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[Weaver]

Yes, I know there presumably isn’t effectively such a thing as SNR if you are using a fibre link.

My question, if you have dodgy fibre, say a bad splice or damage to fibre optic link or who knows what, just something not right which means that you have a non-negligible bit error rate: what metrics are there for the link? What kind of numbers can be seen to diagnose the physical layer?

[psychopomp1]

Fibre light loss is to be expected at various points in the FTTP (GPON) network though I don't think Optical SNR is the terminology used to describe the light loss. When i had FTTPoD installed, as part of the final testing phase I saw the Openreach bod go around with his meter gadget measuring light loss at the external CSP, DP, splitter and agg nodes. There is a certain tolerance Openreach work to, if the light loss exceeds those then they have to re-splice the connection. No idea on what the actual numbers are, sorry! 

[jelv]

Does excessive light loss cause errors or is it the case that it either works or stops working altogether?

[psychopomp1]

Does excessive light loss cause errors or is it the case that it either works or stops working altogether?

It simply works or it doesn't, nothing in between

[aesmith]

I've seen errors if the fibre is "bad", but I'm not complete sure that these were caused by loss of signal rather than dispersion or reflection etc.   What fibre services don't so is negotiate different speeds based on the quality of the signal.