[OptiX OSN 1800V] SPAN_LOSS_EXCEED_EOL on DFIU board Highlighted

Hi guys!!

I would like to share with you an issue about the SPAN_LOSS_EXCEED_EOL alarm.

Description

SPAN_LOSS_EXCEED_EOL (Span fiber loss exceeding EOL) alarm is generated when the actual fiber loss is larger than the EOL value.

Condition:

The customer has a fiber span between 2 NEs in O&M:

SITE 1 -> OADM -> OSN 1800V

SITE 2 -> OADM -> OSN 8800 T32 + 4 x OSN8800 Universal Platform Subrack (one of the UPS is from/to SITE 1)

The design of the link and equipments is as follows:

Symptom:

The SPAN_LOSS_EXCEED_EOL alarm appears on the DFIU board in the SITE 1. This alarm indicate that span fiber loss exceeding EOL value in the unidirectional span from SITE 2 to SITE 1:

The customer sended an "alarm report":

and Optical Doctor (OD) "Setting Fiber Parameter" information:

The customer indicates that reflectometric measurements (with OTDR) and power measurements (with TX/RX optical power meter) have been performed on both unidirectional spans.

As a result, the customer don't detect any failure event in the fiber, the losses are identical and the distance is the same on both unidirectional spans.

Finally, the customer wants to know why there is a difference of 6.5dB between the current attenuation of both unidirectional spans.

Impact:

If the line loss is excessively large, service signals may be unstable and BER may increase. Then an alarm indicating excessively low path optical power may occur and the performance may be abnormal.

The customer doesn't detect any degradation in the services through this link.

 Severity:           

Mayor

Root Cause:   

1. Check whether the line fiber connections are normal, whether the fibers are aged, and whether the fiber connectors are clean.

The customer checked it and everything was OK.

2. Check whether the insertion loss of the board is normal.

It is verified, after carefully studying the unidirectional span from SITE 2 to SITE 1, that the designed loss (EOL) are incorrectly calculated.

In this case, SITE 1 / OSN 1800V has a DCM (Dispersion Compensation Module) installed in the input OBU, as shown in the following figure:

DCM is a passive element that is used to compesate for positive dispersion thus helping to maintain the shape of the signal. DCM isn't manageable, so it doesn't appear in the NCE. As a result, the Optical Doctor doesn't take into account the insertion loss introduced by DCM when calculating the current attenuation.

NOTE: DCU (Dispersion Compensation Unit) is a kind of board that has the same funcion as a DCM module, but is manageable by the NCE and as a result Optical Doctor (OD) takes its insertion loss into account when calculating the current attenuation in the unidirectional span from SITE 1 to SITE 2.

As HedEx indicates (OSN 8800 Product Documentation : Commissioning (U2000) > Configuring NE and Network (Network Management Center), when a DCM is installed, the designed loss (EOL) should be calculated as follows: EOL = Design fiber loss + Maximum insertion loss of the DCM.

Therefore, the span has a distance of 56.673 Km and DCM module will introduce insertion loss of 6.4dB, as shown in the following table (from HedEx):

As a result, the correct designed loss (EOL) for unidirectional span from SITE 2 to SITE 1 are:

EOL = Design fiber loss (0.25 db/Km x 56.673 Km + 3dB) + Maximum insertion loss of the DCM (6.4dB)

EOL = 17.1dB + 6.4dB = 23.5dB

Solution:

The value of designed loss (EOL) is modified in the NCE:

and SPAN_LOSS_EXCEED_EOL alarm is cleared:

The difference between the designed loss (6.4dB) and the difference between the current attenuations in both unidirectional spans (6.4dB approx.) is symmetric.

It's confirmed that it was a configuration mistake in the NCE manager and not a problem in the span.

Now the customer knows that the difference of 6.4dB is due to the insertion loss of the DCM module.

Conclusion:

Issue solved.

The end!