[j-nsp] acceptable/good laser receive power in case of different interfaces

[Keegan Holley]

2011/8/2 Martin T <m4rtntns at gmail.com>

> What is the acceptable Rx power in case of SFP/XFP? For example, here
> are XFP Tx and Rx signals from six FXP's:
>
> 1:
> Laser output power                        :  1.2920 mW / 1.11 dBm
> Laser rx power                            :  0.0285 mW / -15.45 dBm
>
> 2:
> Laser output power                        :  0.6420 mW / -1.92 dBm
> Laser rx power                            :  0.3054 mW / -5.15 dBm
>
> 3:
> Laser output power                        :  0.4230 mW / -3.74 dBm
> Laser rx power                            :  0.5092 mW / -2.93 dBm
>
> 4:
> Laser output power                        :  0.4180 mW / -3.79 dBm
> Laser rx power                            :  0.4208 mW / -3.76 dBm
>
> 5:
> Laser output power                        :  1.0920 mW / 0.38 dBm
> Laser rx power                            :  0.1801 mW / -7.44 dBm
>
> 6:
> Laser output power                        :  0.7680 mW / -1.15 dBm
> Laser rx power                            :  0.3337 mW / -4.77 dBm
>
>
> Is there some sort of pattern? It looks like if the Rx signal is
> lower, the Tx is higher? And what can one consider a decent Rx laser
> power level?
>
>
>
I don't think anyone explained this explicitly but the output power is just
that.  The power of the output laser or led in the sfp local to the box,
unhindered by cabling distances, attenuation or anything else.  They should
be high compared to the the receive number and relatively constant across
like sfp's (usually).  They will vary based on sfp type single-mode vs.
multi-mode, and distance rating LR, ER, SR etc.  There really isn't a
perfect number for every application.  For example if you take two sets of
single-mode LX optics and plug one set into a 5km dark fiber run and the
other into a 3m jumper your receive number will be higher on the shorter
cable because less of the signal is lost to fiber distances.  Vendors will
publish what is acceptable for their equipment, but even then variation
within those numbers could be a cause for concern.  For example if you come
across a link that was -2 or -3dbm one day and suddenly jumps to -6.  -6
might be within the acceptable range for the vendor and sfp, but chances are
the fiber is dirty and taking errors.  Someone may have been walking around
with your new jumper (true story) in their pocket unprotected and then
installed it without cleaning or caring.  Light levels are truly relative,
however low light levels will usually be accompanied by other problems such
as errors and/or upset customers.

HTH,

Keegan