Which module is triggering the optical port LOS alarm

An optical port LOS (Loss of Signal) alarm is typically triggered by the module that is not receiving an optical signal, often the transceiver (SFP/SFP+) or the upstream optical line terminal (OLT) port.

Understanding LOS Alarms

A Loss of Signal (LOS) alarm occurs when an optical module detects no incoming optical signal. This can be caused by:

• Fiber cuts or breaks along the link
• Disconnected or improperly seated transceivers
• Transmitter failure at the upstream module
• Severe signal degradation below the receiver sensitivity threshold (e.g., -28 dBm for GPON Class B+ modules) In OTN systems, LOS is classified at the OTU layer, indicating that the Optical Transport Unit cannot detect any valid optical signal, which may propagate alarms to higher layers .

Determining the Triggering Module

1. Check the LOS Indicator on the Module: The module showing the LOS alarm is usually the one not receiving the optical signal. For example, if an ONU's LOS LED is blinking red, it indicates the ONU is not receiving light from the OLT .
2. Inspect Fiber Connections: Verify that the fiber patch cords are intact, connectors are clean, and there are no severe bends or breaks. A faulty fiber can cause the LOS alarm to appear on the receiving module .
3. Measure Optical Power: Use an Optical Power Meter (OPM) to measure the incoming light at the module. If the power is below the receiver sensitivity, the LOS alarm is triggered at that module .
4. Check Upstream Modules: If the receiving module is fine, the issue may originate from the transmitting module (e.g., OLT or EDFA Tx port) due to shutdown, misconfiguration, or hardware failure .
5. Alarm Correlation in Network Devices: In Cisco NCS platforms, alarm correlation algorithms suppress lower-priority alarms, allowing the network administrator to focus on the root cause module. The LOS alarm is typically active on the port where the signal is lost, while other alarms may be suppressed .

Summary

The module triggering the LOS alarm is generally the receiving optical module that detects no signal. This could be:

• ONU SFP/SFP+ module
• OTN OTU port
• Cisco NCS line card or transceiver port To isolate the root cause, inspect the fiber, measure optical power, and verify upstream transmitter functionality. Alarm correlation tools in network devices can help identify the primary module responsible for the LOS event .