In an ideal world, network installers would simply complete their work and conduct successful certification tests every time. However, we don't live in a perfect world. Tests of some percentage of installed links will always results in failures. Unfortunately, it's the failed tests that can really drive up costs for installation contractors. Knowing what to do when the test fails often can be the make-or-break difference between turning a profit on a network installation or suffering a loss due to excessive troubleshooting expenses.
The key to cost-effective troubleshooting lies in a combination of both technical expertise and specialized tools, along with the knowledge of when and how to use them. Installers need to follow a logical process of looking for the most likely problem issues first, before investing a lot of time and money in pursuing more complex possibilities.
This article provides an exploration of the critical troubleshooting issues that come into play whenever a link fails the testing process. The first steps should be to focus on quickly and systematically eliminating the most likely problem sources, such as bad connectors, cross-termination or other workmanship issues that typically are found within a few meters of the link's end points. When necessary, installers then need to be able to efficiently "drill down" in order to discover and diagnose those faults that are more difficult to find and fix. These types of faults can result from conditions such as scrapes, stretches, abrasions, or splices in copper cabling or from cuts, scrapes, or coupling losses in optical cabling. In this second-stage troubleshooting process, installation technicians often need to be able to "see into the link" - not only to identify problem areas but also to pinpoint specific fault locations along the length of the cable.
It is also important that capital investments should be targeted toward test equipment that is tailored for efficient field-level diagnostics. Cot-effective troubleshooting requires specialized diagnostic tools, which bring together comprehensive testing and analysis capabilities within easy-to-use feature sets. Installers typically do not have the training or the time to conduct extensive ad-hoc diagnostic procedures using general-purpose analysis systems. This article also outlines key capabilities and features that are required for effective field troubleshooting of both copper and fiber links. Some of the specific techniques discussed include time domain reflectometry (TDR) and optical TDR, as well as alternative specialized technologies for diagnosing and finding fiber link faults without incurring the high cost of OTDR systems.
In addition to describing the types of testing methodologies and their specific applications, the following sections provide detail on the underlying technologies, performance requirements, diagnostic parameters, and real-world usage tips. The discussions cover both theory of operation and practical application for each technique, with a focus on matching the most appropriate technologies to each troubleshooting challenge, in order to help the reader make field-oriented bottom-line decisions while maximizing capital test equipment investments.
First, eliminate obvious failures
For the most part, the reasons for test failures today primarily are workmanship issues at the point of installation. Almost all cable manufacturers have refined their processes to approach or achieve six-sigma quality levels, producing millions of feet of cabling with very consistent characteristics and capabilities. Therefore installers can generally have a high level of confidence in the integrity of their raw bulk cable. The vast majority of failures are introduced through the process of pulling, cutting, and terminating the physical lengths of cabling to form specific network configurations.
To a great extent, today's cabling also is impervious to inadvertent damage from bends, kinks, twists, or even knots along the length of the link-leaving the installer to look first at the endpoint terminations and/or any known splices within the link. Some of the most frequent failure modes involve simple miswiring of the connectors, punching a pin down incorrectly or mixing an "A" wired panel with "B" jacks. A general misunderstanding of industry-accepted termination and color-coding conventions can result in workmanship that is both faulty and hard to diagnose. As a widening variety of vendors, such as electricians and general contractors, have gotten into the LAN cabling market to expand on their core services, it has become even more important to focus on these workmanship standards and to develop easy-to-use tools and methods for fault resolution.
For obvious workmanship issues such as miswires, the most appropriate troubleshooting method is visual inspection. In these cases, some basic test tools can help installers know where to look, such as being able to determine whether an "open" is on the display handset side of the link or on the remote side. But once the installer knows where to look, they must understand their craft enough to know how to spot faulty terminations. Most importantly, the best preventative measure for poor workmanship is rigorous installer training and rigid adherence to quality standards and methods.
In addition, there are still some issues with regard to component matching of connectors manufactured by different vendors-especially in Category 6 or Category 7 environments. While most connector and cable suppliers will claim full cross compatibility and such claims are generally true now for Category 5e, installation contractors should still make it a practice to ask their vendors about any specific recommendations regarding optimal matching between components.
If you've used the correct connector hardware and have verified the wiremap integrity, then it's time to drill a little deeper into the next level of potential problem areas. Some of the more subtle failure modes at the link endpoints can involve issues such as excessive untwisting or retwisting 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 plugging a Category 5 jack into a Category 6 panel. Overall, crosstalk issues typically are the most common failure mode after basic wiremap workmanship issues. For crosstalk problems, a good multi-function diagnostic tool with near-end crosstalk testing capabilities can assist the installer by quickly spotting the problem and identifying the affected pairs. In most instances, the first response to a near-end crosstalk failure is to simply remove the offending connector and reterminate it.
Here again, it is important to keep in mind that the overwhelming majority of crosstalk problems will occur in proximity to the endpoint terminations on the network link. Therefore, most installation contractors can get by perfectly well with near-end crosstalk and don't need to pay for esoteric feature that claim to spot crosstalk in the middle of a link. Time-domain reflectometry is not needed to troubleshoot near-end crosstalk, but it does offer a cost-effective alternative that can be used to locate a full range of different fault conditions, including crosstalk, anywhere along the length of a link.
In most cases it does not make sense to invest in overly sophisticated test equipment that is capable of performing quality assurance on the raw cable itself. That is fundamentally the job of the cable providers, and they do it very well. Instead, installers should generally rely on the overall integrity of the cable and focus their capital investments on tools that will help find faults within the field-installed network links.
