Fault and diagnosis using cat6 cabling

Symptom

A wiring engineer and system integrator use cat6 lines to deploy a new telecom building in a city. Although there are not many points, there are only 1800 points, but during the acceptance test, the pass rate is only 80%, and the remaining 20% nearly 360 links are unqualified. If all the 360 links are reworked, the computation will be a huge loss. Therefore, the company decided to first conduct product quality spot checks on the remaining six categories of lines and modules to determine whether the problem was caused by the product, and then arrange how to replace or repair these unqualified links.

The test results are as follows: for the 10 products, each of which is 90 meters out, the on-site authentication test is performed after the Basic Link of the 6 categories of products is connected, as a result, 7-volume products were unqualified. Because the engineer is also the product agent of the manufacturer, the sales representative of the manufacturer cannot explain the test results. Then we conducted the second spot check. As a result, there were still 6 unqualified 90-meter simulation links for the 10-volume products. We asked "Network Hospital" to help "catch ghosts ".

Diagnosis Process

After arriving at the site, some unqualified links were obtained, and a total of 20 links were obtained. All the results were unqualified. Open the parameters saved in the DSP4000 of the cable tester and check that the main unqualified parameters include the return loss "RL" and the overall attenuation string bypass ratio "PSACR", which accounts for about 80% of the total, the second is the near-end crosstalk "NEXT", the comprehensive equivalent remote crosstalk "PSELFEXT", and the comprehensive near-end crosstalk "PSNEXT. Re-check the links previously tested by the engineering engineer. The results are basically consistent with the above results, and the results are still unqualified.

In order to confirm whether the cable products, connectors, connection modules, and other problems of the manufacturer are involved, we recommend that cabling engineers compare the products supplied by another cable manufacturer they represent with the cables used in this project.

One hour later, the engineering engineer suggested to create a standard 90-meter basic link for 20 pairs of cable products provided by the other two cable manufacturers, each with 10 links. The test results of these links are as follows: the link pass rate is 80% for product A and 70% for product B, and 20% of the qualified parameters are close to the edge of the test standard, the main parameters such as "RL" and "NEXT" are generally only 0.5 ~ About 1.3 surplus. The instrument adds a "*" prompt in the upper-right corner of the qualified indicator, indicating that the parameter is very close to the edge of the unqualified.

From this point of view, the other two cable manufacturers provide similar product qualification rates. In addition, the failure of the manufacturer's products, a total of three products have a low pass rate. Does this mean that all the products provided by the three cable manufacturers are faulty? Based on logical analysis, there can be only one of the following possible reasons: first, there is indeed a problem with the product quality, but in this case, why is the ratio so consistent? The possibility does not seem very high; the second reason is that there is a problem with the testing instrument or the testing environment, such as the deviation or damage of the instrument, and the testing environment has a large amount of electromagnetic interference sources or interference signals. The construction site is 400 metres apart from the test site, and the electromagnetic environment is similar. There are no other neighbors using special electromagnetic equipment, large power equipment, and high-power radiation sources, this does not seem to happen either. The third reason is that there are problems with the construction methods, construction tools, construction techniques, and on-site testing methods, however, the construction engineers are all employees with at least one year of construction experience. And the cabling staff have been working for the company for two and a half years, so there should be no technical problems. There is no problem in the visual inspection of the cabling tool, and the cabling tool in the project construction is not the same tool when the test link was just created.

For the time being, we assume that there is no problem with the product. We will use another DSP4000 cable tester carried by ourselves and a cable Tester of the same model provided by the engineering engineer for comparative testing. The test results are quite consistent, indicating that the tester is normal. To locate the fault location, use the "HDTDX" high-precision Time-Domain Crosstalk Analysis Function and the "HDTDR" high-precision time-domain reflection Analysis Function in the DSP4000 cable tester to analyze the fault map, the result shows that the "prominent position" of the unqualified parameter is in the position of the connection plug-in and connection module. This indicates that either the connection plug-in and connection module have quality problems or there is a problem with the construction process. Next, replace the connection plug-in and connection module in the unqualified link before testing. As a result, all 10 links of the three products are qualified, and only one of the two products is unqualified. Repeat the unqualified links for the third test again, and all the results pass the test. Then, we carefully tested the link with the 20% parameter close to the edge, and all the results were retested !!

This shows that the installation and construction process of connectors, connection modules, and cables is an important reason for the failure of the link authentication test and a hiding place of the "Little Devil.

In the next step, in order to verify whether it is possible for electromagnetic interference and other reasons, return to the project site and select 20 links that were originally unqualified for testing, as shown in the following code ", re-create the plug-in and connection module to "confirm the authenticity". All the four cables except for the four cables are qualified. The faulty cable is diagnosed by the instrument. The result shows that the return loss "RL" and the crosstalk "NEXT" are not uniform at the end of the cable. The cable is removed to observe the obvious twisting and scratches, the metal tubes are exported vertically. All cables are qualified after replacement.

Diagnostic comments

The construction process of Integrated Wiring seems simple, but the requirement is not low. During the construction of three types of cables, a large number of wiring suppliers use temporary construction personnel. After two hours of training, they will work on their posts. The engineering acceptance rate is still relatively high. In the construction process of cat5e and cat5e, process problems began to appear and were reflected to the final test results, which gradually attracted the attention of the project engineers, however, it is generally insufficient to cause serious consequences for the failure of such a large area of up to 20% of links. In the construction process of cat6 lines, the requirements for the construction process are placed in a very important position, A slight carelessness in cabling, cabling, and module installation will result in the failure of the on-site verification test of the entire link, which is unexpected at the beginning of product promotion by engineering vendors and manufacturers. In fact, the method for diagnosing specific fault locations is very simple, using the high-precision Time-Domain Crosstalk Analysis Technology "HDTDX" and high-precision time-domain reflection Analysis Technology "HDTDR" for fault diagnosis, you can easily display the actual location of the fault. The construction personnel can take immediate repair measures accordingly, such as replacing the cable, module or re-processing according to the prompts of the instrument. Generally, satisfactory results can be obtained, instead of waiting for the on-site Authentication Test and acceptance.

Diagnostic recommendations

Re-create the unqualified 360 links strictly, and take the same improvement measures for the 360 links whose parameters are close to the edge of 2 dB to ensure the engineering quality. Re-build unqualified links such as modules and connectors to replace the cables. In addition, strict training of the construction team and strict emphasis on the construction process must also be taken seriously.

Postscript

One week later, the engineer informed that after the "back-to-furnace", all 1800 links were accepted. In addition to re-processing the unqualified 360 links, 360 links with edge values were re-created, nearly 40 links were replaced, and a comprehensive authentication test was re-conducted, as a result, the surplus amount of parameters is greater than 3 dB. They intend to use this project as a model demonstration project for the company's business promotion.