DDM levels & thresholds in practice

DDM/DOM is the primary diagnostic tool for an optical link — and it has well-known limits that catch people out. Knowing them turns "the levels look fine" into an actual diagnosis.

Where thresholds come from

The High/Low Alarm and Warning thresholds are programmed by the module manufacturer into the module's EEPROM (the A2h block on SFP-class modules, per SFF-8472). The switch only reads and compares against them. A module with generous thresholds will never raise an alarm on a link that is, in fact, marginal.

Rx is real, Tx often is not

• Rx power is a genuine measurement of light arriving at the receiver.
• Tx power on many inexpensive modules is a fixed constant written to memory, or simply reported as n/a. Do not treat a steady Tx figure as proof the laser is fine.

"In range" does not mean "healthy"

A textbook case: a 30 km dark-fibre span run on 10 km-rated optics worked for a year without a single DDM alarm — while quietly dropping packets the whole time. The receiver was living at the edge of its sensitivity; the thresholds simply were not set where the trouble was.

Reading the numbers

• The more negative the dBm, the weaker the signal: −20 dBm is far weaker than −5 dBm.
• Asymmetry is a clue. If Rx is low on one end but normal on the other over the same fibre, the problem is a module or connector cleanliness on the weak side — not the path.
• Compare against the installation record: Tx/Rx noted when the link was first brought up, ideally alongside an OTDR trace.

Commands by platform

Monitor, don't just check

• Poll optical levels over SNMP and graph them — a slow downward drift in Rx is visible weeks before a failure. LibreNMS is a frequently recommended choice.
• Record a link passport at installation: Tx/Rx on both ends plus an OTDR trace.
• Enable BFD on important links so degradation is detected in milliseconds rather than by user complaints.

In CodingBox

The DDM screen shows live values with the module's own Alarm/Warning thresholds drawn on the charts, polls at a configurable interval and exports the log to CSV — which is exactly what a bench "link passport" needs. For the meaning of each parameter see DDM basics.

Expected windows per module type: Typical values. Turning Tx and Rx into measured loss and margin: Rx power & link budget. What the ±3 dB accuracy and the −40 dBm floor really mean: Accuracy & limits.