Time synchronization with ripple flooding in wireless sensor networks

Abstract

WSNs (Wireless Sensor Networks) are networks of many sensor nodes that are distributed and wirelessly connected with multi-hop fashion. Each node in the WSNs normally battery operated has at least one sensor, an embedded processor, and a low-power transceiver. In WSNs, coordination of wake-up and sleeping time, TDMA schedules, ordering of collected sensor data and events and cooperation of multiple sensor nodes require sensor network nodes to have a precise common timescale. The purpose of the coordination of wake-up and sleeping time is to minimize nodes’ duty cycle for energy efficiency. Because of energy efficiency, precise common timescale over the network is very important. Time synchronization with a GPS receiver or the highly accurate crystal oscillators like OCXO, TCXO are feasible, but generally are ruled out in sensor node design due to excessive form factor, tight cost budget, and additional power consumption. One of methods to achieve the precise common timescale over nodes is time synchronization. However, since nodes in WSNs have a cheap and imperfect hardware clock, each clock in the nodes may drift away each other in time. For keeping a precise common timescale over the nodes, we need to periodically compensate drift and offset between each clock of nodes networked with multi-hop by using time synchronization protocols. Several time synchronization protocols have been proposed to achieve the precise common timescale over the nodes, but we still have problems to be solved; power consumption grows linearly with the required precision and sync error grows with network size. For keeping a highly precise common timescale in the network, a high-frequency clock is required and a node with the high-frequency clock consumes more power than a node with the low-frequency clock. A recent research proposes a possible solution of the power consumption problem by combining a low-frequency clock for a common operation and a high-frequency clock for a specific ...