DDM basics

Digital Diagnostic Monitoring (DDM), also called DOM, lets a transceiver report its own operating condition in real time. CodingBox reads these values on the Check transceiver screen.

The four core parameters

• Temperature — the module's internal temperature, in °C.
• Supply voltage — the 3.3 V rail as measured inside the module.
• Laser bias current — the drive current of the transmit laser, in mA.
• TX / RX optical power — transmitted and received light power, in dBm.

Warning and alarm thresholds

Each parameter carries factory-set thresholds. Values are classified against them:

Reading RX power

Received optical power is the most useful field for diagnosing a link:

RX power = -3.2 dBm   → healthy
RX power = -18 dBm    → near receiver sensitivity, expect errors

A low RX power usually points to a dirty connector, a bent fibre or too much distance — not necessarily a faulty module.

Going deeper

• DDM parameters & units — raw encoding and conversion to °C, V, mA, dBm.
• Thresholds, alarms & flags — where limits live and what the host acts on.
• Internal & external calibration — why some hosts show absurd values.
• Per-lane diagnostics: QSFP & CMIS — multi-lane monitors and VDM.
• Monitoring DDM in production — SNMP, link passports, trending.

Further reading

• Typical values by module type — what Tx, Rx, bias and temperature should look like for SX/LX/SR/LR/ZR, 100G, 400G, PON, AOC.
• Accuracy and limits — ±3 dB, average vs OMA, floors, where DDM is absent or fake.
• Rx power and the link budget — measured loss, margin, both directions, overload, penalties.
• Tx bias and laser ageing — the monitor that predicts failure.
• Temperature and voltage — what they change and what they reveal.
• VDM and FEC metrics — pre-FEC BER, eSNR and host FEC counters on PAM4 links.
• Reading DDM with tools — CLI, ethtool, SNMP OIDs, OpenConfig paths, decoding a raw dump.