When a computer is connected to a computer network, it first encounters communication line and channel transmission problems. At present, computer communication is divided into two types: wired communication and wireless communication. Wired communications use cables, optical cables, or telephone lines to act as transmission conductors, while wireless communications use satellites, microwaves, and infrared rays as transmission conductors.
The choice of network communication lines must consider the performance of the network, the price, the rules of use, the ease of installation, scalability, and other factors.
The cables used in network wiring systems are usually divided into twisted pair, coaxial cable, large logarithmic wire, optical cable, etc. There are many varieties and models of cables supplied on the market, and engineering and technical personnel should be purchased according to the actual engineering needs, mainly considering their role, model, variety and main performance.
At present, there are products from foreign companies in the cable market, and there are also products from domestic companies. In terms of copper cable quality and performance, domestic products have caught up with or exceeded the products of foreign companies, typical of the cables produced by Shanghai Tiancheng Group, whose performance has exceeded the requirements of wiring standards and similar products of foreign companies.
Twisted pair (twistedpair, TP) is one of the most commonly used transmission media in integrated wiring engineering. Twisted pair is made up of two copper conductors with an insulating protective layer. The two insulated copper wires are twisted together at a certain density to reduce the degree of signal interference, and the radio waves radiated by each wire in transmission will be canceled out by the radio waves emitted by the other wire. Twisted pair wires are generally made up of two insulated copper conductors of No. 22, No. 24 or No. 26 intertwined with each other. If one or more pairs of twisted pairs are placed in an insulated sleeve, it becomes a twisted pair cable, and within the twisted pair cable (also known as the twisted wire cable), different pairs of wires have different twisting lengths, usually twisted lengths of 38.1 to 140mm, twisted in a counterclockwise direction. The length of the torsion strand of the adjacent line pair is more than 12.7mm, the denser the general torsion line, the stronger its anti-interference ability, compared with other transmission media, twisted pair in the transmission distance, channel width and data transmission speed are limited, but the price is relatively low.
At present, twisted pair can be divided into unshielded twisted pair (UTP) and shielded twisted pair (STP), the outer layer of shielded twisted pair cable is wrapped in aluminum foil, the price is relatively higher.
Although twisted pair is mainly used to transmit analog sound information, it is also suitable for the transmission of digital signals, especially for the transmission of information over shorter distances. During transmission, the attenuation of the signal is relatively large and the waveform is distorted.
The bandwidth of a local area network using twisted pair depends on the quality of the wires used, the length of the wires, and the transmission technology.
Because twisted pairs radiate around when transmitting information, it is easy to be eavesdropped, so it is shielded at an additional cost to reduce radiation (but not completely eliminate). This is what we often call shielded twisted pair cable. Shielded twisted pairs are relatively expensive and more difficult to install than unshielded twisted pair cables.
Twisted pair cables have the following advantages:
1) Small diameter, space-saving;
2) Light weight, easy to bend, easy to install;
3) Minimize or eliminate crosstalk;
4) Flame retardant;
5) Independence flexibility for structured integrated cabling.
1. Classification of twisted pairs
1) Category 1 line: Used for telephone voice communication, not for computer network data communication.
2) Category 2 line: Transmission frequency is 1MHz, used for voice transmission and data transmission with a maximum transmission rate of 4Mbps, commonly used in older token networks using the 4Mbps specification token delivery protocol.
3) Category 3 line: used for voice transmission and data transmission with a maximum transmission rate of 16Mbps, mainly used for 10BASE-To
4) Category 4 cable: This type of cable has a transmission frequency of 20MHz for voice transmission and data transmission with a maximum transmission rate of 20Mbps, mainly used for token-based LAN and 10BASE-T/100BASE-T.
5) Category 5 wire: This type of cable increases the winding density, coats a high-quality insulating material with a transmission rate of 100MHz for voice transmission and data transmission with a maximum transmission rate of 100Mbps, mainly used for 100BASE-T and 10BASE-T networks. This is the most commonly used Ethernet cable.
