Light-weight time synchronization for Wireless Sensor Networks

Using a Wireless Sensor Network (WSN) for monitoring single source events, such as volcanic activity, involves many challenges. One of these challenges is that of accurate timer synchronization of the data collecting nodes, so as to ensure that there is a consistent view of the data retrieval times. In this paper we discuss a novel approach for an one-way time synchronization protocol that relies on the use of cross-layer timing information to estimate the accumulated delay since a block of data is created until it is received at the sink node. The accumulated delay is computed using the Application, Network and Link layers in the 802.11b technology. At each node which belongs to a route a Protocol Data Unit (PDU) inserted between the User Datagram Protocol (UDP) header and the application payload, is manipulated in order to accumulate the calculated delay. This delay is iteratively incremented along the path until it is finally provided to the receiving application. The developed protocol provides a fairly accurate estimation of the delay a message experiences through a WSN, adding no extra packets and little data overhead. Our experimental evaluation showed that it is able to determine the end-to-end delay of most messages with an error inferior to 700μs in one hop scenarios and to 1500μs in two hops scenarios.