Long Distance White Rabbit Distribution in a Coherent Network Within Sweden

Magnus Bergroth, Ragnar Sundblad, Sven-Christian Ebenhag

Abstract:	One of the nationwide fiber optic data networks in Sweden uses technology with data modulation in both polarization and phase. The network is operated by the Swedish University Data Network (SUNET). With a limited number of clients connected to the network, each client is assigned a personal wavelength. By using dynamically Reconfigurable Optical Addand-Drop Multiplexers (ROADMs), each wavelength can theoretically be connected to any other client within the network. The coherent modulation formats also enable signal recovery by electronic digital signal processing after detection with no optical dispersion compensation used. Using ROADMs, the time transfer settings can be dynamically changed and optimized depending on time requests. Without dispersion compensating fibers, the asymmetries in the duplex fiber are minimized, enabling improved performance for two-way time or frequency transfer. The studies in this paper focus on the Precision Time Protocol (PTP) and the specially developed White Rabbit (WR) profile transported in SUNET’s network. The locations used in the comparison are equipped with WR equipment and cesium atomic clocks. The transmission distances vary up to the longest distance of 699 km. By using the coherent network and its ROADMs, a duplex time and frequency network between cities can be easily established. The study has established a link in the network and uses it as redundancy between the cities and as a complement to GNSS. Initially, it was expected that there would be jumps and interruptions at short intervals, but data sets longer than 6 months show that this does not happen very often and that such events can then be handled. The duplex link varies about +-5ns /6 months. The second part of the study focuses on a 440 km bidirectional link that has been operational for more than 4.5 years on a research basis. The study shows very good results such as a variation that is smaller than the method the link is compared to, i.e. +-2,5ns, and that asymmetries can be minimized.
Published in:	Proceedings of the 57th Annual Precise Time and Time Interval Systems and Applications Meeting January 26 - 29, 2026 Hyatt Regency Orange County Anaheim, California
Pages:	28 - 37
Cite this article:	Bergroth, Magnus, Sundblad, Ragnar, Ebenhag, Sven-Christian, "Long Distance White Rabbit Distribution in a Coherent Network Within Sweden," Proceedings of the 57th Annual Precise Time and Time Interval Systems and Applications Meeting, Anaheim, California, January 2026, pp. 28-37. https://doi.org/10.33012/2026.20490
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