Efficient troubleshooting

In an ideal world, network installers would simply complete their work and conduct successful certification tests every time. But, more realistically, knowing what to do when the test fails can make the difference between turning a profit on a network installation or suffering a loss because of excess troubleshooting expenses.

Cost-effective troubleshooting is a combination of technical expertise, knowledge of when and where to use specialised tools, and following a logical sequence of looking for the most likely problem first, before pursuing more complex possibilities. This article explores the critical troubleshooting issues that come into play when a link fails the testing process, and provides detail on the requirements of field troubleshooting, diagnostic parameters and real-world usage tips.

Eliminate the obvious failures
Today most cable manufacturers produce high-quality cable with consistent characteristics. The majority of failures are introduced through the processes of pulling, cutting and terminating the physical lengths of cabling to form specific network configurations.

To a great extent, cable can withstand damage from bends, kinks, twists or even knots along the length of a link – so the installer should look first at the endpoint terminations of the link. Some of the most frequent failures involve mis-wiring of connectors, punching down incorrectly or mixing wiring specifications.

A misunderstanding of industry-accepted termination and colour-coding conventions can result in workmanship that is both faulty and hard to diagnose.

As a wider variety of installers – such as electricians and general contractors – are entering the LAN cabling market, it is becoming even more important to focus on these workmanship standards and develop easy-to-use tools and methods for fault resolution.

For obvious workmanship issues such as mis-wires, the best troubleshooting method is visual inspection. Basic test tools can help installers know where to look, but they must then understand how to spot faulty terminations.

Another potential problem area is component matching of connectors made by different vendors – especially in Category 6 or 7 environments. Whereas most connector and cable suppliers will claim full cross compatibility, and such claims are generally true now for Cat 5e, installation contractors should still check with their vendors about any specific recommendations regarding optimal matching between components.

Care must also be taken regarding cabling standards, which can vary between countries. For example, in the UK and US, TIA "Category" standards are the accepted norm. However, in mainland Europe, both ISO "Class" and CENELEC "EN" standards are the preference.

The installer must understand the requirements of the manufacturer's warranty in field certification testing.

The importance of training
If after checking the connector hardware and wire-map integrity, the test still fails, then further investigation is needed. Fault identification is simple, as the installer uses sophisticated test equipment. But how can the fault be pinpointed? And more importantly, what is the cause of the fault?

All installers, it is assumed, have attended an installation training course. However, the advent of Category 6 introduced major changes to installation practices, and many technicians may not have been updated. The responsibility is with the installer to refresh and update their skill sets regularly, and there are now a number of highly reputable training companies in the UK offering courses backed by internationally recognised authorities (City & Guilds, BICSI, BTEC).

Pinpointing the problem
Some of the more subtle failure modes at the link endpoints can involve issues such as excessive untwisting or re-twisting of the wiring pairs, which can degrade the integrity of the cable and cause crosstalk problems. Changing the relationship between twisted pairs within the cable can alter the capacitance characteristics enough to change the impedance and create crosstalk. You can also experience a crosstalk failure by erroneously terminating a Cat 5 jack to a Cat 6 panel. Overall, crosstalk issues are the most common failure mode after basic wire-map workmanship issues. For crosstalk problems, a good multi-function diagnostic tool with Near End Crosstalk Testing (NEXT) capabilities can assist the installer by quickly spotting the problem and identifying the affected pairs. In most instances, the first response to a NEXT failure is to simply remove the offending connector and re-terminate it.

Remember that most crosstalk problems occur in proximity to the endpoint terminations on the network link. Therefore, NEXT is sufficient for most installation contractors and it is not necessary to pay for esoteric features that claim to spot crosstalk in the middle of a link.

In most cases, installers do not need overly sophisticated test equipment to test the raw cable. Instead, they should rely on the overall integrity of the cable and focus their capital investments on tools that will be able to help find faults within the field-installed network links.

Using TDR: when, why and how
Using the 80-20 rule, it is a reasonable estimate that 80% of failures occur within a few metres of the link's endpoints. However, the other 20% somewhere in the middle of the link can often require 80% of the troubleshooting time. If the failure sources for a link cannot be readily identified at or near the endpoints, then installers need to go to the next level of sophistication in order to "see into the link" and find fault conditions that are hidden inside the cabling jacket. This can be especially important if the cable has been pulled through walls, ceilings, cabling ducts, etc.

Faults anywhere along a length of cable can be detected and precisely located through the use of a good Time Domain Reflectometer device. TDR is the analysis of a cable (metal or fibreoptic) by applying pulsed signals on one end of the cable and examining the reflection of that pulse, using a technique similar to radar.

In copper cabling, TDR measures cable length and locates specific areas of impedance mismatch by transmitting a pulse down the cable and monitoring the cable in order to detect any reflections of the transmitted pulse. Any problems or anomalies in the cable will result in measurable differences in impedance.

If a cable is metal and has at least two conductors, it can be tested by a TDR. TDRs will troubleshoot and measure all types of twisted pair and coaxial cables, both aerial and underground. Based on the cable's nominal velocity of propagation (NVP), which is put into the TDR before testing, the unit can measure the length accurately.

TDRs will show impedance mismatches wherever they occur down the line, allowing for the precise location of conditions that could introduce fault conditions, such as cable type changes, breaks, water ingress or loose connectors.

In order to optimise user productivity, the TDR capability in cable testers should include easy-to-understand graphical display of impedance vs length characteristics to quickly pinpoint the distance-to-fault conditions. In addition, TDR cable test functions should also incorporate flexible pan and zoom functions to enable the operator to quickly target detailed sections of either the horizontal axis (cable length) or vertical axis (impedance). Although the TDR functionality is not usually required during routine pass/fail testing, it should be readily accessible at the operator's discretion to perform diagnostics on cabling problems, whenever more in-depth analysis is required.

The integration of full-featured TDR capabilities directly into multi-function field test equipment, such as the LANTEK 6/7 series, makes it possible for field installers to leverage the device's familiar operating interface, while also accessing the power of TDR. All the user needs to do is input the cable's NVP and the LANTEK can provide an accurate and easily understandable picture of the TDR trace, showing the distance to the event as well as the event's magnitude.

For field-level troubleshooting, the ability to discern differences in the magnitude can be helpful for locating "return loss" failures and identifying marginal problems or latent issues even when the overall link may not be exhibiting a "hard failure" on total impedance tests. Visual displays can enable the user to easily identify the precise distance to an anomaly or "event" anywhere along the length of cabling (see figure 1). Although this does not tell the installer exactly what is wrong, showing them where to look is a major benefit for streamlining the troubleshooting process.

Field-orientated tools and methods
Effective troubleshooting will always require a complementary interaction between the diagnostic tools and the field technician. There will never be a "universal automatic bug-fixing device" that can solve problems without the involvement of a knowledgeable hands-on user. Likewise, even the most expert technician would be lost without the proper tools to help locate and diagnose fault conditions – especially when they are buried within a length of installed copper or fibre cable.

Installation companies need to equip their staff with cost-effective, easy-to-use copper and fibre diagnostic tools, which can be directly integrated into their multi-function certification and test handsets. Then, when failures occur, the troubleshooting process can become a seamless extension of the test activity by pinpointing likely failure sources and profiling link characteristics. After all, experience has shown that fixing problems quickly can provide major savings in both time and expense – and rapid pinpointing of exact fault locations is the first vital step to the quick and efficient correction of network failures.