Monitoring DDM in production

Reading DDM once tells you a module's state; trending it tells you its future. Most optical failures announce themselves weeks in advance — as a drifting Rx power, a climbing bias current or a rising pre-FEC BER — to anyone who is recording the numbers.

Bench, CLI or NMS

Collecting over SNMP

Most platforms expose DDM through the standard ENTITY-SENSOR-MIB (entPhySensorValue, with type/scale/precision per sensor) and through vendor MIBs (Cisco entity sensors, Juniper DOM MIB, Arista, Huawei). Open-source NMS such as LibreNMS auto-discover optical sensors and graph them; Zabbix and Prometheus/Grafana work with the same OIDs or with streaming telemetry. Poll every 1–5 minutes — DDM values change slowly, and modules answer the two-wire bus at a leisurely pace.

The link passport

At installation, record for both ends of every link:

• Tx power, Rx power, bias current, temperature (the DDM set);
• module vendor / part / serial;
• fibre length and, where available, an OTDR trace.

This baseline is what later readings are compared with. Without it, "Rx is −12 dBm" is just a number; with it, "Rx has dropped 2.5 dB since March" is a work order.

Alerting: deltas, not just thresholds

Module thresholds are the maker's limits (Thresholds & alarms); your alerts should be tighter and relative:

Reacting quickly

• BFD on important links detects a degraded path in milliseconds, long before routing protocols or users notice.
• Track per-lane values on multi-lane modules (Per-lane diagnostics) — a single lane failing is the usual AI/DC failure mode.
• When a threshold trips, follow the diagnostic ladder: levels → clean → swap → OTDR.

What DDM cannot tell you

• Tx power on cheap modules may be a constant (DDM levels).
• DDM sees light, not bits: a link can have perfect levels and still error from dispersion, FEC mismatch or a polarity fault — pair DDM with error counters.
• Resolution is 0.1 µW: nothing below −40 dBm is measurable.

In CodingBox

CodingBox is the bench side of this process: a configurable polling interval, a measurement log and CSV export produce the link passport before installation and let a pulled module be compared with its own history in the code database.

Collection details — CLI commands, ethtool, ENTITY-SENSOR-MIB and vendor OIDs, OpenConfig paths: Reading DDM with tools. What to alert on for PAM4 links: VDM & FEC metrics.

Vendor-by-vendor SNMP objects, OpenConfig telemetry paths and syslog messages that carry the values discussed here: Management & monitoring.

Slow plant faults that trends reveal — contamination, closure ingress, seasonal swings — and the triage order once a trend breaks: Fibre plant faults.

DDM in the context of the whole active infrastructure — amplifier telemetry, OSC, OCM, RFTS, alarm hierarchies and correlating module data with fibre data: Monitoring & management.