Time signal distribution in communication networks based on synchronous digital hierarchy

A new method that uses round-trip paths to accurately measure transmission delay for submicrosecond time synchronization in synchronous digital hierarchy (SDH) networks is proposed. Since the proposed system uses virtual container-3 (VC-3) signal as the time reference signals, existing SDH equipment can be used without any changes. The feature of this method is that it separately measures the initial round-trip path delay and subsequent variations in round-trip path delay. The initial uncertain delay due to the administrative unit-3 (AU-3) pointer operations in SDH equipment is determined by controlling the reference clock of the equipment. The characteristics of the equipment delay caused by the pointer operations are also described. In an experiment with actual SDH equipment, the error in initial delay determination was suppressed to 30 ns. The delay variation measurement of the round-trip path is not affected by the pointer operation in the proposed method. The precision of delay variation measurement can be better than 10 ns.