ONS 15454 LOS-O minor alarm problem

Question

Hello everybody, wish you a nice day!
I have 2 ONS 15454 at 2 site in linear mode.
ONS 1 (side A) -------- line 1 -------- ONS 2 (side B)
ONS 1 (side B) -------- line 2 -------- ONS 2 (side A)
3 days ago, I got the LOS-O alarm. I have followed the instruct from troubleshooting guide but could not clear this alarm.
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(In this firgure, dont care about side A, my trunk line is cut . The problem is on side B, it pops some minor alarm but the connection is still ok)
ALARM:
You can see that there are: SPANLEN-OUT-OF-RANGE and LOS-O. The problem is the power on line RX on OSC-CSM card is below the configured threshold, right? I have monitored the power and the value is -19.5 dbm.
Then I go to Provisioning --> WDM-ANS --> side B --> Rx --> Threshold.OSC LOS is imported as -23.5 dbm.
I also checked the span loss, the measured spanloss is 15.2 (resolution 1.5). It is between the Min exp span loss (15) and Max exp span loss (17).
So what is the problem, please help me to clear this alarm. Thank you very much.

Answer 1

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You are right. the alarm is shown as the power is below the threshold.
If we have OSC LOS threshold set to -23.5 and the total Rx power is Rx power is about -23.5 (on the 40 km span) then we definitely below the threshold as we need to subtract the payload power from the total to get OSC.
Average loss on a fiber is 0.3dB per km, hence the expected loss on 40km would be around 12dB. I doubt the OS Tx power on the remote end is -11dBm (-23 + 12) so most likely that is the fiber/connector issue.
Swap Line Rx and Tx fibers on both switches on OSC-CSM card to verify if the problem follows the fiber or stay with the cards.
Regards,
Alex.