Basically I am having a problem with a fiber optic cable connecting 2 switches not working, hired a professional to help fix it and in the end it is still not working.

I need someone to look over the details that I am writing here and hopefully help me solve the problem. Or determine if the professional was not competent enough for the job requiring me to find another professional. At the bottom of the post I will write all the specs of my equipment and such.

I take care of a network with a star topology network design. Just over a week ago one of the branches of the fiber network went down. The fiber that went down is a fiber that connects a building to the central fiber hub.

When this branch went down I did all the basic tests that I could think of to troubleshoot the problem. Although I am not a fiber optic technician and don’t have any testing or termination kits. What I did do was I replaced the switches on both ends and replaced the fiber modules on both ends with spare parts. I also checked all the splices in the fiber and made sure all the connectors where ok. Doing all of this did not fix the problem, so to be sure I then re-checked all the spare parts on other fiber cables to make sure the switches and optic modules were in working order and the result was that the parts were good.

This was all I could think of to do that I was able to do myself and based on the results I tentatively concluded that the cable was probably damaged. So at this point I called a fiber optic professional to come and test / repair the cable. The professional spent a day and a half on the cable and was not able to get it working.

In the end this is what they told me. They said that the fiber did not have good light flow so they replaced some of the connectors on the cable at the splices. They told me that once they were finished replacing connectors there was enough light flowing to allow a gigabyte of data flow. But despite this they were unable to get the green light to show up on the 2 switches verifying connectivity.

Since they were unable to get connectivity they began to theorize of what could be the problem. One of their theories was regarding signal loss from splices. The fiber cable in question has 3 splices, and each splice has of course 2 ends connecting together for a total of 6 connectors. Then add that to the cable having 2 connectors on it that plug into a switch at either end, bringing the total number of connectors to 8. They told me that a cable should never have more than 4 dB loss which means 5 connectors because each connector creates a .75 dB loss. With that figure the fiber currently has 6 dB loss.

What is odd to me about this theory is that the cable has been operating perfectly for about 3 years like this. When I told them this they said that perhaps it was on the edge the whole time and for some reason something happened that tipped the scales just enough to break the alleged flaky connection.

They had another theory which is they thought perhaps one of the switch was broken but as I said I replaced the switches, ports, and optical modules already. At this point I am not sure what to do. I could put a new fiber in the ground and redo it all. But this is very time consuming and costly. So I am trying to find out if anyone else has any ideas if even if someone can tell me that the fiber professional was competent and the information was correct. Perhaps I need to hire someone else to come out here and get it working.

Before I posted on this website I spent about a day researching about Fiber optics, cabling and about dBs. But I know that fiber can be complex so I got a little information but it is nowhere near enough to really know what the problem is or if the professional was competent or not.

In my research I found some information that does contradict what they were saying about 4dB loss being the maximum recommended, because it depends on what the minimum transmit power and minimum receive sensitivity of your devises and distance and other factors.

Please keep in mind that this has been working perfectly for 3 years up until 8 days ago.


Length of Fiber: 1000 feet

Fiber Type: Multimode 62.5/125 µm

Fiber Modules: Netgear 1000BASE-SX SFP GBIC Module - model # AGM731F

    Output Optical Power (Avg.)   Minimum -9.5    ----   Maximum -3

    Sensitivity (Avg.)            Minimum –N/A   ---   Maximum -17

Switch #1 Netgear L3 Managed Gigabit Switch - Model # GSM7312

Switch #2 Netgear ProSafe 24 Port 10/100 Smart Switch with 2 Gigabit Ports - Model # FS726T

# of Connectors: 8 SC or LC Connectors in the line on each strand.

The professional told me with his testing meeter he had enough light passing for gigabyte of signal.

Do I need to find another professional who is more skilled? Do I need to bite the bullet and just berry a new cable in the ground? Do I need to buy new switches? Etc.

Thanks for your help Thomas

4 Answers 4


Connect your replacement switches back-to-back with a fiber patch cable and verify for sure that their fiber ports work. If they do, then double check the patch cords you're using at each end in the same manner. If you really want to slog through it yourself and have the right patch cables you can use the switches as cheap testers to test at each of the splices (assuming you have connectors on the splices).

