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Twisted pair, not so simple

Contributed by Shanghai Jibu Automation Technology Co., Ltd. 2020/1/8 15:58:41

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  • Keywords: twisted pair definition parameters
  • Abstract: Twisted pair (TP) is the most commonly used transmission medium in integrated wiring engineering. It is composed of two copper wires with an insulation layer. The two insulated copper wires are twisted together at a certain density, and the radio waves radiated by each wire during transmission will be canceled by the radio waves sent from the other wire, effectively reducing the degree of signal interference.

Twisted pair

Twisted pair (TP) is the most commonly used transmission medium in integrated wiring engineering. It is composed of two copper wires with an insulating and protective layer. The two insulated copper wires are twisted together at a certain density, and the radio waves radiated by each wire during transmission will be canceled by the radio waves sent from the other wire, effectively reducing the degree of signal interference.

Twisted-pair wires are generally made of two 22-26 insulated copper wires intertwined with each other. The name of "twisted-pair wires" is also derived from this. In actual use, the twisted pair is wrapped in an insulated cable sleeve by a plurality of pairs of twisted pairs. If one or more pairs of twisted pairs are placed in an insulating sleeve, they become twisted pair cables [1], but in everyday life, the "twisted pair cables" are generally referred to as "twisted pairs."

Compared with other transmission media, twisted pair cables have certain limitations in terms of transmission distance, channel width, and data transmission speed, but the price is relatively low.

principle

Twisted pair is a twisted pair of insulated metal wires. In this way, not only can a part of the electromagnetic interference from the outside be avoided, but also the mutual interference between multiple pairs of twisted pairs can be reduced. The two insulated wires are twisted together, and the interference signal acts on the two twisted wires (this interference signal is called common mode signal). The common mode can be used in the differential circuit that receives the signal. The signal is eliminated to extract a useful signal (differential mode signal).

The function of the twisted pair is to make the noise generated by the external interference on the two wires (in the professional field, useless signals are called noise) the same, so that subsequent differential circuits extract useful signals. The differential circuit is a subtraction circuit. The in-phase signals (common mode signals) at the two input terminals cancel each other (mn), and the inverted signal is equivalent to x-(-y), which is enhanced. In theory, m = n and x = y in twisted pair and differential circuits, which is equivalent to the interference signal being completely eliminated and the useful signal doubled, but there are some differences in actual operation.

In a cable bushing, different pairs have different twist lengths. Generally speaking, the twist length is within 38.1mm ~ 140mm, twisted in the counterclockwise direction, and the twist length of adjacent pairs is 12.7mm. Within. The twisted pair length of a twisted pair is called the pitch. The smaller the pitch (the denser the twisted pair), the stronger the anti-interference ability.

classification

Classified by the presence or absence of shielding

According to the presence or absence of the shield layer, the twisted pair is divided into a shielded twisted pair (STP) and an unshielded twisted pair (UTP).

The shielded twisted pair has a metal shield between the twisted pair and the outer insulation jacket. Shielded twisted pair is divided into STP and FTP (Foil Twisted-Pair). STP means that each line has its own shielding layer, and FTP only works when the entire cable has a shielding device and both ends are properly grounded. Therefore, it is required that the entire system is a shielding device, including cables, information points, crystal heads, and distribution frames. At the same time, the building needs a good grounding system. The shielding layer can reduce radiation, prevent information from being eavesdropped, and prevent external electromagnetic interference from entering, so that shielded twisted pair has a higher transmission rate than similar unshielded twisted pair. However, in actual construction, it is difficult to completely ground all of them, so that the shielding layer itself becomes the largest source of interference, resulting in performance that is far worse than unshielded twisted pair. Therefore, unless there is a special need, usually only unshielded twisted pair is used in the integrated wiring system.

