Adaptive Synchronization in IEEE802.15.4e Networks

Industrial low-power wireless mesh networks are shifting towards time-synchronized medium access control (MAC) protocols which are able to yield over 99.9% end-to-end reliability and radio duty cycles well below 1%. In these networks, motes use time slots to communicate, and neighbor motes maintain their clocks' alignment, typically within 1 ms. Temperature, supply voltage, and fabrication differences cause the motes' clocks to drift with respect to one another. Neighbor motes need to re-synchronize periodically through pairwise communication. This period is typically determined a priori, based on the worst case drift. In this paper, we propose a novel technique which measures and models the relative clock drift between neighbor motes, thereby reducing the effective drift rate. Instead of resynchronizing at a preset rate, neighbor motes resynchronize only when needed. This reduces the minimum achievable duty cycle of an idle network by a factor of 10, which in turn lowers the mote power consumption and extends the network lifetime. This Adaptive Synchronization is implemented as part of IEEE802.15.4e in the OpenWSN protocol stack and is validated through extensive experimentation.