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Why fiber links fail: dirty connections and poor polishing can be identified with cable testing tools

Communications News, July, 2005 by Harley B. Lang

Most information transport systems contain fiber links that are critical to total network performance. The cost of a failed fiber link, as measured in network downtime, is enormous. Ensuring that top-quality cabling is properly maintained becomes essential to network owners.

To help understand the causes of fiber failures, an independent study of private network owners and installation contractors was conducted to ascertain why fibers fail. The study revealed that fiber inspection and certification tools not only identify the problems, but in many cases fix them, as well.

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The most common fiber type being installed is 62.5 mm multimode fiber. Single mode was not far behind, however, especially among contractors installing original cable. 50 mm multimode fiber, particularly that which is laser optimized, is growing in popularity. Plastic fiber is rarely used. ST and SC connectors are still the primary connectors of choice installed today.

Network owners and contractors cited dirty connector end faces as the number one problem encountered during the fiber installation process. The most common reasons for fiber failures were cited as dirty connections and poor polishing. Broken connectors, mislabeled cables and shattered end faces were also significant issues.

Knowing what causes fiber failures is only the first step. Being able to see exactly what is wrong is critical to fixing the problem.

TROUBLESHOOTING PROBLEMS

Troubleshooting fiber link problems is the last step in the installation process. Many times, troubleshooting occurs when the installed link does not pass the loss test detailed in the construction specification. Troubleshooting also occurs at the time of electronic equipment installation.

Oddly enough, fiber end-face inspection is not always carried out during this phase of ensuring fiber performance. For example, only 60% of the troubleshooting calls dispatched by the contractor involve inspection with a microscope. Network owners inspect the fiber end-faces even less-an average of 46% of the time.

The most popular tool for checking the condition of the fiber end face is the hand-held portable microscope. Generally, installers use 100-power microscopes, with a few using 200-power microscopes.

While 100-power magnification devices are commonly used to look at connector end faces, these low-powered scopes will not always show particles of dirt or scratches on the connector end face. Higher-powered instruments, such as 250x, often reveal dirt in the fiber core where the signal is going to travel. In general, 250x scopes are appropriate for multimode installations, while a 400x scope is recommended for single-mode applications.

One of the primary reasons for not inspecting the end face is that access to the end-face terminations in a patch panel is difficult and time consuming. Hand-held microscopes, while inexpensive and simple in their operation, are bulky and require a straight line of sight. An average time of about 10 minutes per connector was reported for disconnecting the fiber and creating enough space for the scope for it to be used.

A cost-effective way to inspect these fiber ends is with the use of a video microscope. A video microscope provides the reach to get into the patch panel, as well as equipment ports. Video microscopes are available with magnification up to 400x with a variety of connector types, including small form-factor connectors.

Video microscopes also eliminate the chance of damaging your eye with harmful infrared light. Viewing end faces in this way can be done in one-tenth of the time required when using a traditional microscope. That means that six connectors can be viewed in six minutes with a video microscope vs. an hour with traditional microscopes.

Video microscopes can also be used on active electronics, since they protect the user from infrared light. If a port on a 100BASE-FX 24-port switch is not working, for example, you could view the port for dirt with a video microscope, even if the equipment is turned on and all other ports are active. In this way, you may be able to identify the trouble and work with the electronics manufacturer to rectify the problem. In the study, 67% of the installation contractors have encountered problems with dirty fiber ports on electronics, while 44% of the network owners had the same experience.

One often-overlooked aspect of dirty fiber connectors is cross-contamination, where one end face gets the mating connectors dirty. This also applies to the connectors on test cables. If you test with a dirty test cord, you not only have higher loss readings, but also contaminate the fiber end faces you are checking. Conversely, remember that your test cord can pick up dirt from a connector.

Cleaning the end face of a patch cord is just as important as cleaning permanently installed cable. Patch cords have been found to be a source of many troubles in the network. Unknowingly, the installer or network owner connects systems to devices and misses inspecting more than half of the potential problems--connections on the patch cords and the equipment ports. Some optical time domain reflectometers (OTDR) incorporate an automated port quality check that warns the user when the OTDR port is dirty or contaminated.

Technology Industry

Why fiber links fail: dirty connections and poor polishing can be identified with cable testing tools

Communications News, July, 2005 by Harley B. Lang

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