The TDR: when, why, and how
Using the 80-20 rule, it is a reasonable estimate that 80% of failures occur within a few feet of the link's endpoints. However, the other 20% somewhere in the middle of the link can often require more than 80% of the troubleshooting time. If the failure sources for a link cannot readily be 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 in real-world situations where the cable has been pulled through walls, ceilings, cabling ducts, and other pathways.
Faults anywhere along a length of cable can be detected and precisely located through the use of a good time-domain reflectometry (TDR) device. TDR is the analysis of a cable (metal or fiber-optic) 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 fast rise-time pulse down the cable under test and then monitoring the cable for constant voltage in order to detect any reflections of the transmitted pulse. Any problems or anomalies in the cable that change the capacitance, inductance, or resistance will result in measurable differences in impedance. A useful analogy is to think of impedance mismatches as "disruptions in the flow or back-pressure" that alters the actual time for propagating the pulses versus the nominal propagation rate.
These impedance mismatches anywhere along the length of the cable cause reflections that are then displayed on the TDR's output. A significant reflection also always occurs at the end of the cable.
If a cable is metal and it 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 dialed into the TDR prior to testing, the unit can measure the time it takes for the transmitted pulse to be reflected from the far end of the cable. By manipulating the instrument's controls, the absolute length can be calculated quite accurately.
Impedance mismatches will also be identified wherever they occur down the line, allowing for precise location of conditions such as cable-type changes, bad vampire taps (in coaxial cable), and pair splits. TDRs are used to locate and identify faults in all types of metallic paired cable, including both major or minor cabling problems such as sheath faults, broken conductors, water ingress, loose connectors, crimps, cuts, smashed cables, or shorted conductors. TDR analysis can be especially valuable for locating cable quality problems, such as corrosion, stretching, crimping, incomplete shielding, and other defects that are visually undetectable but can be "seen" by the TDR.
Historically, TDRs have been considered relatively sophisticated devices, reserved for only large companies and high-level engineers. Although TDRs have become proven, widely used tools in the telephony industry, until recently LAN network installers have been slow to adopt TDR technology due to the complexity of operation and the high cost of the instruments. Today TDR functions integrated directly into cable testers have become more advanced, with easier operation and much simpler interpretation of results.
To optimize user productivity, the TDR capability in cable testers should include easy-to-understand graphical display of impedance (or percentage of reflection) versus 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 normally required during routine pass/fail testing, it should be readily accessible at the operator's discretion to perform on-the-fly diagnosis of cabling problems, whenever more in-depth analysis is required.
The integration of full-featured TDR capabilities directly into multi-function field test equipment makes it possible for field installers to leverage the devices' familiar operating interface, while also accessing the power of TDR. All the user needs to do is dial in the cable's NVP and the tester can provide an accurate and easy-to-understand 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 quite 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. While this doesn't tell the installer exactly what is wrong, showing where to look is a major benefit for streamlining the troubleshooting process.
Pinpointing fiber faults
Optical time-domain reflectometers (OTDRs) operate on the same principles as TDRs, however OTDRs focus on measuring backscatter in fiber-optic links as compared to measuring impedance in copper links. Backscatter or reflected light can occur at any point in the link where the propagation of light is disrupted by dirt, an air gap or other anomaly. This may be a connector, a splice, or end of fiber (EOF).
The thorough cleaning of fiber connections can be a major workmanship issue in fiber-optic networks. All too often, installers who are unfamiliar with the physics of light propagation will assume that if the connectors and launch cables appear clean to the naked eye, then the connection will provide acceptable performance. Having a field inspection microscope can be a useful tool for spotting problems when testing and troubleshooting fiber links.
For most contract network installers, a full OTDR system can be both too difficult to learn and use, as well as too expensive for routine deployment. While an OTDR can provide a variety of detailed data about the intricate light-propagation characteristics within a fiber link, much of that information is not particularly useful for field troubleshooting. Field technicians need targeted capabilities that can quickly profile relevant failure modes and provide location information that enables them to quickly find and fix the fault. In addition, they need fiber diagnostic tools that can be cost-effectively integrated with existing equipment and familiar procedures.
Today products are available that are not OTDRs but provide field-oriented fiber troubleshooting functions that can be integrated with and displayed on familiar handsets. These tools can provide troubleshooting interfaces for multimode or singlemode fiber and use a tester's existing TDR circuitry for analyzing and displaying the results. These tools provide an easy-to-use troubleshooting capability and illustrate distance-to-fault by providing magnitude and distance of reflective events. They also can accurately measure total fiber length without require a remote handset or a second operator. Additionally, the tools use 3-meter launch cords as opposed to the extra length of coiled launch fiber typically needed for full OTDR analysis.
The graphical display capability can be useful for getting a clear and complete picture of reflection points along the entire fiber link, which can guide and evaluate troubleshooting processes.
The width of the light pulse also plays an important role in the use of fiber diagnostic tools. Shorter pulses typically provide greater resolution and have shorter dead zones, however they have less dynamic range and more noise. In contrast, longer pulses have less resolution and wider dead zones but can provide better dynamic range and less noise. Depending upon the overall fiber link's length or the number and location of connections, either a short or long pulse can offer better data.
The bottom line
When all is said and done, 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 the length of installed copper or fiber cable.
In the final analysis, installation contractors need to equip their staff with cost-effective, easy-to-use TDR and fiber diagnostic tools, which can be directly integrated into their multi-function certification and test handsets. That way, whenever failures occur, the troubleshooting process can become a seamless extension of the test activity by pinpointing likely failure sources and profiling link characteristics. Instead of having to disrupt their workflow and/or swap over to unfamiliar or overly complex analysis methods, field technicians can use in-depth troubleshooting capabilities as a routine enhancement to their test/certification process. 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.
Taz Cantrell is application engineer with Ideal Industries (www.idealindustries.com), and Dan Munch is principal of Ficopia Training and Consulting.