Two-way time transfer through 2.4 Gb/s optical SDH system

An experiment to transfer time and frequency over 2.488 Gb/s synchronous digital hierarchy (SDH) systems using a 175-km commercial optical fiber cable has been set up by Communication Research Laboratory (CRL) and Nippon Telegraph and Telephone Corporation (NTT) Laboratory. We confirm that the frequency stability of the time comparison data is 10^-12/square root of tau for averaging times above 10 s. This stability equals that of the cesium frequency standard (HP5071A) used in this experiment. This means that the system can transfer the cesium frequency standard signal without deterioration. The time comparison resolution is of the order of 10^-11 s (square root of time variance). The long-term stability of this system is expected to be better than 1 ns, including the temperature dependence of time information transmitters and receivers. Time comparison results agree well with GPS common-view results. Time accuracy is held to within 32 ns (this includes the anticipated time deviation caused by network elements)