Attack-resilient time synchronization for wireless sensor networks

Study of consensus-based time synchronization in wireless sensor networks

Wireless sensor networks require time synchronization, which is the coordination of events or actions to make a system operate in unison. In this work, real experiments and a theoretical analysis of the behavior of the clock sources, most used in wireless sensor networks, have been carried out. The experiments have been performed on two real platforms from two different manufacturers in real environments with sudden changes in temperature. Complementary metal-oxide-semiconductor oscillators have a low accuracy, bigger than 500 ppm, and a high dependency with temperature. External crystal oscillators have good accuracy, around 20 ppm, and are stable with temperature. Temperature-compensated crystal oscillators are very accurate, around 5 ppm, and the temperature has no influence in their drift. The use of phase-locked loop circuits minimizes the impact of temperature and stabilizes oscillators. We highlight and demonstrate the importance of the early stages of design, especially the selection of the clock source, because that decision has a great impact on the performance of the time synchronization in wireless sensor networks.

Time synchronization, aiming to provide a common timescale among distributed sensor nodes, is a key enabling technology for many applications, such as collaborative condition monitoring and localization detection. Due to the complexity of time synchronization in wireless sensor networks, the Discrete Event Simulator is recommended to adopt resulting from the feature that the behavior of a complex is represented as an order sequence of well-defined event in time. In this paper, the PCO clock is firstly implemented into the open-source software simulator OMNeT++ by using desynchronization mechanism under a realistic scenario, and it can also be simulated at an adjustable and higher resolution. The developed relay node, can either be a Full Function Device or a Reduced Function Device, enables the high scalability for multi-node and multi-hop simulation. In addition, the shared code of this project on the GitHub directly benefits the researchers and engineers in communication.

In wireless sensor networks, efficient usage of energy helps in improving the network lifetime. Many power saving MAC schemes based on TDMA have been proposed in recent years, but none of them particularly discuss the problem of time synchronization. As there are many time synchronization algorithms have been proposed, but they can't be applied to these TDMA MAC schemes directly. In this paper, a lightweight and energy efficient time synchronization (LEETS) scheme that can be applied to all kinds of TDMA power saving MAC schemes was proposed. In the scheme, nodes compete the first slot of each time frame to broadcast local time information, which can be used to realize whole network synchronization. And guard interval is inserted in two consecutive slots to avoid collision may be caused by synchronization errors. The scheme has been simulated and the results show LEETS needs less message exchanges to realize synchronization than other time synchronization schemes. So, It is lightweight and energy efficient.

Time synchronization is important for wireless sensor networks because it facilitates cooperation among nodes and helps raise power efficiency. Time synchronization protocols like TPSN, RBS and FTSP have provided great schemes to fulfill fast synchronization with efficiency. In some applications, nodes might hope to sleep for a long time without timestamp exchanges with other nodes. In that case, accurate time drift prediction is quite necessary. For that purpose, firstly, we propose a time synchronization scheme, which fully utilizes the broadcast nature. The scheme achieves time synchronization with fewer timestamps compared with RBS and TPSN. Secondly, we introduce a method to find relative time drift rate on the fly. Thirdly, we introduce a scheme to predict time drift rates of the next few hours. We also analyze a few factors that deteriorate frequency drift or time drift rate. The diurnal periodical environment trend, instead of mathematical extrapolation, is used for time drift rates prediction of the next few hours.

Chapter 11 - Node Compromise Detection in Wireless Sensor Networks

Time synchronization for wireless sensor networks (WSNs) has been studied in recent years as a fundamental and significant research issue. Many applications based on these WSNs assume local clocks at each sensor node that need to be synchronized to a common notion of time. Time synchronization in a WSN is critical for accurate time stamping of events and fine-tuned coordination among the sensor nodes to reduce power consumption. This paper proposes a bidirectional, reference based, tree structured time synchronization service for WSNs along with network evaluation phase. This offers a push mechanism for (i) accurate and (ii) low overhead for global time synchronization. Analysis study of proposed approach shows that it is lightweight as the number of required broadcasting messages is constant in one broadcasting domain.

SummaryChannel allocation in the wireless sensor networks is one of the important issues, and these networks must allocate shared channels between sensor nodes in the transmission phase. This paper proposes a novel protocol for channel allocation, so it focuses on energy efficiency and collisions avoidance. The main idea of this protocol is based on the Guaranteed Time Slot approach by using novel high‐rate collaborative codes. The sum rate of the proposed collaborative codes is double than the conventional codes, thus increasing the capacity of codes and allowing more nodes to simultaneously transmit at a given time slot. The proposed protocol has a cluster‐based structure, so the sensor nodes of each cluster can communicate together only inside of the any cluster. The protocol performs clustering (grouping) of the nodes based on the geometric distance from the coordinator node and hence allocating different transmission power to each cluster. Because of the difference in transmission power among the groups, the same codes can be reused in the adjacent groups increasing the overall sum rate of the codes. The performance of a proposed protocol is evaluated through extensive simulation and analysis. It is shown from the results that the proposed scheme outperforms significantly the existing ZigBee Guaranteed Time Slot allocation schemes as well as the conventional collaborative codes. Copyright © 2016 John Wiley & Sons, Ltd.

