Time Travels in the L-Band: Deploying a National White Rabbit Infrastructure over Active DWDM Networks#

Michal Hazlinsky (CESNET)

• Why: A blueprint for running nanosecond-grade time distribution on an active, revenue-carrying DWDM network instead of dedicated dark fiber — plus one of the best examples of real NetDevOps automation at the optical layer (L0).
• What: CESNET distributes time traceable to UTC(TP) with better than 1 ns stability to 22 locations, with White Rabbit running in the L-band (1570/1571/1572 nm) coexisting with 8 C-band data channels on the same fiber of its 5,700 km multivendor network (Cisco, Ribbon/ECI, CzechLight). Spectrum-sharing was designed in from procurement: custom channel filters per line (8 C + 4 L channels, <1 dB insertion loss), bidirectional single-fiber transmission to defeat WR's path-asymmetry problem, and CzechLight SDN bidirectional EDFAs for long-haul.
• How: Open-source SDN-controlled BiDi amplifiers (NETCONF/RESTCONF, YANG model, IETF YANG-Push streaming telemetry with sub-second latencies) managed via Ansible with a NetBox inventory — actual DevOps playbooks at the photonic layer. Calibration is treated as a first-class engineering problem: measuring Tx/Rx filter delay asymmetry, chromatic-dispersion asymmetry per line, and a field procedure using the Safran Z16's port-mode survey plus switched Tx/Rx wavelengths so a second WR switch and external counter aren't needed on site. Lessons: transceiver Tx-power variance means power budgets must be recalculated on replacement (keep fixed attenuators in reachable locations), and logical topology reconfiguration can require WRS reboots.
• Where applicable: Requires a spectrum-sharing design (reserved L-band channels and filters) — cheapest on greenfield builds — and CESNET's own CzechLight hardware, but the hardware designs and software are open. The calibration methods and the L-band coexistence technique are broadly reusable; "standard APIs are MUST HAVE" is the summary lesson.