Evaluating Time Synchronization Using Application-Layer Time-Stamping

Time synchronization is an important topic in wireless sensor networks (WSNs) and in networking in general. Most of the synchronization algorithms in WSNs are sender-receiver protocols that involve some message exchange between a pair of nodes to calculate the difference in local clocks (clock drifts). Such protocols rely on MAC-Layer time stamping to reduce synchronization errors to a minimum. Although very accurate, such approaches need intervention in lower layers of the network stack to enable MAC-Layer time stamping of the exchanged packets, which makes applying such protocols hard or even impossible on some platforms. In this paper, we study time synchronization using application-layer time stamping only which relieves the requirement of MAC-layer time stamping. We introduce a simple protocol and implement it on Arduino and XBee. Our results show that sub-millisecond accuracy can be achieved on average for a 3-hop network, and that interference can increase the average synchronization error slightly over 1 ms. We believe such accuracy level is acceptable for a wide-range of applications in WSNs especially.