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

Gerhard Stein – Flexoptix GmbH, Thomas Weible – Flexoptix

• Why: Optics power is a growing DC OpEx line; this is real measurement work on what transceiver telemetry (VDM/TEC current) can and cannot save you.
• What: Bench results across 100G QSFP28, 400G coherent ZR, and 800G optics on Cisco/Juniper gear. The punchline inverts the thesis: laser and TEC are negligible power drawers — the MCU/DSP are the real consumers — so laser-temperature optimization (~200mW average gain from a 20K temperature drop) isn't worth chasing. The real saving is operational: shutting a port forces the optic into low-power mode at ~7–20× lower consumption. Switch behavior differs (Juniper supplies optic power even with no RX; Cisco's Nexus is smarter here), and VDM support is patchy — two of three switches tested expose DDM only, and the one CMIS VDM implementation bakes in a 50°C laser-temp assumption.
• How: CMIS VDM parsing beyond DDM, TEC current and laser power via V×I, SFF-8636/CMIS power-class budgeting, plus a heat-transfer analysis (Nusselt/Prandtl/Reynolds) showing fan speed and case temperature dominate the thermal picture.
• Where applicable: Any operator with >100G optics fleets; vendor-authored (Flexoptix sells programmable optics) but measurement-driven with named DUTs and honest caveats. Next steps teased: port-shutdown automation across SONiC/Mellanox, and water cooling.