Unshielded twisted pair (Unshielded Twisted Pair, abbreviated UTP) is a data transmission line, consisting of four pairs of transmission lines of different colors, widely used in Ethernet and telephone lines [4]. Unshielded twisted pair cable has the following advantages: 1. Unshielded jacket, small diameter, saves space occupied, low cost; 2. Light weight, easy to bend, easy to install; 3. Minimizes or eliminates crosstalk; 4. Flame retardant; 5. Independence and flexibility, suitable for structured integrated wiring. Therefore, in the integrated wiring system, unshielded twisted pair is widely used.

Classification by frequency and signal-to-noise ratio

Twisted pair cables are commonly classified into three types, five types and super five types, and six types. The former has a smaller diameter and the latter has a larger diameter. The specific models are as follows:

1) Type I line (CAT1): The maximum frequency bandwidth of the cable is 750kHZ, which is used for alarm systems or only for voice transmission (a type of standard is mainly used for telephone cables before the early 1980s), not for data transmission .

2) Type 2 line (CAT2): The maximum frequency bandwidth of the cable is 1MHZ, which is used for voice transmission and data transmission at a maximum transmission rate of 4Mbps. It is common in the old token network using the 4MBPS standard token transfer protocol.

3) Type 3 cable (CAT3): refers to the cable specified in the ANSI and EIA / TIA568 standards. The cable has a transmission frequency of 16MHz and a maximum transmission rate of 10Mbps (10Mbit / s). It is mainly used for voice and 10Mbit / s Ethernet. (10BASE-T) and 4Mbit / s token ring, the maximum network segment length is 100m, and the connector in the form of RJ has faded out of the market.

4) Category 4 line (CAT4): The transmission frequency of this type of cable is 20MHz, which is used for voice transmission and data transmission at a maximum transmission rate of 16Mbps (referring to 16Mbit / s token ring), which is mainly used for token-based local area networks And 10BASE-T / 100BASE-T. The maximum network segment length is 100m, and the connector in the form of RJ is not widely used.

5) Category 5 cable (CAT5): This type of cable has increased winding density. The jacket is a high-quality insulating material. The maximum frequency bandwidth of the cable is 100MHz and the maximum transmission rate is 100Mbps. It is used for voice transmission and the maximum transmission rate is 100Mbps data transmission, mainly used for 100BASE-T and 1000BASE-T networks, the maximum network segment length is 100m, using RJ-type connectors. This is the most commonly used Ethernet cables. In twisted-pair cables, different pairs have different twist lengths. Generally, the twisting period of 4 twisted pairs is within 38.1mm, twisted counterclockwise, and the twisted length of 1 twisted pair is within 12.7mm.

6) Category 5 line (CAT5e): Category 5 has less attenuation, less crosstalk, and has a higher attenuation-to-crosstalk ratio (ACR) and signal-to-noise ratio (SNR), smaller delay errors, and performance. Greatly improved. Category 5 cables are mainly used for Gigabit Ethernet (1000Mbps).

7) Category 6 cable (CAT6): The transmission frequency of this type of cable is 1MHz ~ 250MHz. The Category 6 cabling system should have a large margin at 200MHz when the comprehensive attenuation crosstalk ratio (PS-ACR) is provided. Five types of bandwidth. The transmission performance of Category 6 cabling is much higher than the Category 5 standard, and it is most suitable for applications with transmission rates higher than 1Gbps. An important difference between Category 6 and Category 5 is that it improves the performance in terms of crosstalk and return loss. For new generation full-duplex high-speed network applications, excellent return loss performance is extremely important. The basic link model is cancelled in the six types of standards. The wiring standard adopts a star topology. The required wiring distance is: the length of the permanent link cannot exceed 90m, and the channel length cannot exceed 100m.

8) Super Category 6 or 6A (CAT6A): The transmission bandwidth of these products is between Category 6 and Category 7, the transmission frequency is 500MHz, the transmission speed is 10Gbps, and the standard outer diameter is 6mm. Like the seven types of products, the state has not yet issued a formal test standard, but there are such products in the industry, and each manufacturer announces a test value.