6) Cat 5 ultra-cable: The line cable has a reduced attenuation, less crosstalk, and has a higher attenuation to crosstalk ratio (ACR) and signal-to-noise ratio (Structural Return Loss), a smaller time delay difference, and greatly improved performance. Cat 5 cables are mainly used for Gigabit Ethernet (1000Mbps).
7) Category 6 cable: The transmission frequency of this type of cable is 1~ 250MHz, and the Comprehensive Attenuation CrossTalk Ratio (PS-ACR) at 200MHz should have a larger margin, which provides 2 times the bandwidth of Super Class 5. The transmission performance of Cat 6 cabling is much higher than that of the Cat 5 standard, and it is most suitable for applications with transmission rates higher than IGbps. An important difference between Category 6 and Category 5 is that it improves performance in terms of crosstalk and return loss, and excellent return loss performance is extremely important for the new generation of full-duplex high-speed network applications. The basic link model is eliminated in the 6 types of standards, and the wiring standard adopts a star topology, and the required routing distance is: the length of the permanent link cannot exceed 90m, and the channel length cannot exceed 100m. Category 6 lines are divided into 6E and 6EA. The 6E transmission frequency is 200MHz, and the 6EA transmission frequency is 250MHz.
8) Category 7 line: The line class cable is mainly to adapt to the application and development of 10 Gigabit Ethernet technology, but it is no longer an unshielded twisted pair, but a shielded twisted pair, so its transmission frequency can reach at least 600MHz, which is more than 2 times that of the Cat 6 line and the super 6 class line. Category 7 lines are divided into 7F and 7FA. The 7F transmission frequency is 600MHz and the 7FA transmission frequency is 620MHz.
9) Class 8 lines: The international standard has basically identified class 8 wiring, and class 8 lines are divided into 8.1 and 8.2, 8.1 to be compatible with class 6, and 8.2 to be compatible with class 7.
The types of 4 pairs of twisted pairs used in computer network integrated wiring are shown in Figure 1.
Figure 1 Types of twisted pairs used in computer network engineering
The physical structure of The Class 3, Class 5, and Cat 5 Lines 4 pairs of unshielded twisted pairs is shown in Figure 2.
Fig. 2 Physical structure of Class 3, Class 5, and Cat 5 superline 4 pairs of unshielded twisted pairs
The color composition of the 4 pairs of twisted pair conductors is shown in Table 1.
Table 1 4 pairs of twisted pair conductor color composition
| Line pairs | Color coding |
| 1 | White/Blue//Blue |
| 2 | White /orange//orange |
| 3 | White/Green//Green |
| 4 | White/Brown//Brown |
2. Parametric nouns for twisted pairs
For twisted pairs (whether Class 3, Class 5, Class 6, Class 7, Class 8, or shielded or unshielded), users are concerned about parameters such as attenuation, near-end crosstalk, DC resistance, characteristic impedance, distributed capacitance, and so on.
(1) Attenuation
Attenuation is a measure of signal loss along a link. Attenuation varies with frequency, so attenuation should be measured over all frequencies over the application range.
(2) Proximal crosstalk
Near-end crosstalk loss is a measure of signal coupling from one pair of lines to another in a UTP link. This is a key performance metric for UTP links and one of the most difficult to measure precisely, especially as the frequency of the signal increases.
Crosstalk is divided into near-end crosstalk (NEXT) and far-end crosstalk (FEXT), the tester mainly measures NEXT, due to line loss, fext's magnitude has less impact. Ignored in Category 3 and Category 5 systems.
NEXT does not represent the crosstalk value generated at the near-end point, it only represents the crosstalk value measured at the near-end point. This value varies with the length of the cable, and the longer the cable, the smaller it becomes. At the same time, the signal at the sender side will also be attenuated, and the crosstalk on other line pairs will be relatively small. Experiments have proved that the NEXT value measured only within 40m is more realistic, if the other end is an information socket farther than 40m, it will produce a certain degree of crosstalk, but the tester may not be able to measure this crosstalk value. For this reason, it is best to measure NEXT at both endpoints. Today's testers are equipped with equipment that allows the NEXT value at both ends of the link to be measured.
