---
title: "DOM Deep Dive: What Every Parameter Actually Tells You About Your Link"
slug: "dom-digital-optical-monitoring-guide"
category: "Diagnostics & Monitoring"
tags: ["DOM", "DDMI", "digital optical monitoring", "SFF-8472", "diagnostics", "link troubleshooting"]
seo_focus_keyword: "DOM digital optical monitoring transceiver diagnostics"
word_count_target: 1200
difficulty: intermediate
---

Digital Optical Monitoring — also called DDMI (Digital Diagnostic Monitoring Interface), or simply DOM — is one of the most useful diagnostic tools in optical networking and one of the most underused. Most engineers know it exists and can recite "check DOM" as troubleshooting advice. Fewer can look at a set of DOM values, understand which ones are meaningful in context, and correctly distinguish a transceiver that's about to fail from one that's slightly out of optimal operating condition but stable.

The SFF-8472 standard defines the DOM interface for SFP/SFP+ modules. QSFP and QSFP28 use SFF-8636, and newer CMIS (Common Management Interface Specification) covers QSFP-DD, OSFP, and beyond. The measured parameters are largely consistent across standards: temperature, supply voltage, TX bias current, TX power, and RX power. Here's what each actually means and how to interpret it.

**Temperature**

The reported temperature is measured at the module's internal monitor circuit, not necessarily the optical subassembly or the laser junction itself. It reflects the thermal environment the module's electronics are experiencing.

Normal operating range for commercial-grade modules is 0–70°C case temperature, with the internal sensor typically reading 5–15°C above ambient due to self-heating. A 25°C ambient datacenter environment typically produces internal module temps of 35–45°C. Industrial-grade modules are rated to −40°C to +85°C.

What temperature anomalies tell you: consistently high temperatures (>65°C internal) suggest inadequate airflow in the cage, a cage with blocked front bezel area, or a very high-power module in a thermally stressed chassis position. Temperatures that drift steadily upward over weeks without HVAC changes suggest slow cage blockage or degrading module thermal contact. Temperatures that spike suddenly without environmental explanation can precede module failures — thermal runaway in the laser driver circuit is a failure mode that DOM temperature can catch early.

**Supply voltage**

The supply voltage measurement reads the 3.3V supply rail powering the module's electronics. Nominal is 3.3V; acceptable range is typically 3.135V to 3.465V (±5%).

Undervoltage conditions (supply below 3.1V) cause instability in the laser driver circuits and TX power fluctuations. Overvoltage above 3.465V can damage module components over time. In practice, supply voltage issues usually trace back to the host switch's SFP cage power delivery or a long cable run with voltage drop for active copper or active optical cables.

A supply voltage that's consistently at the low end of spec across all modules in a chassis — say, 3.18–3.20V — and normal at 3.28V for modules in a different chassis is worth investigating. The switch's power supply regulation quality varies by vendor and platform, and some older chassis show supply droop under high module count loads.

**TX bias current**

This is the DC current flowing through the laser diode to establish its operating point. It's one of the most diagnostically valuable DOM parameters because it reflects the laser's actual operating condition.

Laser diodes age. As they age, they require increasing bias current to maintain the same output power. The automatic power control (APC) circuit in the transceiver increases bias current to compensate for reduced laser efficiency. TX bias current that's trending upward — even if TX power remains stable — is an early indicator of laser aging.

Typical bias currents: 10G DFB laser for LR/ER applications runs 40–70 mA nominal. At end of life, bias current may climb to 90–110 mA before the APC circuit can no longer compensate and TX power starts dropping. An SFP+ LR module showing 95 mA bias current when it was 50 mA at installation three years ago has burned through most of its compensation headroom and is a candidate for proactive replacement.

Short-reach VCSEL lasers (used in 850nm SR applications) have different bias characteristics: typically 4–8 mA, lower temperature sensitivity, and different aging profiles. Sudden jumps in VCSEL bias current are less gradual — they often indicate a mode stability issue rather than smooth aging.

**TX power**

TX power is the optical power in dBm being launched from the transceiver's transmitter port into the fiber. This is the most directly actionable DOM parameter for link health.

Each transceiver has specified TX power bounds. A 10GBASE-LR module specifies TX power between −1 and +3.5 dBm. A reading of +2 dBm is nominal. A reading of −4 dBm on that same module is already outside specification and indicates either laser degradation or APC circuit failure.

TX power should be stable over time. Gradual downward drift combined with rising bias current, as described above, is classic laser-end-of-life. Sudden sharp drops in TX power without corresponding bias current changes often indicate contamination on the optical connector face — the transceiver is trying to maintain laser power, but the dirty connector is absorbing or scattering light.

TX power fluctuations — power that varies by more than 0.5 dBm over seconds or minutes — indicate laser instability. This can be thermal (not enough time at operating temperature, first-order thermal stabilization not complete), mechanical (fiber connector not properly seated, cable strain inducing microbending), or electrical (noisy supply rail causing laser driver instability).

**RX power**

RX power is the optical power in dBm being received at the module's input port. This measures what's arriving from the far end after traversing the fiber path.

RX power combined with TX power from the far-end DOM gives you the end-to-end link loss, which you can compare against your expected loss from the link budget calculation. If your calculated path loss is 5 dB and the measured loss (far-end TX minus near-end RX) is 8 dB, something in the fiber path has changed — likely a dirty or damaged connector, a degraded splice, or fiber damage.

Low RX power — below the receiver sensitivity specification — will cause bit errors and eventual link failure. High RX power — above the receiver's input overload level — causes saturation and nonlinear distortion that also generates errors. Both are detectable from DOM before they reach the alarm threshold on the link itself.

**Using DOM to diagnose link issues before traffic impact**

The most valuable DOM workflow is trending, not spot-checking. A single DOM reading tells you the current state. DOM readings recorded over time — daily, or correlated with your monitoring system's polling — tell you trajectory.

Build a baseline for every transceiver in your critical links: TX power, RX power, bias current, temperature, and supply voltage at initial installation. Then monitor for:

TX power declining more than 1 dB from baseline: investigate laser health, check bias current trend.

RX power declining more than 2 dB from baseline with stable far-end TX: check fiber path for new connectors, moved cables, or physical changes in the cable route.

Bias current increasing more than 15 mA from baseline with stable TX power: flag for replacement within 6–12 months.

Temperature increasing more than 10°C from baseline: check chassis airflow and cage blockage.

Supply voltage drifting more than 0.15V from baseline: investigate chassis power delivery.

**Alarm and warning thresholds**

SFF-8472 defines four threshold levels for each DOM parameter: high alarm, high warning, low warning, low alarm. These are programmed by the transceiver manufacturer and accessible via the EEPROM. Most monitoring systems expose only whether a parameter is "in alarm" — but reading the actual threshold values is informative. A TX power low warning threshold set at −4 dBm on a module specifying −1 to +3.5 dBm nominal is a loose threshold that won't warn you until the module is well outside specification. Tighten your monitoring system's alert policy to match the module specification, not just the manufacturer's programmed thresholds (which are often set conservatively to minimize false alarms).

DOM is not a crystal ball. Catastrophic failures — connector fractures, fiber cuts, sudden laser failure from electrostatic damage — don't announce themselves in DOM trends. But the slow degradation modes that account for the majority of transceiver failures leave clear fingerprints. If you're not regularly reading and trending DOM data on production links, you're leaving predictive diagnostics on the table.