9) Category 7 cable (CAT7): Transmission frequency is 600MHz, transmission speed is 10Gbps, standard outer diameter of single wire is 8mm, standard outer diameter of multi-core wire is 6mm.

The larger the type number, the newer the version, the more advanced the technology, the wider the bandwidth, and of course the more expensive the price. These different types of twisted pair labeling methods are specified as follows. If it is a standard type, it is labeled according to CATx. For commonly used Category 5 and Category 6 cables, the outer sheath of the line is labeled CAT 5, CAT 6. If it is an improved version, it will be marked by xe. For example, the super five categories are marked as 5e (letters are lowercase, not uppercase).

Regardless of the line, the attenuation increases with increasing frequency. When designing the wiring, consider that the attenuated signal should have sufficient amplitude so that it can be detected correctly at the receiving end under the condition of noise interference. The high-speed (Mb / s) data that a twisted pair can transmit also has a great relationship with the encoding method of digital signals.

Sequence standard

In North America, the three most influential integrated wiring organizations in the world are as follows. ANSI (American National Standards Institute, American National Standards Institute) TIA (Telecommunication Industry Association, American Communications Industry Association) EIA (Electronic Industries Alliance, American Electronics Industry Association). Because the TIA and ISO organizations often coordinate in the formulation of standards, the differences between the standards promulgated by the TIA and the ISO are not great. In North America, and even globally, the most widely used twisted pair standards are ANSI / EIA / TIA-568A and ANSI / EIA / TIA-568B (actually ANSI / EIA / TIA-568B.1, referred to as T568B ). The main difference between the two standards is the core sequence:

The line order definitions of EIA / TIA 568A are green-white, green, orange-white, blue, blue-white, orange, brown-white, and brown, and their numbers are shown in the following table:

Green and white

green

Orange and white

blue

Blue and white

Orange

Brown and white

Brown

1

2

3

4

5

6

7

8

The line order definition of EIA / TIA 568B is orange-white, orange, green-white, blue, blue-white, green, brown-white, and brown, and their numbers are shown in the following table:

Orange and white

Orange

Green and white

blue

Blue and white

green

Brown and white

Brown

1

2

3

4

5

6

7

8

According to the 568A and 568B standards, the contacts of the RJ-45 connector (commonly known as the crystal head) are used in the network connection. For the transmission signal, their functions are: 1, 2 for sending, 3, 6 for receiving. , 4, 5, 7, 8 are two-way wires; for the twisted-pair wires connected to it, in order to reduce mutual interference, the standard requires that 1, 2 must be a twisted pair of wires, and 3, 6 must also be twisted. A pair of wires, 4, 5 are tangled with each other, and 7, 8 are tangled with each other. It can be seen that in fact, the two standards 568A and 568B have no essential difference, except that the line order of the eight twisted pairs when connecting RJ-45 is different. In actual network engineering construction, the 568B standard is mostly used.

Production steps

The production method of the most basic straight-through type 5 cable is described below. The production method of other types of network cables is similar, except that the jumper method is different.

Step 1: Use twisted-pair network cable pliers (other cutting tools are also available of course) to cut one end of category 5 twisted-pair cable (preferably cut a length of network cable that meets the requirements of the wiring length), and then insert the trimmed end into In the notch used by the network cable pliers for stripping, make sure that the network cable cannot be bent.

Step 2: Hold the crimping pliers slightly and rotate it slowly (no need to worry about the skin of the core wire inside the network cable, because there is a certain distance between the two blades of the wire stripping, this distance is usually the diameter of the 4 pairs of core wires inside ), Let the blade cut off the protective rubber of the twisted pair, and remove the rubber. Of course, you can also use a special stripping tool to peel off the protective rubber. Note: The stripping length should usually be exactly the length of the crystal head. This can effectively avoid the trouble caused by too long or too short stripping. Stripping too long is not beautiful, on the other hand, because the network cable cannot be caught by the crystal head, it is easy to loosen; Make good contact with the core of the network cable.

