Frequency and Time Synchronization in Digital Communications Networks

摘要

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 synchronization 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 ps in conventional portable cesium beam standards. The time accuracy in NTT's digital networks is expected to 100 ns to 600 ns.