Time and frequency transfer using asynchronous fiber optical networks: progress report
Övrigt konferensbidrag, 2009
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.