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I live in a dorm where the network is mostly resident-managed and budget is limited.

The network is a basic star topology, with a central switch in the server room branching out to switches that feed two hallways each, with no redundancy. The links from the server room to the hallway switches are fiber optical runs, running at 1 Gbps.

Yesterday, one of the links suddenly degraded heavily, and begun intermittently losing large amounts of packets. The link still somewhat works, but there are periods ranging from a few seconds to upwards 15 minutes where almost all packets are lost. There are no signs of faults in router or switch software or hardware. Our testing shows it is most likely the fiber run that is failing.

We have tried degrading the link to 100 Mbps, this does not improve the quality. We still haven't finally determined the fiber link to be the fault, but intend to make a final check for that later.

There is no option of running a copper cable.

I suspect that it may somehow be caused by the winter weather, although there shouldn't be a good explanation for this, I assume there is proper insulation where the cable runs. The real question here is: Is cold weather a plausible explanation for this sudden fault, and in that case, can we expect the link to improve again once the weather gets warmer?

Update: It turns out that the fiber pair is in fact fine and it has to be a failing client causing the problem. We should be able to handle that.

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Have you traced the fiber line to make sure someone do something stupid like bend it around a corner or pinch it into a too tight crevice in the wall? – Zypher Jan 7 '10 at 18:56
Everywhere the fiber is exposed is behind locked doors, so nobody without business should have had access. The run has worked flawlessly for years. – user31053 Jan 7 '10 at 19:27
up vote 1 down vote accepted

I've seen fiber do weird things when it fails. Usually though it just drops, period, and won't sync up. You're saying that it just "degrades".

Have you determined that it isn't being flooded with something? It's odd that it works but is just dropping packets. No excess network traffic? Mainly because I wouldn't be surprised seeing as this is a dorm room you're getting degradation from.

Is it possible that winter weather causes something? It's possible. Depends on how exposed the conduit is. Glass is glass, if exposed to extreme temps it could do something odd. Ideally it's not exposed to such circumstances, though.

One thing to try if you're not using VPNs or special network configuration is switch the working fiber on the switch with the non-working one and see if the behavior suddenly shows up on the other dorm while the other one has it go away. That will tell you if it's the fiber connection on the switch and not the fiber run. If it's a Cisco switch with modules you can switch the modules on the dorm side to see if it's the hardware causing issues.

I'd definitely rule out someone trying to use file sharing or torrent software though. Any of that can bog down a switch fairly easily, depending on the hardware.

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It would be very hard for a single room to overload the fiber link, as the links to individual rooms are 100 Mbit. The traffic graphs towards the Internet don't show any irregular traffic patterns. I forgot to mention, both fibres seem affected, traffic in either direction is lost so there is no fallback. It's good to know that the cold may affect it, unfortunately the forecast shows the freezing weather may continue for another two weeks. – user31053 Jan 7 '10 at 19:37
Not if the routing tables are being overwhelmed. We've had that happen with malware that hits hundreds of IP's in a very short span of time. For the fibers, you mean two pair (one to building one, one to building two) are affected, or one building is being affected? It's possible that cold can cause it. But it shouldn't be. If it's that exposed, you're going to probably have trouble over time and it should be replaced with new fiber (better insulated) or a radio link of some kind, depending on the configuration of the site. – Bart Silverstrim Jan 7 '10 at 20:00
Better insulated meaning the conduit path, not some special fiber clad in space age cladding :-) – Bart Silverstrim Jan 7 '10 at 20:01
It's just one fiber pair that is affected, this pair is servicing two hallways in the building. (There is only one building.) The remaining 7 fiber pairs seem to be unaffected. I'm not sure how thorough a malware check has been performed, so another may be in order. – user31053 Jan 7 '10 at 20:30
If your switch supports it, you can mirror traffic to another port. Then monitor traffic through traffic sniffing. You see a lot of weird traffic via all email, or random streams of requests all over with no rhyme or reason, chances are malware or torrents can be contributing. SNMP may help also with gathering stats on how much traffic is going by protocol where and on what port. – Bart Silverstrim Jan 7 '10 at 21:03

If parts of the system are getting very cold, there could be mechanical changes due to thermal expansion. I seem to recall that fiber is very sensitive to alignment issues.

Though I would be surprised to find that an issue with this cause was intermittent.

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