Fluke: Detailed description of important parameters for twisted pair wires Testing

The preferred LAN cabling system is the twisted pair cable system. Among them, the mainstream choice is a system with over 5 categories or higher performance.

For cabling systems, the most basic test conducted by the installer is to use the connectivity tester to verify the end-to-end connection of the link. These testers provide complete wiring diagram tests, using TDR technology to measure the length and other additional information. This type of instrument is very helpful for testing voice lines, rapidly checking data links, and the rapidly growing residential LAN cabling market.

Currently, the standard requirements for cabling systems are measured with several parameters to enable certified cabling systems to meet certain transmission performance requirements. Some tests are required worldwide, while others are different. Each standard has its specific pass/failure limit value, which depends on the Link category and the definition of the link model.

All standards perform three tests on the installed link level requirements. The first is wiring diagram testing. The wiring diagram test is used to verify the connectivity between each pin end and end of the cable link and check the winding problem. Any wrong wiring format, such as open circuit, short circuit, cross-connection, reverse connection, and string winding, should be able to be detected.

Attenuation is another important parameter for determining the performance of the LAN cabling system. Any electronic signal may cause energy loss during transmission after it is sent from the signal source, which is no exception for LAN signals. Attenuation increases with the increase of temperature and frequency. High-frequency signals are more severe than low-frequency signals. This is also the reason why the link has the correct wiring diagram and the 10Base-T network runs very well, rather than working properly in the 100Base-T network. For Category 5 cabling systems, the performance of products of various manufacturers in terms of attenuation is very close. Cables with better attenuation performance than those required by Category 5 standards generally increase the core wire diameter or slightly increase the characteristic impedance.

The most important parameter for determining the performance of the network cabling system is the near-end crosstalk NEXT ). Crosstalk is caused by a pair of wire signals generating radiation and sensing other adjacent lines. Crosstalk also changes with frequency. Similar to attenuation, the three types of lines can well support 10 BASE-T applications, but cannot be used in 100Base-T networks.

Keeping the line-to-line close stringing and the balance between the line-to-line pairs can effectively reduce the string bypass. A relatively small twist margin can form the opposite direction of the electromagnetic field to effectively offset the influence of each other, thus reducing the line to the outward radiation. The twisting distance of Category 5 cables is smaller than that of Category 3 cables, and the consistency of the twisting distance is better than that of Category 3 cables. Insulation Materials with better performance are also used, these further suppress crosstalk and reduce attenuation. TIA/EIA-568-B standards require that all UTP connections at the end of the untwisted part cannot exceed 1.3 cm 0.5 inch ).

The standard also requires measurement of the link length. Length Measurement seems simple, but it is actually very knowledgeable. In the standard, the length of the permanent link cannot exceed 90 meters, and the length of the channel cannot exceed 100 meters. The precise measurement length is affected by several factors, including the rated transmission speed of the cable NVP, the length of the stranded wire and the outer leather sheath, and pulse scattering along the length direction.

When the length is measured using an on-site testing instrument, the time delay is usually measured, and the length value is calculated based on the preset signal speed.

Rated transmission speed NVP) represents the signal transmission speed in the cable, expressed as a percentage of the speed of light. Incorrect NVP settings are common errors. If NVP is set to 75% and the actual NVP value of the cable is 65%, the measurement has an error of more than 10% before it starts. In addition, the NVP between each pair of lines may be different and may change with the frequency. For the three and mixed five lines, the maximum NVP value of the line pair may be 12% different.

In addition, the copper core wire in UTP is twisted, so its actual length is longer than the jacket of the outermost layer of the cable. For a 305 m cable, the core line may be 310 m.

Based on the above reasons, the measurement result of the length obtained from the tester can only be a good approximate value rather than a precise value.

Attenuation crosstalk ratio ACR) represents the ratio of valid signals to noise in the link. Simply put, ACR is the ratio of attenuation to NEXT, which measures the ratio between the signal that passes through attenuation at the far end and the crosstalk noise. For example, a lecturer lectures in front of a teacher. The goal of the lecturer is to allow the trainee to hear his speech clearly. The volume of the lecturer is an important factor, but more importantly, the difference between the volume of the lecturer and the background noise. If the lecturer speaks in a quiet library, he can even hear it in whispers. Imagine what would happen if the same lecturer spoke at the same volume in a busy football field. The lecturer will have to increase his volume so that his voice needs to signal) the difference from the crowd's background noise) to be heard. This is ACR.

In fact, the development of application technology still needs to measure new parameters. Return Loss, expressed in dB) form, is a reflection of some signals caused by mismatch of characteristic impedance. Ripple loss is an increasingly important parameter when trying to obtain better UTP system performance. High-performance UTP manufacturers will pay special attention to ensure the consistency of the special impedance of the cable, and all components must have a good matching. Therefore, the ripple loss is not a big problem when five types of systems are just developed, and it is very important in cat5 and cat6 cabling systems.

Overall close-end crosstalk PSNEXT) is actually a calculated value, rather than a direct measurement result. PSNEXT is calculated based on the formula that each line is affected by the NEXT of the other three lines. PSNEXT and FEXT) are very important parameters used to ensure that the performance of the cabling system can support applications with four-wire simultaneous transmission, like Gigabit Ethernet.

Remote crosstalk FEXT) similar to NEXT, it is the crosstalk measured from the local end and measured at the remote end.

FEXT itself is not very valuable. Because FEXT is greatly affected by the length of the cable link, signal strength, including crosstalk, will be affected by the attenuation size after the signal source is sent. That's why the measurement of equivalent remote crosstalk ELFEXT is more meaningful. The attenuation is taken into account in the ELFEXT results, so this parameter is more practical. For the same reason, there is also comprehensive equivalent remote crosstalk PSELFEXT ).

The requirements for testing 6-class cabling systems in three aspects are currently not available in Level II precision instruments:
1. test the accuracy required for all new parameters
2. Test the dynamic range of FEXT and return loss as required by 6 types of systems
3. 250 MHz bandwidth

This causes many installers to replace the current tester with a brand new Tester that supports more than 5/6 testing requirements to keep up with the development of the cabling system.

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