If the switches do work, get a better fiber contractor in there. It sounds like your contractor was dodgy. I agree w/ the other posters re: their meter. My experience with contractors testing fibers for me is that their meter knows far, far more than I do about a given fiber.

  • Thanks... this did the trick. I was able to track the problem down by doing what you suggested. It turns out that in the end after they found the fiber end or ends that were problematic. They then did not connect the cables back together correctly which I was able to discover by following your instructions and fixed it. Much appreciated. Additionally I wish I could vote other answers as good as well because other people gave good advice also. But your answer is what helped me actually fix the problem.
    – Thomas
    Aug 16, 2009 at 18:53

I'm a little surprised at the answer your tech gave you. I'll accept that he may have told you more than you relayed, but...

If I use test gear on a fiber run, it will tell me more than just "enough light for 1g". If there are problems, a TDR will tell exactly where the problem is and likely what it is as well. People with good testing equipment and knowledge of how to use it can look at a plot from where the ring out a fiber run and tell you about how far down the cable each of your splices is, and then where the fiber is broken. A pro won't just ask you how many splices you have and tell you what your loss is over a run, they'll tell you exactly what it is from their gear.

Of course, now that they've taken your money and mucked with your fiber plant, all bets are off. Re-terminating fiber isn't trivial.

As far as what you can do / need to do: If you figure out that it is the long fiber run and not your equipment or patches, you can re-terminate the fiber without digging up the existing run or running a new run. A TDR will tell you if there is a break at an end near a patch or in the middle. If someone ran a backhoe through your fiber, they should pay for the repairs (dig up the hole, take the fiber apart, clean the ends, splice in a patch). If it is at either end, you trim the ends and re-terminate. These sorts of splices shouldn't eat much into your light budget. Again, the TDR tells you roughly where in the fiber to look, and what to look for. I can't think of many circumstances where you would ever need to replace a fiber run instead of just cleaning up the ends.


Even if you put a new fiber into the ground, you're going to need an expert to do it, so what I would do is find an expert that's willing to do an evaluation of your current condition, and apply the charge for that diagnostic session to reparing/replacing the line, if necessary.

No offense, but you aren't qualified to evaluate or replace the line. Neither am I. That's why God invented contractors. Follow all the standard precautions for hiring a contractor, like getting references and asking about their work on similar setups. If everything is looking favorable, ask them to come in for a meeting, and make sure someone in the meeting on your side is a good judge of character. That person may be you. Whoever it is, go with their gut.

Once you've got your contractor, trust them, because they're the expert brought in, but don't be afraid to assert your needs. Their job is to do the job satisfactorily such that you can do your job.


Disclaimer : I'm no fiber expert. But I would say get a second contactor in there to do another re-evaluation. Long run the cost of that if they can find the issue will be cheaper then re-running the line so its worth the expense. Also what he says doesn't make sense even to someone with limited knownledge as myself. If they have good equipment, which a reputable guy should, they should be able to determine exactly how far out the issue is. What about the guys that did the initial install? Are they available for hire?

Did this fibre run only have the minimum number of fibre threads you needed? Many places I have checked out (ie Data Centers) typically run fiber with extra glass that not hooked up and just hanging there. The purpose being if something happens to the in use ones, you can hook up a new set, or you need more capacity. Do you have that now? If not when/if you run a new line you might want to do that.

I don't entirely belive what they say about the signal loss from splices, they run fiber lines across the ocean, needing to pull the cables up and splice them for repairs after a shark chomps on them and they work fine. Sure the cable itself is heavier but at the core the fibre itself "should" be the same. But like I said, I'm no expert.

  • He said they're using multimode fiber, and a 1km run is somewhat marginal. For instance, the wikipedia entry states the maximum run is 600m. I personally have seen longer than 1km runs work, but I would have specified only singlemode for those runs. I agree about the multiple strands, typically the armored fiber bundles come in multiple pairs because nobody pays to dig the trench and only runs 1 pair. But, it's possible the parent bought a pair from someone who had extra. Not enough information to say what's really going on here...
    – chris
    Aug 9, 2009 at 3:12
  • Like I said I never claimed to be an expert :) Hopefully between everyone he'll have enough information get it working. Aug 9, 2009 at 7:48

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