[c-nsp] Possible causes for fiber link flap?

Peter Rathlev peter at rathlev.dk
Thu Oct 10 03:42:36 EDT 2013

On Wed, 2013-10-09 at 13:57 +0100, Phil Mayers wrote:
> On 09/10/13 10:31, Peter Rathlev wrote:
> > within tolerances. The total link loss (according to DOM) is 0.6 dB in
> > one direction and 2.4 dB in the other direction. Budget should be at
> > least around 8 dB.
> As others have suggested, this seems like a bad fix to me. How sure of 
> the OTDR trace are you?

Our own OTDR equipment is sometimes unrealiable (e.g. cannot see the 1km
delay line sometimes) and we have not yet received full data from the
splicing team. They're just saying it "looks good". :-)

We have two different runs between the two sites in question and we will
now try swapping these to see if the problem follows the fiber strands
or the equipment.

> There could be a micro-bend; we've seen similar things where long fibre 
> runs get fiddled with at some mid-point, and are then very sensitive to 
> physical movement (as the nudge puts the micro-bend "over the edge").
> Or just a plain bad splice!

We'll ask the splice team to open up the vault and take a look again.
Hopefully it's something simple like a bended cable. :-)

The frequency of events is rising by the way:


The graph is event time on the X-axis and distance to last event on the
Y-axis. A linear regression (management types like those, right?) shows
that the link will be unusable around next midnight CEST. ;-)


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