The fastest fiber fault to fix is the one you never open a panel for. Before you pull a patch cord or swap an optic, the optic itself will tell you what is wrong — if you ask it. Every modern SFP and QSFP reports DOM values (Digital Optical Monitoring, also called DDM): transmitted power, received power, temperature, supply voltage, and laser bias current. Reading them takes a minute from the switch CLI, costs nothing, and answers the first question on any fiber fault: is the problem the optic, or the span behind it?
A tech who swaps the optic first and reads DOM second has already wasted the part and the maintenance window.
The five values
Tx power (dBm) is how much light this optic is sending. Healthy is inside the range on the optic's datasheet — for example, roughly -7 to -1 dBm on a typical 10G short-reach optic. If Tx power is out of range, the laser is failing or degraded, and this optic is the suspect.
Rx power (dBm) is how much light is arriving at this optic. Healthy is several dB above the receiver sensitivity on the datasheet — typically around -11 dBm for a 10G short-reach optic and around -14 dBm for a 10G long-reach optic. But never judge Rx power alone: if the far end's Tx power is also low, the far optic is the fault, not your span. Check both ends before touching the fiber.
Temperature (°C) is the optic's own temperature. Commercial optics are rated 0 to 70 °C. Above range usually means blocked airflow, a hot slot, or an optic cooking itself — check the blanking panels and the fan before blaming the transceiver.
Supply voltage (V) is the power the slot feeds the optic: 3.3 V, within ±5% (3.135 to 3.465 V). Out of range means the slot or the switch power is the problem, not the optic. Reseat it, try another slot, and if the voltage follows the slot, escalate to the platform team.
Tx bias current (mA) is how hard the laser is being driven. This is the one most techs never look at, and it is the one that saves an outage. A laser near end of life drives its bias current up to hold Tx power steady, so the link looks fine until it suddenly is not. A bias current climbing over weeks while Tx power holds steady means: plan the replacement now, do not wait for the alarm.
How to read DOM
The command depends on the platform, but the values are the same everywhere. On a Cisco switch: show interfaces transceiver detail. On Arista: show interfaces transceiver dom. On Juniper: show interfaces diagnostics optics. On a Linux server NIC: ethtool -m eth0.
Read both ends of the link. A one-ended DOM read is half the evidence, because a low Rx on your end is either your span or the far end's Tx, and only the far end's numbers tell you which. If the optic reports no DOM at all, it is either a very old optic or a third-party optic with DOM disabled — worth one line in the ticket, because it means the next fault on that link will have no self-reported evidence.
The decision flow
- Rx low, far-end Tx normal → the span. Light leaves the far end fine and arrives weak. Clean and inspect the connectors, then compare the measured loss against the loss budget. Do not swap the optic.
- Tx low on this end → this optic. The laser is not sending enough light. Swap the optic, re-read DOM on the new one, and record both readings so the ticket shows the before and after.
- Rx low and far-end Tx low → the far-end optic. The far laser is the fault. Escalate with both ends' numbers attached — the far-end team can verify from the ticket without re-measuring.
- Bias current climbing, Tx still in spec → a dying optic, not a dead one. Record the bias trend and raise a planned replacement.
- Temperature high → airflow, not the optic. A hot optic in a hot slot will fail again after the swap.
- Voltage out of range → the slot. Reseat, try a known-good slot, and escalate if the voltage follows the slot.
Alarms vs warnings
DOM carries two sets of thresholds per value: a warning and an alarm. A warning means the value is drifting toward the edge of spec — worth a ticket line and a planned look. An alarm means it is out of spec now. The common mistake is treating a warning as noise and only acting on the alarm: by the time Rx power alarms, the link has already been marginal for days, and every ticket written in that window says "intermittent" because nobody read the warning.
When you open a fiber ticket, copy the DOM values into it even when they are healthy. A healthy baseline is what makes the next fault diagnosable in minutes instead of hours.
Recording DOM in the ticket
A DOM reading without its thresholds is just numbers; with them, it is a verdict. The minimum useful line records both ends, all five values, and the call:
SW-04 Gi1/0/12 ↔ SRV-22 eth0, 10G-SR, 85 m OM4.
This end DOM: Tx -3.2 dBm, Rx -6.8 dBm, temp 41 C,
Vcc 3.31 V, bias 6.8 mA.
Far end DOM: Tx -3.0 dBm, Rx -7.1 dBm.
Rx 4.3 dB above -11.1 dBm sensitivity → optics healthy;
fault is in the span, not the optic.
The shape to copy: link, optic type, both ends' DOM, Rx margin above sensitivity, and the verdict — optic, span, airflow, or slot. Seven fields, and the next person on the ticket can see in one line whether the optic was ever the suspect, without re-reading anything.
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