Recent advances in micro-electromechanical (MEMS) technology have led to the development of small, low-cost, and low-power sensors Wireless sensor networks (WSNs) are large-scale networks of such sensors, dedicated to observing and monitoring various aspects of the physical world In such networks, data from each sensor is agglomerated using data fusion to form a single meaningful result, which makes time synchronization between sensors highly desirable This paper surveys and evaluates existing clock synchronization protocols based on a palette of factors like precision, accuracy, cost, and complexity The design considerations presented here can help developers either in choosing an existing synchronization protocol or in defining a new protocol that is best suited to the specific needs of a sensor-network application Finally, the survey provides a valuable framework by which designers can compare new and existing synchronization protocols

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Clock synchronization for wireless sensor networks: a survey

Time synchronization is an important issue in multihop ad hoc wireless networks such as sensor networks. Many applications of sensor networks need local clocks of sensor nodes to be synchronized, requiring various degrees of precision. Some intrinsic properties of sensor networks, such as limited resources of energy, storage, computation, and bandwidth, combined with potentially high density of nodes make traditional synchronization methods unsuitable for these networks. Hence, there has been an increasing research focus on designing synchronization algorithms specifically for sensor networks. This article reviews the time synchronization problem and the need for synchronization in sensor networks, then presents in detail the basic synchronization methods explicitly designed and proposed for sensor networks.

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Time synchronization in sensor networks: a survey

Clock synchronization is a critical component in the operation of wireless sensor networks (WSNs), as it provides a common time frame to different nodes. It supports functions such as fusing voice and video data from different sensor nodes, time-based channel sharing, and coordinated sleep wake-up node scheduling mechanisms. Early studies on clock synchronization for WSNs mainly focused on protocol design. However, the clock synchronization problem is inherently related to parameter estimation, and, recently, studies on clock synchronization began to emerge by adopting a statistical signal processing framework. In this article, a survey on the latest advances in the field of clock synchronization of WSNs is provided by following a signal processing viewpoint. This article illustrates that many of the proposed clock synchronization protocols can be interpreted and their performance assessed using common statistical signal processing methods. It is also shown that advanced signal processing techniques enable the derivation of optimal clock synchronization algorithms under challenging scenarios.

Clock Synchronization of Wireless Sensor Networks

This paper presents lightweight tree-based synchronization (LTS) methods for sensor networks. First, a single-hop, pair-wise synchronization scheme is analyzed. This scheme requires the exchange of only three messages and has Gaussian error properties. The single-hop approach is extended to a centralized multi-hop synchronization method. Multi-hop synchronization consists of pair-wise synchronizations performed along the edges of a spanning tree. Multi-hop synchronization requires only n-1 pair-wise synchronizations for a network of n nodes. In addition, we show that the communication complexity and accuracy of multi-hop synchronization is a function of the construction and depth of the spanning tree; several spanning-tree construction algorithms are described. Further, the required refresh rate of multi-hop synchronization is shown as a function of clock drift and the accuracy of single-hop synchronization. Finally, a distributed multi-hop synchronization is presented where nodes keep track of their own clock drift and their synchronization accuracy. In this scheme, nodes initialize their own resynchronization as needed.

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Lightweight time synchronization for sensor networks

Recent advances in electronics and wireless communication technologies have enabled the development of large-scale wireless sensor networks that consist of many low-power, low-cost, and small-size sensor nodes. Sensor networks hold the promise of facilitating large-scale and real-time data processing in complex environments. Security is critical for many sensor network applications, such as military target tracking and security monitoring. To provide security and privacy to small sensor nodes is challenging, due to the limited capabilities of sensor nodes in terms of computation, communication, memory/storage, and energy supply. In this article we survey the state of the art in research on sensor network security.

Security in wireless sensor networks

Time synchronization is important for any distributed system. In particular, wireless sensor networks make extensive use of synchronized time in many contexts (e.g. for data fusion, TDMA schedules, synchronized sleep periods, etc.). Existing time synchronization methods were not designed with wireless sensors in mind, and need to be extended or redesigned. Our solution centers around the development of a deterministic time synchronization method relevant for wireless sensor networks. The proposed solution features minimal complexity in network bandwidth, storage and processing and can achieve good accuracy. Highly relevant for sensor networks, it also provides tight, deterministic bounds on both the offsets and clock drifts. A method to synchronize the entire network in preparation for data fusion is presented. A real implementation of a wireless ad-hoc network is used to evaluate the performance of the proposed approach.

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C. Veerarittiphan

Papers

Simple, accurate time synchronization for wireless sensor networks