Reference tables for attenuation and NEXT test values are shown in Tables 2 and 3.
Table 2 Attenuation limits at each frequency when various connections are the maximum length
| Frequency (MHz) | Maximum attenuation 20°C | |||||||||
| Channel (0.10m) | Link (90m) | |||||||||
| 3 categories | 4 categories | Category 5 | 5E | 6 categories | ||||||
| 4.2 | 2.6 | 2.5 | 2.1 | 3.2 | 2.2 | 1.9 | ||||
| 7.3 | 4.8 | 4.5 | 6.1 | 4.3 | 3.5 | |||||
| 8 | 10.2 | 6.7 | 63 | 6.3 | 5.7 | 8.8 | 6.0 | 5.0 | ||
| 10 | 11.5 | 7.5 | 7.0 | 6.8 | 5.6 | |||||
| 16 | 14.9 | 9.9 | 9.2 | 13.2 | 8.2 | 7.1 | ||||
| 20 | 11.0 | 10.3 | 9.0 | 7.9 | ||||||
| 25 | 11.4 | 10.1 | 8.9 | |||||||
| 31.25 | 12.8 | |||||||||
| 62.5 | 18.5 | 16.5 | 16.7 | 14.4 | ||||||
| 100 | 24.0 | 21.3 | 21.6 | |||||||
| 200 | 31.5 | 27.1 | ||||||||
| 250 | 36.0 | 30.7 |
Table 3 NEXT test limits at specific frequencies
| Frequency (MHz) | Min. NEXT/20°C | |||||||||
| 39.1 | 53.3 | 60.0 | 65.0 | 40.1 | 54.7 | |||||
| 29.3 | 43.3 | 50.6 | 53.6 | 45.1 | 51.8 | 54.8 | 64.1 | |||
| 24.3 | 38.2 | 45.6 | 48.6 | 58.2 | 25.9 | 40.2 | 47.1 | 50.0 | 59.4 | |
| 22.7 | 36.6 | 44.0 | 47.0 | 56.6 | 38.6 | 45.5 | 48.5 | 57.8 | ||
| 19.3 | 33.1 | 40.6 | 43.6 | 53.2 | 21.0 | 35.3 | 42.3 | 45.2 | 54.6 | |
| 31.4 | 39.0 | 42.0 | 51.6 | 33.7 | 40.7 | 43.7 | 53.1 | |||
| 37.4 | 40.4 | 52.0 | 42.1 | 51.5 | ||||||
| 35.7 | 38.7 | 48.4 | 37.6 | |||||||
| 30.6 | 33.6 | 43.4 | 32.7 | |||||||
| 30.1 | 39.8 | 32.3 | 41.8 | |||||||
| 34.8 | 36.9 | |||||||||
(3) DC resistance
A DC loop resistance consumes a portion of the signal and converts it into heat, which refers to the sum of a pair of wire resistors, ISO/IEC 11801 specifications must not be greater than 19.2Ω. The difference between each pair must not be too large (less than 0.1Ω), otherwise it indicates poor contact and the connection point must be checked.
(4) Characteristic impedance
Unlike loop DC resistance, the characteristic impedance includes resistance and inductive and capacitive reactances with frequencies from 1 to 100MHz, which are related to the distance between a pair of wires and the electrical properties of the insulation. Various cables have different characteristic impedances, for twisted pair cables, there are 100Ω, 120Ω and 150Ω (domestic does not use or produce 120Ω cables).
(5) Attenuation crosstalk ratio (ACR)
In some frequency ranges, the ratio of crosstalk to attenuation is another important parameter that reflects the performance of a cable. ACR is also sometimes expressed as a signal-to-noise ratio (SNR), which is calculated from the difference between the worst amount of attenuation and the value of the NEXT amount. A larger ACR value indicates greater ability to combat interference, and the system requires at least greater than 10dB.
