Troubleshooting fiber

Summary:So you've replaced your copper cables with fiber optics, but now you're having problems. Learn how to troubleshoot faults in fiber optic cables, and how to narrow down possible causes.

Fiber optic cable was once reserved for high-performance needs, but today it’s turning up in all kinds of networks. If you’re familiar with copper cable, you’ll quickly discover that fiber optic cable is a completely different animal. Not only is the installation process different for fiber, but also the troubleshooting process. Fiber optic cable is also far more fragile than copper cable, so there are more potential causes of trouble. I’ll discuss common fiber optic cable problems and how to diagnose and repair them.

The most common causes of fiber optic malfunctions
Anyone who’s ever done network troubleshooting knows it’s a complicated process, so it’s helpful to know where to start looking for a problem. To help you make an educated guess about the cause of your network’s troubles, here are some of the most common fiber optic cable problems with their possible causes:

  • Broken fibers because of physical stress or excessive bending
  • Insufficient transmitting power
  • Excessive signal loss due to a cable span that’s too long
  • Excessive signal loss due to a contaminated connector
  • Excessive signal loss due to faulty splices or connectors
  • Excessive signal loss due to having too many splices or connectors
  • Faulty connection of fiber to the patch panel or in the splice tray

Typically, if a connection is completely dead, it’s because of a break in the cable. However, if the connection is intermittent, there are several possible causes:
  • The cable’s attenuation may be too high because of poor quality splices or too many splices.
  • Things like dust, fingerprints, scratches, and humidity can contaminate connectors.
  • There is low transmitter strength.
  • There are bad connections in the wiring closet.

Gathering information
When I’m called in to fix a problem in an environment that I’m not thoroughly familiar with, the first thing I do is gather some basic information about the problem’s symptoms and possible causes. As with any other medium, the key to getting good information is knowing the right questions to ask. Here are a few questions that can get you started.

Has anyone disconnected, reconnected, or moved the PC recently?
It’s important to find out if the PC has been disconnected or has moved recently. If the fiber optic cable has been disconnected from the PC, it could be that the cable was never reconnected, was reconnected incorrectly, or was contaminated during the time that it was disconnected.

Have there been any changes to the PC’s hardware?
Upgrading a PC’s hardware can also cause problems. The cable could have been disconnected and either not reconnected, reconnected improperly, or contaminated prior to being reconnected. It’s also entirely possible that the cable was never disconnected during the hardware upgrade. If that’s the case, there’s a chance that the cable could have been overextended when the PC was moved, or the PC could have been accidentally smashed against the back of the desk or the wall, damaging the connector.

It’s also possible that the cable was never damaged or hooked up incorrectly, but rather that the new hardware is preventing the NIC from working correctly. The exact method by which you’d determine whether or not the NIC is having problems varies with your operating system. However, if you’re using Windows 9x, Me, 2000, or XP, you can use the Device Manager to examine each of the system’s hardware devices to see what is and isn’t working.

I also recommend temporarily removing the new hardware and returning the system to its previous state, if possible. This could show if the system’s new hardware is causing the problem, or if the problem is related to something else.

Has any furniture been moved recently?
I can’t count the number of times that I’ve seen copper network cables damaged because someone scooted a desk out from the wall or because the cleaning crew moved a desk to vacuum. When you move a desk without disconnecting the network cable, the cable may become overextended, or the desk could smash or crimp the cable. If something like that can destroy a copper cable, imagine what could happen to a fiber cable, which is made out of glass.

Has anyone, such as the telephone company, been performing any work in the building?
This may seem like a strange question to ask, but it’s my experience that network problems often coincide with visits from the phone company. I’ve had phone company technicians cut cables, disconnect cables, and do just about anything else that you can imagine. I won’t pretend to understand the logic behind this behavior, but if you’re having network problems and someone tells you that someone was in the building doing phone or electrical work yesterday, then you’ve got a good place to start your troubleshooting.

Has the cable been stepped on, had a chair rolled over it, or suffered any other physical stress?
It never fails to amaze me what end users will admit to when you ask the right questions. If you ask a user in a nonthreatening manner about any physical stress that the cable might have endured, they’ll usually tell you. Remember that most users have no idea that a fiber cable can be destroyed if stepped on or bent too sharply, or if they roll a chair over it.

Topics: CXO

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