Clocks, power and synchronization in duty-cycled wireless sensor nodes

Recent advances in CMOS VLSI ICs and micro-electromechanical technology have led to development of small, low-cost, and low-power multifunctional sensors. Wireless sensor networks (WSNs) are large-scale networks of such sensors, dedicated to observing and monitoring various aspects of the physical world. Some intrinsic properties of WSNs including limited resource of energy, storage, computation, and bandwidth, make traditional synchronization methods unsuitable for WSNs. Time synchronization as an important issue consists of giving all sensor nodes (SNs) of the WSN a common time scale to operate. The common time scale is usually achieved by periodically synchronizing the clock of each SN to a reference source. In this manner the local time seen by each SN of the network is approximately the same, and time synchronization allows the entire system to cooperate. This paper gives a brief look to the time synchronization problem and the need for synchronization in WSNs. Then it points out that clock systems become a bottle-neck, after that it presents the available current clock technologies, next it examines the influence of these clock technologies, and finally provides guidelines for WSN developers who must choose among the different clock synchronization techniques.