Optical transceiver question

Wed Sep 7 20:23:30 UTC 2016

We recently purchased some generic optics from a reputable reseller that
were marketed to reach 60 km.

But what we found, based on the spec sheets, is that it could only reach
that distance if the optics were transmitting on the high side of the
transmit power range. 

For example, if the TX range was 0 to +5 dBm and minimum RX power was -20
dB, the designed optical budget should be no more than 20 dB (0 - -20).
Based on the wavelength the appropriate loss would be 0.4 dB/km and results
in only 50 km, not 60 km.  To get 60 km it would need 24 dB of link margin,
and that would only be attainable if it was transmitting on the high side,
at +4 dBm.

Is it an industry practice to market distance based on the hot optics, not
on the worst case, which is minimum TX power?

Frank