Step 3: After peeling off the skin, you can see 4 pairs of 8 core wires of twisted pair network cable, and you can see that the color of each pair is different. Each pair of two core wires is composed of a core wire dyed with a corresponding color plus a white interphase core wire dyed with a little corresponding color. The colors of the four full-color core wires are: brown, orange, green, and blue.

Step 4: Untie each pair of cables that are entangled with each other. After unraveling, arrange several sets of cables in order and straighten them according to the rules. When arranging, you should pay attention to avoid too much winding and overlapping of lines. After arranging the cables in order and straightening them, because the cables are entangled with each other before, the cables will have a certain bend. You should straighten the cables as much as possible and keep the cables flat. The method of pulling the cable straight is also very simple. Hold the cable with both hands and then apply force in two opposite directions, and pull it up and down.

Step 5: After arranging the cables in order and straightening them straight, you should check them carefully, and then use the cutting edge of the crimping pliers to trim the cable parts neatly.

Step 6: Insert the organized cable into the crystal head. It should be noted that the side of the crystal head with the molding spring sheet facing downwards and the side with the pins facing upwards, so that the end with the pins pointing away from itself and the end with the square hole facing itself. At this time, the leftmost foot is the first foot, the rightmost foot is the eighth foot, and the rest are arranged in order. When inserting, pay attention to slowly and forcefully insert the 8 cables along the 8 wire trunks in the 45 head of the IU at the same time, all the way to the top of the wire trunk. Note: When cutting, it should be inserted horizontally, otherwise the different cable lengths will affect the normal contact between the cable and the crystal head. If the protective layer has been peeled off too much before, you can cut the excessively long thin wires here, leaving about 15mm of the outer protective layer removed. This length is just enough to insert the thin wires into their respective troughs. If the section is left too long, the crosstalk will increase because the cables are no longer twisted, and the cable may come out of the crystal head because the crystal head cannot press the sheath, resulting in poor contact or even interruption of the line. . Before the last step of crimping, you can check from the top of the crystal head to see if each group of cables is firmly pressed against the end of the crystal head.

Step 7: Press the line. Before the last step of crimping, you can check from the top of the crystal head at this time to see if each group of cables is firmly pressed at the end of the crystal head. After confirming that it is correct, you can insert the crystal head into the 8P slot of the crimping pliers. After inserting the crystal head, firmly clamp the pliers. If the strength is not enough, you can use both hands to press together. This pressing process makes the crystal head The pins that protrude outside are all pressed into the crystal and the inside of the head. After applying force, you can hear a slight "snap".

Step 8: After the thread is pressed, all the pins protruding from the crystal head are pressed into the crystal, and the plastic buckle on the lower part of the crystal head is also pressed onto the gray protective layer of the network cable. At this point, the crystal head is finished.

Performance

For twisted pair cables, users are most concerned about several indicators that characterize their performance. These indicators include attenuation, near-end crosstalk, impedance characteristics, distributed capacitance, and DC resistance.

1. Attenuation

Attenuation is a measure of signal loss along the link. The attenuation is related to the length of the cable. As the length increases, the signal attenuation also increases. Attenuation uses "db" as the unit, which indicates the ratio of the signal strength at the source transmitting end to the signal strength at the receiving end. Because attenuation varies with frequency, attenuation should be measured over all frequencies in the application range.

