Time and frequency transfer using asynchronous fiber optical networks: progress report

SP Technical Research Institute of Sweden has since 2004 been running a project with the aim of performing time and frequency transfer using commercial asynchronous fiberoptical networks. The project is motivated by the need for an alternative and complementary time transfer method on a national basis with the goal of reaching accuracy and stability comparable to satellite-based methods. Previous results using an OC-192/STM-64 10-Gb/s packet over SONET/SDH network, show that time transfer accuracy of the order of a few nanoseconds is possible on baselines exceeding 500 km [1]. The method is based on passive listening on existing data traffic and the detection of certain bit sequences in the SDH frame headers continuously transmitted by the network routers. By using two-way time transfer, it is possible to estimate and compensate for symmetric delays in the optical fibers. The method relies on that time dependent residual delays are small or can be can be compensated for and constant residual delays can be calibrated. This paper briefly revises the method and presents new results in comparison with the GPS carrier-phase technique, with focus on residual effects due to temperature variations which have shown to have significant impact on the stability and accuracy. It also discusses hardware miniaturizations as well as new ideas for active time transfer using bit-sequence generators/transmitters in dedicated wavelength slots of the optical network. Finally, the use of a subset of the IEEE standard 1588-2008 (Precise Time Protocol, PTP) for data transport is briefly discussed