[c-nsp] Interpreting DOM outputs

[Anton Kapela]

Rob,

> I have few LX SFPs with DOM but I'm unsure if I reading outputs correctly.
[..]
> Tx Power '-4.9' better than '-6.9' (i.e. signal is stronger if TX
> Power is '-4.9' comparing to '-6.9')
> Rx Power '-9.6' is btter than '-11.2' (i.e. signal at receiver in case
> of '-9.6')

LX can vary anywhere from -7 ish (sometimes I've seen things out of
the box at -9 via DOM/DDM), up to as much as -4 (near what you're
apparently being shown here). DDM is 'best-effort' in the sense the
measurements accuracy is not terribly high. Its precision, however,
can be good enough that over time, you can tell if things are heading
south for either modules or fiber paths. Events such as bends,
macrobends, patch panels getting disturbed, cuts/breaks, etc. will all
reveal themselves as relative changes to these measurements.

That is, these measurements are best-used as a referential figure, not
absolute -- meaning you ought to start polling & storing them now for
the most utility to be found in troubleshooting later. ;)

-Tk