Decision summary
Digital optical monitoring—often called DOM or DDM—reports operating telemetry exposed by the module management interface. Typical fields include module temperature, supply voltage, transmitter bias and Tx/Rx optical power, plus alarm and warning flags. A normal snapshot is useful, but it does not prove that a link is error-free or stable. A complete review connects telemetry with host status, channel loss, traffic, FEC counters and time.
The available fields depend on the module family and interface. SFP diagnostics are described by SFF-8472, four-lane QSFP management by SFF-8636, and newer high-lane-count modules commonly use CMIS. Reports should identify which interface and revision were read rather than presenting unrelated screenshots as equivalent evidence.
How to read a DOM snapshot
- Temperature and voltage: compare with the exact model specification and the host environment. One value at idle does not represent a loaded chassis.
- Tx power and bias: review per lane where available and look for unexpected imbalance or drift; do not apply one universal threshold to different PMDs.
- Rx power: compare with the receiver range and the calculated or measured channel loss. Connector contamination and patching changes can alter the value.
- Warnings and alarms: retain both the measured value and the module's threshold status. A cleared flag does not replace a stability test.
Minimum BER/FEC report fields
| Field group | Required record |
|---|---|
| Identity | Host, port, software, module part/serial/revision and both link endpoints |
| Link | Speed, full PMD, lane mode, breakout, fiber/channel and active FEC |
| Stimulus | Traffic generator or PRBS pattern, offered load, frame sizes and loopback/topology |
| Time | Start/end timestamps, duration, counter clear point and interruption notes |
| Results | Pre-FEC or raw BER, post-FEC or effective BER, corrected and uncorrectable blocks, per-lane detail when available |
| Telemetry | DOM/CMIS snapshots at start and end, ambient/host condition and raw command output |
Interpretation rules
Do not merge pre-FEC and post-FEC BER. FEC may correct errors that are visible in raw measurements, so the active FEC mode and both counter types are part of the result. A zero displayed counter also needs a duration and counter-clear timestamp; otherwise it has no meaningful exposure window. Where a host provides only corrected/uncorrectable block counters, record that limitation instead of inventing a BER value.
Trend data is stronger than a single screenshot. Capture values under representative traffic and temperature, observe lane balance, and retain the raw output alongside any summary. If DOM appears normal but BER or uncorrectable blocks rise, inspect the channel, FEC configuration, endpoint PMD, connector condition and thermal loading before assigning a module failure.
Primary references
- SNIA SFF-8472 Rev. 12.5a — diagnostic monitoring for SFP-class modules.
- SNIA SFF-8636 Rev. 2.12 — QSFP management interface and monitoring fields.
- OIF CMIS 5.0 — common management interface for newer modules.
- NVIDIA UFM telemetry field definitions — raw/pre-FEC and effective/post-FEC BER terminology.