Services are unavailable because the FEC mode configured on the local 40G regeneration board is inconsistent with that configured on the opposite regeneration board.
Symptom
OptiX BWS 1600G stations A, B, C, and D using NE software version 5.8.7.12 form an end-to-end chain network. Stations A and D are OTM stations configured with the TMX40S boards. Stations B and C are REG stations configured with the LUR40S boards. Two wavelengths (196.0 GHz and 195.9 GHz) are added and dropped at stations A and D and regenerated at stations B and C.
Not all optical fibers between station A and station B and those between station C and station D are connected but the optical fibers between station B and station C are connected. Therefore, links between station B and station C are commissioned. EVOAs and V40s are configured on the main optical paths along all the links. During commissioning, Laser Status is set to Open for the LUR40S boards. After the optical fibers are connected, the links between station B and station C are remotely commissioned. The commissioning result shows that the bit error rates (BERs) before correction of the two wavelengths received at station B are normal; performance of wavelength 195.9 GHz received at station C is normal and the BER before correction is 1E-10. Performance of wavelength 196.0 GHz received at station C is abnormal and the BER before correction is 1E-2.
The value of FEC Type for the LUR40S board configured with wavelength 196.0 GHz changes between FEC and AFEC on the NMS. The value of FEC Type changes frequently for a certain period, and then is always displayed as FEC. At this time, services are unavailable and the system reports an OTU_LOF alarm.
Cause Analysis
Many factors, such as dispersion adjustment, polarization mode dispersion (PMD), adjustment of receiver thresholds, and optical power fluctuation, can affect the performance of a 40G system. Therefore, the LUR40S board uses the FEC auto-sensing function. That is, if the system performance under the current FEC mode cannot satisfy the system requirements, the LUR40S board automatically changes its FEC mode. However, in certain scenarios the FEC mode cannot be automatically changed.
In the network, wavelengths 195.9 GHz and 196.0 GHz received at station C have the same routes and similar optical power. However, their BERs before correction are different. The cause is that FEC Type is set to FEC for the LUR40S board configured with wavelength 196.0 GHz at station C but is set to AFEC for all other LUR40S boards. The reasons for different FEC modes are described as follows:
- An LUR40S board switches the FEC mode based on the service access status at the IN optical interface on the WDM side.
- When two LUR40S boards are connected in a single direction, for example, from the OUT optical interface on the local board to the IN optical interface on the opposite board, the switching process of the FEC mode is as follows (assuming that both boards work in AFEC mode):
- When no service is received at the IN optical interface on the local board, the board switches the FEC mode at the OUT optical interface from AFEC to FEC.
- After finding that no service is available, the opposite board switches the FEC mode at the IN optical interface from AFEC to FEC.
- The local and opposite boards switch their own FEC modes repeatedly even when a service is available at a moment. The local board adapts and regenerates only the upstream service but does not monitor whether the downstream service is normal. Therefore, to commission a service by interconnecting two regeneration boards, you need to disable the FEC auto-sensing function, that is, uniformly set FEC Type to AFEC.
Procedure
- In the NE Explorer, select the LUR40S board. Then choose from the Function Tree.
- Select By Board/Port(Channel).
- On the Basic Attributes tab, query and ensure that Service Type is OTU2e (11.1G) for all LUR40S boards.
- On the Basic Attributes tab, query and ensure that FEC Type is AFEC for all LUR40S boards.
- Then the problem is resolved.
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