Restoring Fiber Optic Networks Rapidly
A consequence of fiber optic systems’ high bandwidth, long distance capability and security is the dependence of users on the non-stop operation of these systems.
A consequence of fiber optic systems’ high bandwidth, long distance capability and security is the dependence of users on the non-stop operation of these systems. They can transmit large amounts of data long distances with immunity from signal degradation and extremely high reliability, so these systems usually carry the most critical data. By critical data, we’re not talking telephone conversations or TV signals (although some viewers would question that conclusion during the Super Bowl or the finals of “American Idol,”) we’re talking about utility network monitoring and control signals, surveillance CCTV systems, traffic control systems, airport monitoring and security, and the like.
The wise user designs a network with backup options. Telcos run dual links, one transmitting data and one on “hot back-up” ready to switch over in milliseconds. Electronics must be installed with duplicate links and all power must be backed up with batteries or fuel cells. Critical systems often add in geographic diversity, two links available running paths that are as widely separated as possible to ensure that if one suffers a failure due to damage to the fiber optic cable plant itself, the other can be switched in immediately. Even with backup, a failure requires immediate restoration, as one must never depend on a single link any longer than necessary.
The biggest cause of outside fiber optic cable plant failure is “backhoe fade”.
In outside plant fiber optic installations, the biggest cause of network failure is what we call “backhoe fade” for buried cables and “target practice” for aerial cables, both of which are pretty self-explanatory. Cables in premises installations are unlikely to be dug up accidentally, but are susceptible to damage when any personnel are working around the fiber optic cables in trays or conduit.
With the current push by landlords to remove abandoned cables to comply with the NEC, the likelihood of damage is much higher as installers cut out the old cables. We have always recommended installing critical indoor fiber optic cables inside bright orange “innerduct” to protect it. Inside telecom 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.
A properly equipped and trained tech is needed for restoration.
The biggest single help in troubleshooting starts with producing good documentation during the installation and keeping it current. Documentation is the most helpful thing you can have when trying to troubleshoot a fiber network. Start with the manufacturer’s datasheets on every component you use: electronics, cables, connectors, hardware like patch panels, splice closures and even mounting hardware. Along with the data, one should have manufacturer’s “help line” contact information, which will be of immense value during restoration.
During installation, mark every fiber in every cable at every connection and keep records using cable plant documentation software or a simple spreadsheet of where every fiber goes. When tested, add loss data taken with an optical loss test set (OLTS) and optical time domain reflectometer (OTDR) data when available. Someone must be in charge of this data, including keeping it up to date if anything changes.
For the electronics, if possible one should have data on the optical power at transmitters and receivers. If that data is not taken during installation and setup, typical data should be available in the equipment manuals.
Outside plant cabling should have maps and photos detailing the routing of the cable, with GPS locations if possible. For premises cabling, drawings of the building noting all cable runs, again with photos if possible, are needed. One needs lists describing the types of cable, installation hardware and test data for restoration. Knowing where every cable goes will keep you from blindly searching for the cables when you try to locate problems. Having original test data will make it much easier to find bad cables.
Next, you need some basic test equipment. An OLTS should have a power meter to use to test the power signal in the transmission link. Troubleshooting electronics is straightforward; start at the receiver and measure the optical power, which may require putting the remote transmitter into a test mode. The power at the receiver is the first piece of data you need. If the power level is correct, the transmitter is transmitting and the fiber in the cable plant is good for that link. If the power is bad, you must test the remote transmitter to see if its power level is correct, in which case the problem is the cable plant, or if not, you have a bad transmitter. If both are OK, but the link is bad, the input or output from the electronics is suspect.
Total failure of all fibers in the cable plant usually means a break or cut in the cable. For premises cables, finding the location is often simple if you have a visual fault locator or 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 should be visible at the location the fiber is cut and allow locating the damaged point. VFLs also can be used to find bad connectors at patch panels.
Aerial cables may require a bucket truck for restoration, like this fiber-connected CCTV security camera.
For longer cables, an OTDR will be useful. Outside plant networks should use the OTDR to document the cable plant during installation, so during restoration a simple comparison of installation with current traces will usually find problems. OTDRs generally do not have adequate resolution for short cables, say less than 30-50 meters, so a VFL will be needed. OTDRs can also find non-catastrophic problems, for example when a cable is kinked or stressed, so it only has higher loss, which can also cause network problems. Remember that OTDRs measure fiber length, not cable length, which is usually 1-2 percent shorter due to the excess fiber in the cable.
Once you find the problem, you have to repair it. Repair requires having the right tools, supplies and trained personnel available. Besides the test equipment needed for troubleshooting, you need tools for splicing and termination, which may include a fusion splicer for outside plant cables. You also need matching components. We always recommend that for every installation, a reasonable amount of excess cable and installation hardware be set aside in storage for restoration. Some users store the restoration supplies along with documentation in a sealed container ready for use. Remember that the fiber optic patchcords that connect the electronics to the cable plant can be damaged also, but are not considered repairable. Just keep replacements available.
Cable cuts or breaks generally can be spliced or re-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 together. You need about 1 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, which hopefully you kept from the leftovers after installation.
What else besides cables and cable plant hardware 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. 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 need to know where to find the test equipment you need.
Do not forget that personnel must be properly trained to use this equipment and do the troubleshooting and restoration. And, of course, they must be available on a moment’s notice. The biggest delay in restoring a fiber optic communications link is often the chaos that ensues while personnel figure out what to do. Having a plan that is known to the responsible personnel is the most important issue.
Major users of fiber optics have restoration plans in place, personnel trained 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. You may find that the cost of having all this equipment is not economic, as OTDRs cost more than $10,000 and fusion splicers can cost twice that. It may be preferable to keep an inexpensive test set consisting of a VFL and OLTS at each end of the link and having an experienced contractor on call for restoration.
A final word, no, a warning, is needed. Fiber optic cable plants do not need maintenance! Inspecting and/or testing them is more likely to cause damage than provide useful information or prevent failures. Install them and leave them alone, unless a failure occurs.
The Fiber Optic Association, the professional society of fiber optics, has available on its website, www.thefoa.org, guides for end users on fiber optic network design and installation. The FOA has certified 24,000 technicians through over 200 approved schools to create a pool of trained, experienced and certified techs who can install and restore networks. You can search for techs or contractors with appropriate experience throughout the world using the FOA’s free online database on its website.
About the Author: Jim Hayes of VDV Works is a writer/trainer and President of The Fiber Optic Association. For the last 30 years, he has been involved with fiber optics and communications. A frequent lecturer on communications, he is also the author of two textbooks, The Fiber Optic Technicians Manual and Data, Voice and Video Cabling, and numerous articles on fiber optics and cabling. Find him at www.JimHayes.com.