The LWF board reports R_LOS because the incorrect fiber type leads to dispersion over-compensation.
Product

OptiX BWS 1600G
Fault Type

Dispersion Abnormity
Optical Transponder Unit
Fiber
R_LOS
R_LOF
Symptom

In a national backbone WDM network, the OptiX BWS 1600G is used. A, B, C, and D are the OTM, OLA, OADM, and OTM stations respectively. The distance between A, B, C, and D is more than 100 km. The 120 km dispersion compensation module (DCM) is added between C and D. A 100 km DCM is configured at the transmit end and a 20 km is configured at the receive end. The OTU boards that are configured at stations A, C, and D are the LWM, LWF, and LQG. The OTU is forced to emit light. After the system commissioning is complete, the LWF boards at stations A, C, and D report R_LOS on the WDM side.
Cause Analysis

The per-channel optical power is very low or no light is received.
The multiplexer/demultiplexer device is faulty, which causes wavelength drop.
The LWF board is faulty.
The OSNR does not meet the design requirements.
There is problem in the dispersion.
Procedure

The LWF boards at stations A, C, and D report R_LOS. Hence, the cause cannot lie in a certain board. Use an optical power meter to measure the optical power at the IN interface of every LWM board. The optical power is about -8 dBm, which meets the requirement. Hence, the problem is not caused by the LWF, D40, M40, or MR2.
In the same system, the problem does not occur on the LWM and LQG at 2.5 Gbit/s. Because the board at 10 Gbit/s has different OSNR and dispersion requirements from the board at 2.5 Gbit/s, the problem must be caused by a dispersion problem.
Remove the 20 km DCM at the receive end at station D. The alarm persists. Then, remove the 100 km DCM at the transmit end at station D, the alarm that is reported at station C changes to R_LOF. Remove the DCM at station C, the alarms on the corresponding LWFs at stations A and D change to R_LOF. Hence, the problem lies in the dispersion. Remove the 100 km DCM and the R_LOF alarm persists. The dispersion to be compensated that is obtained during the design phase cannot be more than 100 km. Check the fibers that are used on site. The G.655 fibers are used. The G.652 fibers, however, are used during the design phase. Hence, replace the fibers between station A and station D. Then, the R_LOS on the WDM side of the LWF is cleared and the problem is solved.
Reference Information

None.