FOA Guide 

Topic: Maintaining Fiber Optic Networks

Table of Contents: The FOA Reference Guide To Fiber Optics

Maintaining Fiber Optic Networks

maintenance testing of fiber optic networks

Some people have suggested that fiber optic networks need periodic maintenance, including microscopic inspection of connectors and mating adapters and even insertion loss testing or taking OTDR traces. That advice is misguided. It could hurt an installer or get them sued by an irate network owner whose network you bring down or cable plant you damage. Do you think the telcos have crews out checking fiber networks to see if the connectors and splices are OK? How about the military on tactical systems? Or is there a Captain Nemo in the Nautilus checking submarine cables? Of course not.

Fiber systems are designed to be installed and never touched unless something damages them – the infamous “backhoe fade” of buried outside plant cables. In the early days of fiber optics, some network owners tried building automatic monitoring systems to keep tabs on the loss of the cable plant. That idea faded when fiber proved to be much more reliable than copper cabling and the network communications manufacturers built monitors for data transmission into their equipment, a more reliable indicator of problems.

Let me give you some reasons why you do not want to try to do maintenance on any fiber optic network:

Most inspection procedures require bringing the network down, unacceptable in almost every instance. Telcos have backup links running alongside operational links and the equipment will switch over to the backup if it senses high errors on the main link. Do you know any premises networks set up like that? Want to bring down an OC192 telco link or gigabit LAN backbone fast? Unplug a fiber optic connector to inspect it with a microscope. See how long it takes the network manager to find you.

Most harm to installed fiber optic systems (and copper also) is done during handling by unskilled or clumsy personnel. We heard of one network that crashed when a executive of a company disconnected a fiber connector to show it to a visitor they were escorting around the facility. We know of workers accidentally backing into patch panels and breaking cables at the junction to the connector. We’ve seen connectors dropped on the floor breaking the ceramic ferrule. We’ve even helped troubleshoot broken fibers in splice closures caused during repairs of other fibers.

It's easy to get dirt into mating adapters or on connectors whenever they are exposed to the air. Fiber technicians are taught to keep connections clean after termination, cover connector ferrules and mating adapters with dust caps and clean the ferrule end whenever it is opened to the air. If dirt is such a big problem (and airborne dirt is the size of the core of singlemode fiber), why risk contaminating operating connectors by exposing them to the air to see if they are dirty? Systems should be designed to protect components from dust in the environment - that's what cable plant hardware is for.

Mating and unmating may wear the connector interfaces, affecting optical performance. Ferrule endfaces rubbing against the mating connector and the outside of the ferrule scraping materials off the alignment sleeve in the mating adapter, especially with adapters using cheap plastic alignment bushings which are good for only a few mating cycles, can cause higher loss.

Safety can also be an issue. Links operating at gigabit and higher speeds generally use 850 nm VCSELs, which are relatively high power lasers at a wavelength near the high end of human eye sensitivity, still visible to some people. Using a high power microscope, like a 400X, concentrates the light into the eye, increasing the risk of eye damage, especially if you are not able to see this wavelength. Should a link being inspected be “hot,” the consequences could be bad. Anyway, a 400X microscope is overkill – it’s the maximum magnification you would use to inspect singlemode connectors during termination; 100-200X is generally considered the maximum for connector inspection.

The fiber link loss may be different when  a link is reassembled after inspection, especially with connectors that have spring loaded ferrules like STs. Inspecting a connection could lead to higher loss than initially measured and potentially affecting data transfer on systems like Gigabit Ethernet and 10G Ethernet where loss margins are very low.

As for testing with an OTDR for maintenance inspection, some telcos do that automatically on spare fibers in outside plant cables that run tens or hundreds of kilometers through desolate regions. An OTDR, unless it is very high resolution, is inappropriate for most premises systems under any circumstances and often causes more problems that it solves.

Finally, if you have a problem with dust in a telecom closet, room or data center, you have a poorly designed facility that should be fixed with proper sealing, filtration and air conditioning. You should not try to fix it with a feather duster.

So what periodic maintenance should be done on fiber optic networks? NONE!

Let's take a different viewpoint - how do you design a system that is going to be reliable and not need maintenance or repair?

That makes a lot more sense. Let's look at some things you can do to ensure reliability.

Design the cable plant to be protected. Bury it deep enough to prevent most dig-ups (1.5m is usually good enough.) Put it in conduit or innerduct where it is exposed. Use racks and patch panels with locking doors.

Ensure the cable was installed properly and is stress-free. Follow guides for pulling tension and bend radius. Do not let long cables hang on connectors causing stress on the connector. Do not bundle patchcords tightly enough to cause stress. Check fibers in splice closures to ensure they are stress free. Vertical cables should be supported about every meter. Ensure that all cables are in operating environments that are within their specifications - both heat and cold can cause problems. Make certain that all cables meet their loss budgets - and fix high problems causing high loss cables. If it's regular fiber, a high-resolution OTDR should show any areas of stress after installation and those areas should be fixed by loosening or re-routing cables. Bend-insensitive fiber requires careful physical inspection.

Carefully inspect and clean - the inspect again and clean again until every connector is perfectly clean. Clean the mating connectors and mating adapters also - dirt is the enemy of good fiber optic connections. Clean communications equipment transceiver ports also.

Keep protective caps on every connector, mating adapter and equipment port. People usually call them dust caps and as one FOA instructor likes to say, we call them "dust caps" because they are generally full of dust. But they do protect the connector or receptacle from further accumulation of dirt or physical damage.

Only use patch panels with doors - lockable doors. This solves two problems - keeping your connectors and cables safe from unauthorized personnel and keeping dirt out of the patch panel.

Put equipment in racks with doors -
lockable doors. Keep unauthorized personnel out!

If MACs (moves, adds and changes) are needed, only allow
authorized personnel to access the cabling systems.

If you have a problem, FOA has a guides to troubleshooting and restoration.


(C)1999-2014, The Fiber Optic Association, Inc.