Understanding and optimising transceiver power consumption using internal metrics#

Gerhard Stein - Flexoptix GmbH

• Why: Optics power is a growing slice of DC energy budgets, and the common assumptions (laser temperature tuning matters, TEC is a big draw) turn out to be wrong. This talk replaces folklore with measurements.
• What: A measurement study of >100G transceivers (100G QSFP28, 400G coherent ZR, 800G QSFP-DD) in Cisco Nexus 93600CD-GX, C9500, and Juniper QFX5120, using Versatile Diagnostics Monitoring (VDM/CMIS) internals. Headline findings: laser and TEC power draw is negligible at any transceiver temperature (the 50 °C laser target makes no efficiency difference; a ~20 °C ambient delta moves only ~200 mW); the real energy consumers are the DSP/MCU. The single biggest lever is administrative: shutting a port forces the transceiver into low-power mode at 7–20× less energy. Also documents real power-per-speed trends (400G FR4 from 12 W → 9 W via DSP node shrink 16→7 nm; 1.6T OSFP 30 → 26 W).
• How: Read VDM metrics (TEC current, laser temperature) alongside DDM; compute P_laser = V_fwd × I_laser and P_TEC = V_trc × I_TEC; measure per-state transceiver draw with a Flexbox research tool (their pytest/PyQt/matplotlib prototype); a "frozen transceiver" experiment with ice cubes validated the temperature insensitivity.
• Where applicable: Any operator with >100G optics. Practically constrained: VDM support is patchy — Juniper QFX5120 (JunOS 25.2R1.9) and Cisco C9500 (IOS XE 17.14.1) expose DDM only; the Nexus 93600CD-GX (NX-OS 10.5.3(f)) supports CMIS VDM. Flexoptix is a vendor and the Flexbox is theirs, but the talk's findings (port shutdown as the dominant lever, DSP as the consumer) transfer to any environment.