News
Most optical network outages are not surprises — they are ignored warnings. A transceiver's Rx power drifts downward by 1 dB over three months. The pre-FEC BER creeps from 10⁻⁶ to 10⁻⁴. A DWDM channel shows 0.5 dB of extra insertion loss after a maintenance window. None of these trigger an alarm on their own. Together, they are a countdown to a link flap at 3 AM. Here are the metrics to monitor — and the thresholds that matter.
| Metric | What It Tells You | Warning Threshold | Critical Threshold |
|---|---|---|---|
| Tx Optical Power | Laser aging, driver circuit health | −1.5 dB from baseline | −3 dB from baseline |
| Rx Optical Power | Fiber loss, dirty connectors, bends | −2 dB from baseline | −4 dB from baseline |
| Pre-FEC BER | Signal quality before error correction | 5× baseline value | 50% of FEC limit |
| Post-FEC BER | Corrected errors (link health) | Any non-zero value | >10⁻¹² (degraded) |
| Laser Bias Current | Laser end-of-life indicator | +15% from baseline | +30% from baseline |
| Module Temperature | Cooling failure, airflow blockage | >65°C (commercial) | >75°C (risk of shutdown) |
Post-FEC BER tells you the link is clean — or already broken. Pre-FEC BER tells you it is about to break. A modern coherent transceiver reports pre-FEC BER continuously via streaming telemetry. When it rises, the cause is almost always physical: a dirty connector, a micro-bend in the fiber, or aging laser output.
The problem: most NMS dashboards only alert on post-FEC BER, because that is what the routing protocol sees. By the time post-FEC errors appear, the pre-FEC BER has been degrading for weeks. Monitoring pre-FEC BER with a trend line catches degradation when the link still has 2–3 dB of margin — enough time to schedule a maintenance window instead of responding to a P1 incident.
Rx power degradation is the number one cause of optical link failures, and it is almost always a connector problem. A single dirty LC connector can add 1–2 dB of loss. Two dirty connectors — one at each end — add 3–4 dB, burning through your entire safety margin.
Best practice: baseline every link's Rx power at install, then set a −2 dB warning threshold six months later. The six-month window allows for initial connector settling. After that, any −2 dB drop is a real degradation that needs investigation.
As a laser ages, it requires more current to produce the same optical output. The transceiver's control loop increases bias current automatically to compensate — but there is a limit. When bias current reaches +30% above the factory baseline, the laser is nearing end of life and should be proactively replaced.
Most operators never check this metric until the link goes dark. A simple trending report — bias current vs baseline, sampled weekly — catches degrading lasers months before failure.
Practical monitoring setup: Pull Tx power, Rx power, pre-FEC BER, laser bias current, and module temperature from every transceiver every 15 minutes via streaming telemetry. Trend them over 90 days. Alert on slope — a 1 dB drop over 90 days is more actionable than a −2 dB absolute threshold that fires at 3 AM. The slope tells you it is time to schedule maintenance; the absolute threshold tells you it is already an emergency.
APEX Group transceivers expose full DOM (Digital Optical Monitoring) via standard I2C/MDIO interfaces — Tx/Rx power, bias current, temperature, voltage, and both pre-FEC and post-FEC BER — compatible with all major NMS and streaming telemetry platforms for proactive optical-layer monitoring.
APEX GROUP — www.apexallinone.com