(6) Cable characteristics
The quality of a communication channel is described by its signal-noice ratio (SNR). SNR is a measure of the strength of a data signal that takes into account the interference signal. If the SNR is too low, it will cause the receiver to be unable to distinguish between the data signal and the noise signal when the data signal is received, eventually causing a data error. Therefore, in order for data errors to be limited to a certain range, a minimum receivable SNR must be defined.
3. The transmission rate of the twisted pair
The International Electrical Industry Association (EIA) defines different quality models for twisted pair cables.
Computer network integrated wiring uses five types of twisted pairs, namely Class 3, Class 4, Class 5, Class 5 (5E), and Class 6, which are defined as:
1) Category 3: refers to the cable currently specified in the ANSI and EIA/TIA 568 standards. The cable has transmission characteristics up to 16MHz for voice transmission and data transmission with a maximum transmission rate of 10Mbps.
2) Class 4: The transmission characteristics of this type of cable are up to 20MHz, which is used for voice transmission and data transmission with a maximum transmission rate of 16Mbps.
3) Category 5: This type of cable increases the winding density, the jacket is a high-quality insulation material, the transmission characteristics of the highest specification of 100MHz, for voice transmission and data transmission with a maximum transmission rate of 100Mbps.
4) Super Class 5: On the basis of Class 5 twisted pair, additional parameters (ps NEXT, ps ACR) and some performance improvements have been added, but the transmission rate is still lOOMbpSo
5) Class 6: Physically different from Super Class 5, line pairs are separated from line pairs, the transmission rate is 250Mbps, and its standard was adopted on June 5, 2002.
4. The twisting distance of the twisted pair
In the twisted pair cable, different pairs have different twist lengths, generally 4 pairs of twisted pairs of twisted length within 38.1mm length, twisted in a counterclockwise direction, a pair of pairs of twisted strands within 12.7mm.
5. The core of the twisted pair
The American Wire Gauge (AWG) is the standard used to measure copper wire diameter and DC resistance. The wire gauge number is from 0000-28, and the correlation between its diameter, DC resistance, and weight is shown in Table 4.
Table 4 American cable wire gauges
| Wire gauge number | Cable DC | DC resistance (Q/km) | Weight (kg/km) | |
| mm | in | |||
| 28 | 0.320 | 0.0126 | 214 | 0.716 |
| 27 | 0.361 | 0.0142 | 169 | 0.908 |
| 26 | 0.404 | 0.0159 | 135 | 1.14 |
| 0.455 | 0.0179 | 106 | 1.44 | |
| 0.511 | 0.0201 | 84.2 | 1.82 | |
| 23 | 0.574 | 0.0226 | 66.6 | 2.32 |
| 22 | 0.643 | 0.0253 | 2.89 | |
| 0.724 | 0.0285 | 41.9 | 3.66 | |
| 0.813 | 0.0320 | 33.3 | 4.61 | |
| 19 | 0.912 | 0.0359 | 26.4 | 5.80 |
| 18 | 1.020 | 0.0403 | 732 | |
| 17 | 1.144 | 0.045 | 16.3 | 9.24 |
| 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 |
| 2.313 | 0.091 | 37.10 | ||
| 2.593 | 0.102 | 3.3 | 46.79 | |
| 2.898 | 0.114 | 59 | ||
| 3.254 | 0.128 | 74.5 | ||
| 0.144 | 1.6 | 93.87 | ||
| 4.118 | 0.162 | 1.3 | 118.46 | |
| 4.626 | 0.182 | 49.00 | ||
| 5.186 | 0.204 | 0.8 | 187.74 | |
| 5.821 | 0.229 | 0.7 | 236.91 | |
| 6.558 | 0.258 | 0.5 | 299.49 | |
| 7.346 | 0.289 | 0.4 | 376.97 | |
| 8.261 | 0.325 | 0.3 | 475.31 | |
| 00 | 9.278 | 0.365 | 0.26 | 600.47 |
| 10.422 | 0.410 | 0.2 | 756.92 | |
| 11.693 | 0.460 | 0.16 | 955.09 |
6. Test data of twisted pair cable
The 100Ω 4 pair of unshielded twisted pairs is divided into Cat 3, Cat 4, Cat 5 and Cat 6. They are constrained by the following specifications: attenuation, distributed capacitance, DC resistance, DC resistance deviation value, characteristic impedance, return loss, near-end crosstalk. Their standard test data are shown in Tables 5 and 6.
