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Noob fibre question, I suspect.

A while ago while there was a trench open between my house and an outbuilding, I took the opportunity to lay a 15 metre length of armoured OM1 fibre (4 core, LC-terminated). I'd intended to use media converters at each end and cat6 for the rest of the infrastructure, the fibre being just to avoid dealing with lightning protection.

Now I'm in a position to finish the link, and I've changed my mind: I've got a switch with SFP ports in the house, and I'm thinking of using a similar one in the outbuilding, so I'm thinking of going fibre all the way. However, this will require a 10 metre extension in the house, and perhaps up to the same again in the outbuilding.

I'm aware that the overall length of the completed link will be perilously close to the maximum length for 10g over OM1 (33 metres). I'd like the option of 10g even if I don't use it straight away.

So, my questions:

  • Can I use OM2/3/4 for the parts of the link in the house / outbuilding and join them to the OM1? (Initial enquiries suggest not because the fibres have different diameters.)
  • What effect do couplers (I'm assuming good quality ceramic ones) have on the link, and in particular on the maximum achievable 10g distance if I'm close to the edge?

Thanks!

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Mixing various MMF fiber grades is generally not a good idea - especially OM1 with anything else. Changing over from one core diameter to another causes a significant signal power loss. Better stick to OM1 and use as few connections as possible.

Very often, you can somewhat or even significantly exceed the reach of a fiber standard, especially with few splices and connectors in between. Your scenario sounds like it's just two LC connects. However, there's no guarantee the link is working without errors.

In contrast to the usually MMF 10GBASE-SR, 10GBASE-LRM runs for 220 m even over OM1 and even over ancient FDDI fiber. Transceivers are slightly more costly but function is guaranteed.

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It is not a binary "It will work at this distance and not work at an inch longer", rather, fiber capabilities are rated at the minimum length they will sustain given throughput at.

Wikipedia lists the types and minimum lengths as follows:

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By adding the additional connectors you are adding more opportunities for problems on the line, so those lengths will be more likely to result in decreases in throughput. For maximum throughput on a given line, you want a minimum of conversions.

However, the line will likely still work, just not at 10Gbps speeds.

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  • Thanks. So OM1 should be able to sustain 10g over up to 33m, but the degradation is gentle beyond that rather than suddenly renegotiating to 1g or anything?
    – cooperised
    Jan 27, 2018 at 9:26
  • And... any idea of the magnitude of effect I should expect from the couplers? Any nice handy rule like "it's equivalent to adding N metres of fibre"?
    – cooperised
    Jan 27, 2018 at 9:27
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    @cooperised: I think the research terms are "insertion loss" and "fibre budget calculator". The coupler's spec sheet might have the former mentioned.
    – u1686_grawity
    Jan 27, 2018 at 13:05

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