Accurate time synchronization in PTP-based industrial networks with long linear paths

Assuring very accurate time synchronization across wide area industrial networks is still an open issue, which even the second version of the Precision Time Protocol (PTPv2) has not been able to solve completely. This is due to the accumulation of many uncertainty contributions when PTP event messages are routed from the master clock to the slave one through multiple network nodes. Peer-to-peer transparent clocks may mitigate this problem. Nonetheless, poor or noisy relative clock rate estimates may drastically reduce the synchronization accuracy on the farthest nodes. In this paper Kalman filters are used to estimate and to compensate, drift rate differences, frequency skews and time offsets between pairs of adjacent transparent clocks. Although the idea of using a Kalman filter for synchronization purposes is not new per se, the proposed solution is specifically tailored to optimize the performance of networks with a long linear topology. Several simulation results confirm the validity of this approach.