An Event -Triggered Time Synchronization Scheme for Sensor Networks

Time Synchronization is one of the problems and still ignored problem in area of wireless sensor network (WSN). After reviewing the existing literatures, it is found that there are few studies that combinely address the problem of energy conservation, clustering, routing along with minimizing the errors due to time synchronization in sensor network. Therefore, this manuscript presents a delay-based routing which considers the propagation delay to formulate a mechanism for delay compensation in large scale wireless sensor network. The prime goal of this technique is to jointly address energy problems, time synchronization, and routing in wireless sensor network. The outcome of the proposed study was found to posse’s minimized communication overhead, minimized synchronization errors, lower energy consumption, and reduced processing time when compared with the existing standards of time synchronization techniques.

Time synchronization is critical to sensor networks at many layers of its design and enables better duty-cycling of the radio, accurate localization, beamforming and other collaborative signal processing. While there has been significant work in sensor network synchronization, measurement based studies have been restricted to very short-term (few minutes) datasets and have focused on obtaining accurate instantaneous synchronization. Long-term synchronization has typically been handled by periodic re-synchronization schemes with beacon intervals of a few minutes based on the assumption that long-term drift is too hard to model and predict. Thus, none of this work exploits the temporally correlated behavior of the clock drift. Yet, there are incredible energy gains to be achieved from better modeling and prediction of long-term drift that can provide bounds on long-term synchronization error across a sensor network. Better synchronization can lead to significantly lower duty-cycles of the radio, simplify signal processing and can enable an order of magnitude greater lifetime than current techniques.We measure, evaluate and analyze in-depth the long-term behavior of synchronization skew and drift on typical Mica sensor nodes and develop an efficient long-term time synchronization protocol. We use four real time data sets gathered over periods of 12-30 hours in different environmental conditions to study the interplay between three key parameters that influence long-term synchronization - synchronization rate, history of past synchronization beacons and the estimation scheme. We use this measurement-based study to design an online adaptive time-synchronization algorithm that can adapt to changing clock drift and environmental conditions while achieving application-specified precision with very high probability. We find that our algorithm achieves between one and two orders of magnitude improvement in energy efficiency over currently available time-synchronization approaches.

Time synchronization is a critical component in many wireless sensor network applications. Although several synchronization protocols have recently been developed, they tend to break down when implemented on networks of heterogeneous devices consisting of different hardware components and operating systems, and communicate over different network media. In this paper, we present a methodology for time synchronization in heterogeneous sensor networks (HSNs). This includes synchronization between mote and PC networks, a communication pathway that is often used in sensor networks, but has received little attention with respect to time synchronization. In addition, we evaluate clock skew compensation methods including linear regression, exponential averaging, and phase-locked loops. Our HSN synchronization methodology has been implemented as a network service and tested on an experimental testbed. We show that a 6-hop heterogeneous sensor network can be synchronized with an accuracy on the order of microseconds.KeywordsSensor NetworkSensor NodeWireless Sensor NetworkTime SynchronizationSynchronization ErrorThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Packet collision in underwater wireless sensor networks (UWSNs) can create inconsistent time stamps leading to inaccurate time synchronization in underwater sensor networks. Most extant time synchronization techniques do not model for the probability of packet collision in UWSNs. This paper proposes a novel non-homogenous Poisson process (NHPP)-based back-off technique that estimates probability of packet collision in UWSN over a set period of time and allows sending of data packets only when probability of collision in a network is low. Any inconsistent time stamps are further identified and removed using Mahalanobis distance technique before linear regression is run. The proposed method performs better in extensive simulations as compared to two similar time synchronization techniques in literature.KeywordsUWSNsNon-homogenous Poisson processPacket collisionTime synchronization

Secure and self-stabilizing clock synchronization in sensor networks

Study of consensus-based time synchronization in wireless sensor networks

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.

Global synchronization is important for many sensor network applications that require precise mapping of collected sensor data with the time of the events, for example, in tracking and surveillance. It also plays an important role in energy conservation in MAC layer protocols. This paper describes four methods to achieve global synchronization in a sensor network: a node-based approach, a hierarchical cluster-based method, a diffusion-based method, and a fault-tolerant diffusion-based method. The diffusion-based protocol is fully localized. We present two implementations of the diffusion-based protocol for synchronous and asynchronous systems and prove its convergence. Finally, we show that, by imposing some constraints on the sensor network, global clock synchronization can be achieved in the presence of malicious nodes that exhibit Byzantine failures.

Time synchronization is essential for several ad-hoc network protocols and applications, such as TDMA scheduling and data aggregation. In this paper, we propose a clustering- based time synchronization framework for multi-hop sensor net- works. We assume that relative node synchronization is sufficient, i.e., consensus on one time value is not required. Our goal is to divide the network into connected synchronization regions (nodes within 2-hops) and perform inter-regional synchronization in O(LLSync) × Niter time, where O(LLSync) denotes the complexity of the underlying low-level synchronization technique (used for single hop synchronization), and Niter denotes the number of iterations where the low-level synchronization protocol is invoked. We propose two novel fully-distributed protocols, SYNC-IN and SYNC-NET, for regional and network synchro- nization, respectively, and prove that Niter is O(1) for both protocols. We exploit the tradeoff between rapid convergence (and consequently energy-efficiency) and perceived accuracy. Our framework does not require any special node capabilities (e.g., being GPS-enabled), or the presence of reference nodes in the network. Our framework is also independent of the particular clustering, inter-cluster routing, and low-level synchronization protocols. We formulate a density model for analyzing inter- regional synchronization, and evaluate our protocols via extensive simulations.

This chapter reviews time synchronization and calibration for wireless sensor networks. Authors show that time synchronization can be considered as a calibration problem and many observations about time synchronization can be transferred to calibration. They discuss applications of synchronized time in sensor networks, present challenges of sensor networks, and discuss why traditional synchronization approaches fail to meet these challenges. Models of sensor nodes, of hardware clocks, and of communication are presented. Authors then give an overview of the various classes of synchronization and present common synchronization techniques. Next, they examine current synchronization algorithms and present common techniques for evaluating synchronization algorithms and selected evaluation results.

Attack-resilient time synchronization for wireless sensor networks

Time synchronization in a wireless sensor network is critical for accurate timestamping of events and fine-tuned coordination of wake/sleep duty cycles to reduce power consumption. This paper proposes TSync, a novel lightweight bidirectional time synchronization service for wireless sensor networks. TSync's bidirectional service offers both a push mechanism for accurate and low overhead global time synchronization as well as a pull mechanism for on-demand synchronization by individual sensor nodes. Multi-channel enhancements improve TSync's performance. We deploy a GPS-enabled framework in live sensor networks to evaluate the accuracy and overhead of TSync in comparison with other in-situ time synchronization algorithms.

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.

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.