Frequency and Time Synchronization in Digital Communications Networks

Frequency distribution performance will be improved with the installation of a new synchronization equipment (slave clock) in Nippon Telegraph and Telephone Corporation's (NTT's) network. In the slave clock system, the PLL has been optimally designed so that the total phase-time variation in the whole network is significantly less than 10 ps. This phase-time variation is recommended by International Telegraph and Telephone Consultative Committee (CCITT). The design method and PLL control algorithm enable both an optimum time constant of the PLL and a frequency departure of 10-12 in a holdover operation. The functions of the frequency distribution system has the intrinsic capability of being expanded and applied to a time transfer system. Since the frequencies are synchronized, a phase-time of the standard frequency signal can be synchronized to a coordinate time scale by an initial time setting, c.g., using portable clocks. Time synchroniza t ion capability is determined by the relative phase-time variation of standard frequency signals and the time accuracy of the initial phase-time setting. Relative phase-time variation can be reduced to within 500 ns by using wander correction. The initial time accuracy is within 0.1 p s in conventional portable cesium beam standards. The time accuracy in NTT's digital networks is expected to 100 ns to 600 n s .