Clock Synchronization for Multihop Wireless Sensor Networks

In wireless sensor networks, more so generally than in other types of distributed systems, clock synchronization is crucial since by having this service available, several applications such as media access protocols, object tracking, or data fusion, would improve their performance. In this dissertation, we propose a set of algorithms to achieve accurate time synchronization in large multihop wireless networks. First, we present a fully distributed and asynchronous algorithm that has been designed to exploit the large number of global constraints that have to be satisfied by a common notion of time in a multihop network. For example, the sum of the clock offsets along any cycle in the network must be zero at any instant. This leads to the concept of ``spatial smoothing.'' By imposing the large number of global constraints for all the cycles in the multihop network, these time estimates can be smoothed and made more accurate. The algorithm functions by simple asynchronous broadcasts at each node. Changing the time reference node for synchronization is also easy, consisting simply of one node switching on adaptation, and another switching it off. It has been implemented on a Berkeley motes testbed of forty nodes, and comparative evaluation against a leading algorithm is presented. Next, considering that most of the clock synchronization protocols that have been developed do not provide means to detect security attacks which could render them useless, we present a secure network-wide clock synchronization protocol. At the same time, this protocol allows the nodes to securely discover the network's topology by detecting and isolating all links that have fallen under the control of attackers. The protocol detects the attacks using only timing information under certain conditions. It has been implemented on an IMote2 testbed of twenty five nodes. Experimental results are provided.not peer reviewe