Abstract
Many clock synchronization protocols based on message passing, e.g., the Network Time Protocol (NTP), assume symmetric network delays to estimate the one-way packet transmission time as half of the round-trip time. As a result, asymmetric network delays caused by either %natural one-way network congestion or malicious packet delays can cause significant synchronization errors. This paper exploits sinusoidal voltage signals of an alternating current (ac) power grid to tame the asymmetric network delays for robust and resilient clock synchronization. Our extensive measurements show that the voltage signals at geographically distributed locations in a city are highly synchronized. Leveraging calibrated voltage phases, we develop a new clock synchronization protocol, which we call Grid Time Protocol (GTP), that allows direct measurement of one-way packet transmission times between its slave and master nodes, under an analytic condition that can be easily verified in practice. The direct measurements render GTP resilient against asymmetric network delays under this condition. A prototype implementation of GTP, based on readily available ac/ac transformers and PC-grade sound cards as voltage signal sampling devices, maintains sub-ms synchronization accuracy for two nodes 30 km apart, in the presence of malicious packet delays. We believe that GTP is suitable for grid-connected distributed systems that are currently served by NTP but desire higher resilience against network dynamics and packet delay attacks.