Understanding and optimising transceiver efficiency using internal metrics for improved power savings#

Gerhard Stein (with Thomas Weible), Flexoptix

• Why: Optics power draw is a real DC cost line at 400G/800G scale, and this is one of the few talks with actual measurements of where transceiver watts go — a topic the PM works on physically and strategically.
• What: A measurement study across 100G QSFP28, 400G QSFP-DD coherent ZR, and 800G QSFP-DD transceivers on Cisco Nexus 93600CD-GX, Catalyst C9500, and Juniper QFX5120. Findings: laser and TEC draw is negligible at any transceiver temperature (laser at its ~50 °C target is most efficient, but it barely matters); the real consumers are the MCU/DSP, flagged as the next research step; shutting a port forces low-power mode and cuts transceiver consumption 7–20x; "doubling the speed does not double the Watts." VDM (CMIS calculated metrics) vs DDM support varies sharply by platform — only the Nexus 93600CD-GX with a 400G coherent module reported a real laser temperature (49.98 °C); the QFX5120 and C9500 expose DDM only.
• How: VDM reads calculated values from module bytes (e.g., P_laser = V_fwd × I_laser, P_TEC = V_Trc × I_TEC) where DDM gives raw currents; power budget and fan/heat-exchange modeling (Nusselt/Prandtl/Reynolds); measurements taken via a Flexbox-based research tool, including a frozen-transceiver experiment showing laser behavior across a ~20 °C differential.
• Where applicable: The port-shutdown power lever and platform-dependent VDM support are broadly useful to any switch operator. Flexoptix is a vendor, but the work is measurement-based and spans three switch vendors' hardware.