2. Near-end crosstalk

Crosstalk is divided into near-end crosstalk and far-end crosstalk (FEXT). The tester mainly measures NEXT. Due to the line loss, the impact of the magnitude of FEXT is small. Near-end crosstalk (NEXT) loss is a measure of signal coupling from one pair to another in an unshielded twisted-pair link. For unshielded twisted-pair links, NEXT is a key performance indicator and one of the most difficult to measure accurately. As the signal frequency increases, its measurement difficulty will increase. NEXT does not indicate the crosstalk value generated at the near-end point, it only indicates the cross-talk value measured at the near-end point. This value varies with the length of the cable. The longer the cable, the smaller it becomes. At the same time, the signal at the transmitting end will be attenuated, and the crosstalk to other wire pairs will be relatively small. Experiments have shown that only NEXT measured within 40 meters is more realistic. If the other end is an information socket longer than 40 meters, it will produce a certain degree of crosstalk, but the tester may not be able to measure this crosstalk value. Therefore, it is best to take NEXT measurements at both endpoints. Most testers are equipped with corresponding equipment, so that the NEXT value at both ends can be measured at one end of the link. The results of the NEXT test refer to the following table:

Attenuation limits at various frequencies when various twisted pairs are connected to the maximum length


Frequency (MHz)

Maximum attenuation (20 ℃)


Channel (100 meters)

Link (90 meters)


Media category

Class 3

Category 4

Category 5

Class 3

Category 4

Category 5


1

4.2

2.6

2.5

3.2

2.2

2.1


4

7.3

4.8

4.5

6.1

4.3

4.0


8

10.2

6.7

6.3

8.8

6

5.7


10

11.5

7.5

7.0

10

6.8

6.3


16

14.9

9.9

9.2

13.2

8.8

8.2


20


11

10.3


9.9

9.2


25



11.4



10.3


31.25



12.8



11.5


62.5



18.5



16.7


100



twenty four



21.6


NEXT attenuation limit at specific frequencies


Frequency (MHz)

Minimum NEXT


Channel (100 meters)

Link (90 meters)


Media category

Class 3

Category 4

Category 5

Class 3

Category 4

Category 5


1

39.1

53.3

60.0

40.1

54.7

60.0


4

29.3

43.3

50.6

30.7

45.1

51.8


8

24.3

38.2

45.6

25.9

40.2

47.1


10

22.7

36.6

44.0

24.3

38.6

45.5


16

19.3

33.1

40.6

twenty one

35.3

42.3


20


31.4

39.0


33.7

40.7


25



37.4



39.1


31.25



35.7



37.6


62.5



30.6



32.7


100



27.1



29.3


The above two indicators are the main content of the TSB67 test, but some types of testers can also give indicators such as DC resistance, characteristic impedance, and attenuation crosstalk ratio.

3.DC resistance

The DC loop resistance consumes a portion of the signal and converts it into heat. It refers to the sum of the resistance of a pair of wires. The DC resistance of the 11801 twisted pair cable must not be greater than 19.2 ohms. The difference between each pair must not be too large (less than 0.1 ohms), otherwise it indicates poor contact and the connection points must be checked.

4. Characteristic impedance

Different from the loop DC resistance, the characteristic impedance includes resistance and inductive impedance and capacitive impedance with a frequency of 1 to 100 MHz. It is related to the distance between a pair of wires and the electrical performance of the insulator. Various cables have different characteristic impedances, and twisted-pair cables are available in 100 ohm, 120 ohm, and 150 ohm.

5. Attenuated crosstalk ratio (ACR)

In some frequency ranges, the ratio of crosstalk to attenuation is another important parameter that reflects cable performance. ACR is also sometimes expressed by the signal-to-noise ratio (SNR: Signal-Noice ratio), which is calculated from the difference between the worst attenuation and the NEXT value. A larger ACR value indicates stronger anti-interference ability. General system requirements are at least greater than 10 dB.

6. Cable characteristics

The quality of a communication channel is described by its cable characteristics. SNR is a measure of the strength of a data signal when considering interfering signals. If the SNR is too low, when the data signal is received, the receiver cannot distinguish between the data signal and the noise signal, and eventually cause a data error. Therefore, in order to limit data errors to a certain range, a minimum acceptable SNR must be defined.

Review editor (Wang Jing)
For more information, please visit Shanghai Jibu Automation Technology Co., Ltd. ( http://c.rgwds.com/?cid=57822 )

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