Table 5 Standard test data for twisted pair cables
| type | Attenuation (in dB) | Distribute capacity (calculated at 1kHz) | DC resistance 201D Measurement Correction Value | DC resistance deviation value 20P measurement correction value |
| W 2.320sqrt (f) + 0.238 (f) | At 33Opf71OOm | At 9.38Q/100m | 5% | |
| W 2.050sqrt (f) +0.1 (f) | ditto | |||
| W 1.9267sqrt (f) +0.75 (f) |
Table 6 Standard test data for twisted pair cables
| Impedance characteristics 1MHz to the highest reference frequency value | Return loss Measuring length 〉100 meters | Proximal crosstalk Deletion length〉100 meters | |
| 1000±15% | 12dB | 43dB | |
| 58dB | |||
| 23dB | 64dB |
7. Twisted pair varieties in weak current systems
The varieties of twisted pair in weak current systems are divided into two categories: shielded twisted pair and unshielded twisted pair. In these two categories, it is divided into: 100Ω cable, two-body cable, large logarithmic cable, 150 ohm shielded cable. There are a variety of specific models, as shown in Figure 3.
Fig. 3 Twisted pair varieties in weak current systems
8. Twisted pair text on appearance
For a twisted pair, the appearance needs to be noted: there is a paragraph of text every two feet. For example, its company's cable, the text is:
XXXX SYSTEMS CABLE E138034 0100
24 AWG (UL) CMR/MPR OR C (UL) PCC
FT4 VERIFIED ETL CAT5 044766 FT 9907
thereinto:
□ XXXX: Represents the company name.
□ 0100: Represents 100Ω.
□ 24: Indicates that the core is number 24 (the core has three specifications: 22, 24, and 26).
□ AWG: Indicates the U.S. cable specification standard.
□ UL: Indicates that passing the certification is a certification mark.
□ FT4: Represents 4 pairs of lines.
□ CAT5: Represents a Category 5 line.
□ 044766: Indicates the number of feet the cable is currently in.
□ 9907: Indicates the year and month of production.
9. Cable fire rating
The insulating materials in communication cables contain chemicals that are used as substances to contain fires. PVC-based cables (trunk, commercial, general purpose and household) use halogen chemicals to contain fires. When PVC burns, it emits halogenated gases (such as chlorine, which quickly absorbs oxygen), which extinguishes the fire and causes the cable to go out on its own. However, at high concentrations, chlorine gas is highly toxic. In addition, when oxygen is combined with water vapor, it produces hydrochloric acid, which is also very harmful to people.
The cable fire rating is divided into supercharging grade, trunk grade, commercial grade, general grade and household grade.
(1) Supercharged stage
The booster stage is the highest grade cable, and when a fan is used to force blow into flames on a bundle of cables, the cable will extinguish itself within 5m of flame spread. The pressurized grade cable uses an insulating material of PTFE, and the chemicals used emit very low levels of smoke when burned or at extremely high temperatures, and the cable does not emit toxic fumes or water vapor.
(2) Trunk level
The trunk grade is the second grade cable, under the condition of forced blowing of the fan, the bundle of cables must extinguish itself within 5m of flame spread, but the trunk grade cable has no smoke or toxicity specifications. Cables of this fire rating are often used in building trunks and horizontal cables.
(3) Commercial grade
Commercial grades are less demanding than trunk grades, and bundled cables must extinguish themselves within 5m of flame propagation, but there is no restriction on forced blowing by any fan. As with the trunk grade, commercial grade cables have no smoke or toxicity specifications. Cables with this fire rating are commonly used in horizontal routing.
(4) General-purpose level
The general purpose grade is similar to the commercial grade.
(5) Home grade
The home grade is the lowest fire rating in communication wiring, and this grade of cable also has no smoke or toxicity specifications and is only used in home or small office systems where each cable is laid individually.
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