commercial, General, and Residential grade) all use halogenated chemicals to retard fire. When PVC burns, it emits halogenated gases (e.g., chlorine), which quickly absorb oxygen, thereby extinguishing the fire and causing the cable to self-extinguish. However, at high concentrations, chlorine gas is highly toxic. Additionally, the combination of oxygen with water vapor generates hydrochloric acid, which is also very harmful to humans.
Cable fire resistance levels are classified into plenum, commercial, general, and residential grades.
(1) Plenum Grade
Plenum grade cables have the highest level of fire resistance. When a fan is used to blow air towards the flame on a bundle of cables, the cables will self-extinguish within 5 meters of flame spread. Plenum grade cables use polytetrafluoroethylene as the insulation material, which emits very low levels of smoke when burning or under extreme heat, and the cables do not release toxic smoke or steam.
(2) Commercial Grade
Commercial grade cables have requirements lower than plenum grade, where bundled cables must self-extinguish within 5 meters of flame spread, but without the stringent requirement for fan-forced air. Like plenum grade, commercial grade cables do not have smoke or toxicity standards. These fire resistance level cables are typically used for horizontal runs.
(3) General Grade
General grade cables are similar to commercial grade.
(4) Residential Grade
Residential grade cables have the lowest level of fire resistance in communication cabling, with no standards for smoke or toxicity. These cables are only used for laying individual cables in homes or small office systems.
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