I. Introduction
Wireless sensor networks (WSNs) consist of lots of small nodes which have communication capacity and computational capability. WSNs are deployed in a specified area to collect interesting information and send it to observer for processing and analysing (in [1]). Because the low cost sensor nodes, which computational capability, storage capacity, wireless communication capacity and the energy is limited, are deployed in uncontrollable and hostile environments, so it is common for faults to occur due to component failure, battery depletion and some unknown reasons. The faults will reduce the reliability of the network and dirty data collected by the faulty nodes will influence the analysis result of the whole network (in [2]). So, the fault detection is an important problem in WSNs research, and we will pay close attention to fault detection method in WSNs.