Time synchronization is extremely important for wireless sensor networks (WSNs). For largescale WSNs, hierarchical clock synchronization, which can effectively save energy by reducing communication overhead, has become an attractive approach in practical networks. Based on the hierarchical network, in this paper, we present a global clock skew estimation scheme with immediate clock adjustment. The maximum likelihood estimator and corresponding Cramer-Rao lower bound of the clock skew are derived under the Gaussian delay model, and the simulation results verify that the proposed method is efficient.
