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enter image description here My network at two points, A and B, have the same connection devices that have been working well until recently. A (A is the internet entry point) transmits internet to B (B is the data centre) and B distributes to other locations, ie. C, D, F,G. A and B are connected by fiber using an OLT.

I never noticed before, but since it stopped working, I only see three lights under TX blinking at both point A and B.

Obviously the two devices are not communicating, but does it mean one of them is broken?

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    Hi Masssly and welcome. How long are the fibres between A and B? My first guess would be damage to those, before suspecting the devices. What is the manufacturer and model? – jonathanjo Oct 23 '18 at 17:57
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    Having dealt with more than 3000 fiber converters, i can surely write, that it is common for converters to stop working suddenly. – user36472 Oct 23 '18 at 18:07
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    I suspect your environment is better regulated than most of mine, which are usually temporary sites, where things tend to break because of feet, tools, and similar -- which means it's basically always a cable of one kind or another. 3000 is a lot of experience, do you know what part of them tends to fail? – jonathanjo Oct 23 '18 at 18:17
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    @jonathanjo 100BASE-FX only uses LEDs which can last for ever - but they can break, too. Burned-out receivers should be very rare due to the low power involved. – Zac67 Oct 23 '18 at 18:21
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    Well, the most errors i've seen was after thunder storms. But i have tried to turn off and turn on a converter at least 20 times, where it did not work afterwards. 3000 is also over a span of around 12 years. – user36472 Oct 23 '18 at 18:39
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As it seems, the fiber link is down. There is no link light for 100BASE-FX on either side as you wrote. Note that usually, both sides' link lights go dark when one of the converters dies or one of the fibers fail.

Possible causes:

1) One of the converters died.

Check out with a spare converter or any other device speaking 100BASE-FX. For a quick test, disconnect the 100BASE-TX side from the network (unless you've got STP active) and use an SC-SC patch cable to connect a converter to itself, it should link with itself. (Some converters require that the 100BASE-TX is linked though.) Looking at the transmitter side even through a digicam is useless due to -FX's long 1300 nm wavelength.

2) The fiber is broken

If a spare/test device links on both converters but not across the fiber, the latter is disrupted. Check the patch cables for damage or tight bends - single-mode is especially sensitive to an undercut bend radius.

Generally, it's a good idea to use pluggable transceivers in managed switches. If you use SFP transceivers with DOM (digital optical monitoring) in a managed switch you can monitor the transmitted and received power on the link.

While you're at it you might want to go for 1000BASE-LX (it looks like you've got single-mode fiber). You might also want to go for 1000BASE-BX and run two links aggregated with failover capability - there are many very affordable 3rd-party transceivers around. -BX is also a solution when one fiber has failed but can't be replaced and the other still works.

  • Option 1 tried, and as shown below, TR and FX are lit up. i.imgur.com/8M244hl.jpg Any advice on how to check if the fibre is broken? – Masssly Oct 24 '18 at 10:22
  • The crude test is to shine a bright flashlight (or a simple fiber tester) into one end - with just 100 m you should be able to see something. A more real test is to run another fiber in parallel and use it as loopback or as 2nd link fiber. Or use bidi transceivers with each fiber alternately. – Zac67 Oct 24 '18 at 11:24

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