Optical Transceiver Power: Reading Rx/Tx dBm Correctly

An interface that shows up does not mean the optics are healthy - it means the receiver can still recover enough signal, possibly with almost no margin left. Digital optical monitoring (DOM, defined by SFF-8472) exposes the transceiver's Tx power, Rx power, temperature, voltage and laser bias current, and reading those numbers correctly is one of the most valuable fiber troubleshooting skills. This guide explains dBm, what typical Tx and Rx power ranges look like for 10G SR/LR optics, how to interpret the alarm and warning thresholds in CLI output, and what to check when power levels drift.

dBm: The Language of Light Levels

dBm is optical power relative to 1 milliwatt: 0 dBm equals 1 mW, every +3 dB doubles the power and every -3 dB halves it. A reading of 466 uW is 10 x log(466/1000) = -3.3 dBm. Because the scale is logarithmic, a 3 dB change is already a factor of two - small-looking dBm differences are large real changes.

Typical Tx and Rx Power Ranges

Absolute numbers depend on the transceiver data sheet, but field experience gives useful rules of thumb for 10GBASE-SR multimode optics:

• Tx power is normally good in the -1 to -7 dBm band for SR optics (check your module's spec; LR modules transmit higher, around -3 to +2 dBm).
• Rx power is normally expected between -1 and roughly -10 dBm for SR.
• Rx around -14 dBm or lower on an SR link usually means a real cable-plant fault: bad splice, dirty connector, or poorly seated jumper.
• Tx or Rx at -30 dBm or below is typically the receiver's noise floor - in practice, no signal at all.

Always compare against the module's own DOM thresholds, which the switch reports next to the live reading.

Reading DOM Output: Values and Thresholds

Cisco-style output shows the measured value plus high/low alarm and warning limits. The reading is healthy when it sits inside the warning band; alarms are the emergency stops:

# show interface transceiver details
Temperature  47.05 C     High Alarm 89.00 C    Low Alarm -9.00 C
Tx Power     -4.87 dBm   High Alarm 1.00 dBm   Low Alarm -13.50 dBm
Rx Power     -7.69 dBm   High Alarm 4.00 dBm   Low Alarm -21.02 dBm
Transmit Fault Count = 0

On Brocade gear the equivalent is sfpshow, which prints power in both dBm and uW with the same alarm/warning columns.

Interpreting Common Patterns

Never judge a module by a single reading - check the current value, its thresholds, and the trend over several weeks. A classic case: Rx drifting from -10 dBm to -16 dBm while the link stays up; cleaning mid-span LC connectors recovered the margin without replacing a good optic.

Related Reading

See our SFP/SFP+/SFP28/QSFP28 transceiver types guide and ethtool network diagnostics for host-side checks.

原文链接：https://community.cisco.com/t5/optics-and-optical-networking/understanding-tx-rx-light-level/td-p/2794532