Synchronization of tiny sensor nodes forming Wireless Sensor Networks (WSNs) is a challenging problem due to frequent topological changes, node failures and power, memory and computation constraints. These difficulties promote a self-organizing solution for the problem of time synchronization in WSNs to be quite desirable. Current self-organizing time synchronization protocols in WSNs have drawbacks: They either provide synchronicity but not a common notion of time for the nodes in the network or they demand keeping track of the time information of the neighboring nodes. The latter drawback becomes quite crucial especially on dense WSNs, which makes available self-organizing time synchronization protocols impractical. This paper provides a novel self-organizing time synchronization protocol for WSNs which does not require keeping track of the neighboring nodes. The main component of our protocol is a computationally light "adaptive-value tracking" algorithm which synchronizes the rate of each sensor node to that of all its neighboring nodes through successive feedbacks. We show by simulations that the proposed protocol achieves a tight synchronization which results in the desired property of global time notion after a finite amount of time on dense networks. Although there is no formal proof yet of the protocol's convergence, we anticipate this protocol could be used as a practical time synchronization approach.

Localization and synchronization are two important issues in communication systems and wireless sensor networks. These two problems are addressed in many applications, and share many aspects in common. However, these two problems are traditionally treated separately. In this paper, we present a unified framework to jointly solve these two problems at the same time. The maximum likelihood joint estimation of location and timing is firstly derived. Then, a more computationally efficient two-stage least square method is proposed. The Cramer-Rao lower bound for the joint localization and time synchronization is also derived. Simulation results show that the performances of the maximum likelihood and two-stage least square estimators can both achieve the Cramer-Rao lower bound.

It is important and challenging to achieve secure time synchronization in wireless sensor networks, which may be deployed in a hostile environment under various malicious attacks. The recently developed average-consensus-based time synchronization protocol (ATS) is a promising alternative as it does not depend on any reference node or network topology, which makes it robust to different kinds of attacks, e.g., denial-of-service, node destruction, etc. However, the in-network information fusion nature of average consensus makes the ATS vulnerable to the well-known message manipulation attacks. In this paper, we focus on how to defend the ATS protocol in wireless sensor networks under message manipulation attacks. We first investigate the impact of message manipulation attacks over ATS, and derive a necessary condition for ATS to converge. Then based on the obtained insights, we propose a novel adjusting parameter checking mechanism which exploits the two-hop neighboring information to dynamically constrain the attackers. We further incorporate all checking processes into the traditional ATS protocol to form a secure average-consensus-based time synchronization protocol (SATS). We prove that SATS guarantees the network time synchronization with an exponentially converging speed.

The paper studies the basic characteristics and the definition of the wireless sensor networks, the time synchronization of wireless sensor network and the algorithm of time synchronization in wireless sensor networks. This paper studies the RBS algorithm and its improved algorithm. The research results have practical significance to the rapid development of network and the future network.

A unified framework to jointly solve the problems of localization and synchronization at the same time is presented in this paper. The joint approach is attractive because it can solve both localization and synchronization using the same set of message exchanges, which is extremely important for energy saving in wireless sensor networks. The inaccuracy of anchor locations and timings is taken into account to provide accurate joint localization and synchronization. The anchor uncertainties are assumed to be bounded, but knowledge of the statistics of anchor uncertainties is not required. The problem is formulated into a linear model with uncertainties on both sides of the equation. A robust joint estimator is then proposed based on minimizing the worst-case mean square error and the solution is obtained by solving a semidefinite programming problem. Simulation results show that the proposed estimator outperforms the traditional least squares estimator at the cost of higher computational complexity.

Accurate time synchronization of ultrasonic TOF measurements in IEEE 802.15.4 based wireless sensor networks

The reduced-rank method exploits the distortion-variance tradeoff to yield superior solutions for classic problems in statistical signal processing such as parameter estimation and filtering. The central idea is to reduce the variance of the solution at the expense of introducing a little distortion. In the context of parameter estimation, this yields an estimator whose sum of distortion plus variance is smaller than the variance of its undistorted counterpart. The method intrinsically results in an ordering mechanism for the singular vectors of the system matrix in the measurement model used for estimating the parameter of interest. According to this ordering rule, only a few dominant singular vectors need to be selected to construct the reduced-rank solution while the rest can be discarded. The reduced-rank estimator is less sensitive to measurement errors. In this paper, we attempt to derive the reduced-rank estimator for the total least squares (TLS) problem, including the order selection rule. It will be shown that, due to the inherent structure of the problem, it is not possible to exploit the distortion-variance tradeoff in TLS formulations using existing techniques, except in some special cases. This result has not been reported previously and warrants a new viewpoint to achieve rank reduction for the TLS estimation problem. The work is motivated by the problems arising in practical applications such as channel estimation in wireless communication systems and time synchronization in wireless sensor networks.