Always on time with the White Rabbit protocol


Researchers of the National Measurement Institute VSL in Delft and the VU University Amsterdam managed to synchronise a clock of the Nikhef, the National Institute for Subatomic Physics, in Amsterdam with the atomic clock of VSL in Delft. The synchronisation took place via a 135 km long optical fibre link using the so-called White Rabbit protocol. This protocol allows clocks to be synchronised with the caesium clocks at VSL. The precise synchronisation between Delft and Amsterdam is a first step. In the future, this technique should be available throughout the Netherlands.

The highly accurate synchronisations are not only important for the scientific world. Accurate time measurements are also crucial for telecom providers, energy companies and financial institutions. The telecom companies, for instance, can achieve better alignment between GSM transmitter masts, while financial institutes can better monitor tampering with financial transactions.

White Rabbit
Dutch time standardCurrently, satellites are used to synchronise the clocks of VSL (the national time standard) with the international time zones. The uncertainty in synchronisation via satellite is approximately 5 nanoseconds. In addition, satellite links are vulnerable and easy to disrupt. Synchronisation via optical fibre cables is a good alternative for medium distances.

The time synchronisation in the optical fibre cable from Delft to Amsterdam takes place via the White Rabbit protocol. This protocol was developed by CERN, German-based GSI, and partners from universities and industry, and was named after the white rabbit from 'Alice in Wonderland.' This rabbit lures Alice into the rabbit hole at the beginning of the book and, despite always wearing a watch, he is always late. The intended accuracy of the White Rabbit protocol is 1 nanosecond, making it five times more accurate than satellite links.

Until recently, White Rabbit was only suitable for short-distance links. For an accurate transport of time through the optical fibre cables, the signal has to go back and forth through the same fibre. This allows for the time signal to be corrected for the delay caused by the transport. Longer links require amplifiers in the fibres to transmit the signal. However, normal amplifiers in optical fibre links are only suitable for one-way traffic. Within the framework of SURFnet’s GigaPort3 programme 'Research on Networks', Eindhoven University of Technology, VU University Amsterdam and SURFnet developed a new amplifier that solves this problem and allows for two-way traffic through the SURFnet network.

About VSL
VSL is the Netherlands’ National Metrology Institute and is an internationally leading knowledge institute in the field of metrology, the science of measurement. In addition to the management and development of the national primary measurement standards, VSL experts provide customised services and calibrate measuring instruments on behalf of customers.

The research into the White Rabbit link is part of a research project within the European EMRP research programme. Dutch partners of VSL in this project are VU University Amsterdam, SURFnet, Nikhef, Eindhoven University of Technology and Delft University of Technology. The Finnish MIKES Institute and CERN also take part in the project.