  Frequency dissemination in phase-stabilized optical fiber networks for
metrological frequency comparisons and precision measurements are promising
candidates to overcome the limitations imposed by satellite techniques.
However, network constraints restrict the availability of dedicated channels in
the commonly-used C-band. Here, we demonstrate the dissemination of an
SI-traceable ultrastable optical frequency in the L-band over a 456 km fiber
network with ring topology, in which telecommunication data traffic occupies
the full C-band. We characterize the optical phase noise and evaluate a link
instability of $4.7\cdot 10^{-16}$ at 1 s and $3.8\cdot 10^{-19}$ at 2000 s
integration time, and a link accuracy of $2\cdot 10^{-18}$, which is comparable
to existing metrology networks in the C-band. We demonstrate the application of
the disseminated frequency by establishing the SI-traceability of a laser in a
remote laboratory. Finally, we show that our metrological frequency does not
interfere with data traffic in the telecommunication channels. Our approach
combines an unconventional spectral choice in the telecommunication L-band with
established frequency-stabilization techniques, providing a novel,
cost-effective solution for ultrastable frequency-comparison and dissemination,
and may contribute to a foundation of a world-wide metrological network.
