Introduction of twisted pair specifications and models
Twisted Pair (TP: Twisted Pairwire) is the most commonly used transmission medium in integrated wiring projects.
Twisted pair wire consists of two copper conductors with an insulating protective layer. Twisting two insulated copper wires together at a certain density can reduce the degree of signal interference. The radio waves radiated by each wire during transmission will be offset by the radio waves emitted by the other wire. Twisted pair wire is generally made of two 22-26 gauge insulated copper wires twisted around each other. If one or more pairs of twisted pairs are placed in an insulating sleeve, it becomes a twisted pair cable. In twisted pair cables (also called twisted pair cables), different pairs have different twist lengths. Generally speaking, the twist length is within 38.1cm to 14cm, twisted in a counterclockwise direction, and adjacent pairs The twisted length is above 12.7cm. Compared with other transmission media, twisted pair cables are subject to certain limitations in terms of transmission distance, channel width, and data transmission speed, but they are relatively cheap. Currently, twisted pair cables can be divided into unshielded twisted pair cables (UTP: Unshilded Twisted Pair) and shielded twisted pair cables (STP: Shielded Twisted Pair). Although twisted pair is mainly used to transmit analog sound information, it is also suitable for the transmission of digital signals, especially for shorter distance information transmission. During transmission, the attenuation of the signal is relatively large and waveform distortion occurs. The bandwidth of a local area network using twisted pair depends on the quality, length and transmission technology of the wires used. As long as twisted pairs are carefully selected and installed, reliable transmission rates of several million bits per second can be achieved over limited distances. When the distance is very short and special electronic transmission technology is used, the transmission rate can reach 100Mbps ~ 155Mbps (megabits per second). Since information is radiated to the surroundings when using twisted pairs to transmit information, the information can easily be eavesdropped, so additional costs must be spent to shield it. The outer layer of shielded twisted pair cable is wrapped with aluminum to reduce radiation, but it cannot completely eliminate radiation. Shielded twisted pair cables are relatively expensive and more difficult to install than unshielded twisted pair cables. Similar to coaxial cable, it must be equipped with special connectors that support shielding and corresponding installation techniques. But it has a higher transmission rate, which can reach 155Mbps within 100 meters. In addition, unshielded twisted pair cable has the following advantages:
- No shielding jacket, small diameter, saving occupied space;
- Light weight, easy to bend and easy to install;
- Minimize or eliminate crosstalk;
- Flame retardant;
- It has independence and flexibility and is suitable for structured integrated wiring.
Specifications and models
EIA/TIA defines five different quality models for twisted pair cables. Comprehensive cabling for computer networks uses categories 3, 4, and 5. The five models are as follows:
- Category 1: Mainly used for transmitting voice (Category 1 standards were mainly used for telephone cables before the early 1980s), not for data transmission.
- Category 2: The transmission frequency is 1MHz, used for voice transmission and data transmission with a maximum transmission rate of 4Mbps. It is common in old token networks that use the 4Mbps standardized token passing protocol.
- Category 3: Refers to cables currently specified in the ANSI and EIA/TIA568 standards. The transmission frequency of this cable is 16MHz, which is used for voice transmission and data transmission with a maximum transmission rate of 10Mbps, mainly for 10base-T.
- Category 4: The transmission frequency of this type of cable is 20MHz, used for voice transmission and data transmission with a maximum transmission rate of 16Mbps, mainly used for token-based LAN and 10base-T/100base-T.
- Category 5: This type of cable has increased winding density and is coated with a high-quality insulating material. The transmission frequency is 100MHz. It is used for voice transmission and data transmission with a maximum transmission rate of 100Mbps. It is mainly used for 100base-T and 10base- T network, which is the most commonly used Ethernet cable.
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, DC resistance, return loss, etc.
(1) Attenuation
Attenuation is a measure of signal loss along a link. Attenuation is related to the length of the cable. As the length increases, the signal attenuation also increases. The unit of attenuation is "db", which represents the ratio of the signal strength at the source transmitter to the signal strength at the receiver. Since attenuation varies with frequency, the attenuation should be measured at all frequencies within 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 existence of line loss, the impact of the magnitude of FEXT is small. Near-end crosstalk (NEXT) loss is a measurement of signal coupling from one pair of wires to another in a UTP link. For UTP links, NEXT is a key performance indicator and the most difficult indicator to accurately measure. As the signal frequency increases, its measurement difficulty will increase. 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 cable length, the longer the cable, the smaller the value becomes. At the same time, the signal at the transmitting end will also be attenuated, and the crosstalk to other line pairs will also be relatively smaller. Experiments have proven that only NEXT measured within 40 meters is more realistic. If the other end is an information socket farther 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 better to perform NEXT measurements at both endpoints.
(3)DC resistance
TSB67 does not have this parameter. The DC loop resistance dissipates some of the signal and turns it into heat. It refers to the sum of the resistance of a pair of wires,
The DC resistance of the 11801 specification twisted pair must not be greater than 19.2 ohms. The difference between each pair cannot be too large (less than 0.1 ohm), otherwise it indicates poor contact and the connection point must be checked.
(4)Characteristic impedance
Different from the loop DC resistance, the characteristic impedance includes resistance, inductive impedance and capacitive impedance with a frequency of 1 to 100MHz. It is related to the distance between a pair of wires and the electrical properties of the insulator.
Various cables have different characteristic impedances, and twisted pair cables are available in 100 ohms, 120 ohms and 150 ohms.
(5) Attenuation crosstalk ratio (ACR)
In certain frequency ranges, the proportional relationship between crosstalk and attenuation is another important parameter that reflects cable performance. ACR is sometimes expressed in terms of signal-to-noise ratio (SNR: Signal-Noice ratio).
It is calculated as the difference between the worst attenuation and the NEXT value. The larger the ACR value, the stronger the ability to resist interference. General system requirements are at least greater than 10 decibels.
(6)Cable characteristics
The quality of a communications channel is described by its cable characteristics. SNR is a measure of the strength of the data signal taking into account interfering signals. If the SNR is too low,
This 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 data errors. Therefore, in order to limit data errors within a certain range, a minimum acceptable SNR must be defined.
(7)Return loss
Return loss (RL, referred to as return loss), as the name suggests, refers to a type of loss. In effect, it measures the proportion of the transmitted signal that is reflected back to the transmitter. So, how does return loss occur? We all know that data cables have (or should have) an impedance of 100 ohms when using unshielded twisted pair. But at a given frequency, the impedance value is rarely exactly equal to 100 ohms, as any impedance diagram can prove. The impedance here refers to the input impedance, Zin, rather than the fitted impedance Zo. Input impedance refers to the impedance we describe in the DataTwist 350 and MediaTwist manuals, and fitted impedance refers to the nearly straight-line impedance recognized by the 568-A. The amount of impedance is determined by the spacing between the centers of the two conductors of the twisted pair. While bonded pairs greatly reduce the variation in spacing between the centers of two wires, it's not perfect yet. Even small changes in center spacing can cause changes in impedance. The impedance at one point in the cable may even reach 105 ohms at certain frequencies. This impedance inconsistency causes a portion of the signal to be reflected back to the source at a specific frequency. This is exactly the same as the echo principle.
Test Data
100 ohm 4 pairs of unshielded twisted pairs are divided into Category 3 lines, Category 4 lines, Category 5 lines and Category 5e lines. The main performance indicators are attenuation, distributed capacitance, DC resistance, DC resistance deviation value, impedance characteristics, return loss, and near-end crosstalk.
