In outside plant fiber optic installations, the biggest cause of network failure is what we call “backhoe fade,” which is pretty self-explanatory. Cables in premises installations are unlikely to be dug up accidentally, but are susceptible to damage when installers are working around the cables in trays or conduit. With the current push by landlords to remove abandoned cables to comply with the new National Electrical Code, the likelihood of damage is much higher as installers cut out the old cables. Inside telecomm closets and computer rooms, it's possible to damage cables, patchcords and connectors as equipment is moved or connections changed. Sooner or later, the end-user will likely be faced with restoring the system.
Efficient fiber optic restoration depends on rapidly finding the problem, knowing how to fix it, having the right parts and getting the job done quickly and efficiently. Like any type of emergency, planning ahead will minimize the problems encountered.
The biggest single help in troubleshooting starts with producing good documentation during the installation. Good documentation is the most helpful thing you can have when trying to troubleshoot a fiber network. Drawings of the building that note all cable runs as well as lists describing the types of cable and test data are important documents for restoration. Knowing where every cable goes will keep you from blindly searching for the cables when trying to locate problems. Having original test data will make it much easier to find bad cables.
Next, you need some basic test equipment. Total failure of a cable usually means a break or cut. Finding the location is often simple if you have a visual fault locator (VFL), which is a bright red laser coupled into the optical fiber. Coupling the VFL light into the cable allows testing continuity. If there is a break or cut, the laser light may be visible at the location the fiber is cut and would assist in locating the damaged point. VFLs also can be used to find bad connectors.
For longer cables, an OTDR will be useful. Outside plant networks always use the OTDR to document the cable plant during installation, so in restoration a simple comparison of traces will usually find problems. OTDRs do not have adequate resolution for short cables, say less than 30 to 50 meters. OTDRs can also find noncatastrophic problems, for example, when a cable is kinked or stressed, it only has higher loss, which can also cause network problems. Remember that OTDRs measure fiber length, not cable length, which is usually 1 to 2 percent shorter due to the excess fiber in the cable.
Once you find the problem, you have to repair it. Cuts or breaks generally can be spliced or terminated. In the outside plant, the splices will be permanent-usually fusion splices just like the initial installation-and sealed in a new splice closure. Sometimes in premises cabling, temporary mechanical splices placed in an enclosure can be used, the fibers can be terminated with connectors or a new cable can be pulled, whichever is more efficient.
One big problem is pulling the two cable ends close enough to allow splicing them. You need about one meter of cable on each end to strip the cable, splice the fibers and place them in a splice closure, which is the reason service loops are recommended for all cables. If the cable ends are too short, you have to splice in a new section of cable.
That's right, you will need a length of cable like the one you are repairing, and you will have to make two splices for each fiber. For each cable type installed at a site, it is advisable to keep several lengths of the cable stored away for just such an emergency.
What else besides cables should be in a restoration kit? You should have a termination or mechanical splice kit and proper supplies. For splices, you need splice closures with adequate space for a number of splices equal to the fiber count in the cable. It's a good idea to have some patchcords stashed away too. All these should be placed in a clearly marked box with a copy of the cable plant documentation and stored in a safe place where those who will eventually need it can find it fast. And you have to know where to find the test equipment you need.
Major users of fiber optics have restoration plans in place and kits of supplies ready for use. It's doubtful that most premises users are ready for such contingencies. Smart contractors doing fiber installations ask their customers about restoration plans, noting they can assist in the planning and be available on short notice for such work when needed. EC
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.