Self Organizing Maps for Synchronization in Wireless Sensor Networks

Impact of time synchronization error on the mode-shape identification and damage detection/localization in WSNs for structural health monitoring

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.

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.

A scheme for WSNs (wireless sensor networks) security is given by dividing sensing tetrahedron into clusters and using the symmetric polynomials in this paper.The sensing tetrahedron is divided into a number of small grids.All those sensor nodes, both ordinary sensor nodes and heterogeneous sensor nodes are distributed in the sensing tetrahedron.In a grid, all ordinary sensor nodes and heterogeneous sensor nodes establish their shared keys through using the symmetric polynomials.All the heterogeneous sensor nodes establish their shared keys through using the symmetric polynomials.At last, all sensor nodes establish their keys directly or indirectly in the whole sensing tetrahedron.Analysis and comparison demonstrate this scheme enhances the WSN security, has good network connectivity, saves node storage, reduces network computing load, and extends the network lifetime.

Study of consensus-based time synchronization in wireless sensor networks

ABSTRACTThe clock in embedded systems usually is driven by a crystal oscillator and implemented via a counter register, such a crystal clock is non-identical and drifting due to the manufacturing tolerance and variation of working conditions. Thus, a common time among distributed wireless sensor nodes, also referred to as Time Synchronization, is required for many time-sensitive wireless applications, such as collaborative condition monitoring, coordinated control and localization. Inspired by fireflies’ behaviour, the Pulse-Coupled Oscillators (PCO) has been proposed for synchronization in complex networks. Since the concurrent transmission of PCO’s Pulses is impossible in Wireless Sensor Networks (WSNs), the desynchronization mechanism is adopted to ensure the implementation of PCO in WSNs. Moreover, due to the uncertainties in radio channels and the complexities of communication protocols and packet-exchange behaviours in wireless networks, it is challenging to have a closed-form solution to the performance of PCO synchronization in WSNs. The realistic software simulation, in particular, the discrete event simulator has been a powerful tool to exam the performance of communication protocols in various scenarios, since an order sequence of well-defined event in time is to represent the behaviour of a complex system. This paper presents the development of a pulse-coupled oscillators time synchronization simulator on the OMNeT++ platform for simulating and studying its behaviour and performance in sensor networks. A clock module with configurable phase and frequency noises, and adjustable and higher resolution is developed to mimic various crystal oscillators in embedded systems, for example, the real-time clock. The developed simulator also supports the full functions devices defined by ZigBee protocol, which allows realistic simulation of multi-hop IEEE 802.15.4 wireless networks. Finally, the intensive simulations of classical PCO with the refractory period in IEEE 802.15.4-based WSNs have been carried out to demonstrate the features and benefits of the developed simulator. It is shown that for the non-identical and time-varying PCO clocks in the WSNs, the achieved synchronization will lose gradually, and the time that maintained synchronization depends on the length of refractory period.

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.

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.

The use of wireless multimedia sensor networks (WMSNs) for surveillance applications has attracted the interest of many researchers. As with traditional sensor networks, it is easy to deploy and operate WMSNs. With inclusion of multimedia devices in wireless sensor networks, it is possible to provide data to users that is more meaningful than that provided by scalar sensor-based systems alone; however, producing, storing, processing, analyzing, and transmitting multimedia data in sensor networks requires consideration of additional constraints, including energy, processing power, storage capacity, and communication. Furthermore, as multimedia sensors produce much more data than scalar sensors, more manpower is required to analyze multimedia data. To overcome these constraints and challenges, this paper aimed to propose a system architecture and a set of procedures for WMSNs that facilitate automatic classification of moving objects using scalar and multimedia sensors. Methods and standards for detecting and classifying a moving object, as well as transmission of the results, are described in detail. The hardware for each sensor node includes a built-in camera, a passive infrared motion sensor, a vibration sensor, and an acoustic sensor. An application using our proposed methods was developed and embedded in the multimedia sensor node. In addition, a sink station was set up and the data produced by the sensor network was collected by this server. The classification performance of the application was tested using video recorded by the sensor node. The effect of the proposed methods on power consumption was also tested and measured. The experimental results show that the proposed approach is sufficiently lightweight to be used for real-world surveillance applications.

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.

Pattern transformation-invariant schemes for wireless sensor networks based on an edge detection gradient-based mechanism

Node placement for connected target coverage in wireless sensor networks with dynamic sinks

Motivated by the importance of the clock synchronization in wireless sensor networks (WSNs), this paper proposes a new research approach and model approach, which quantitatively analyzes clock synchronization from the perspective of modern control theory. Two kinds of control strategies are used as examples to analyze the effect of the control strategy on clock synchronization from different perspectives, namely, the single‐step optimal control and the LQG global optimal control. The proposed method establishes a state space model for clock relationship, thus making dimension extension and parameter identification easier, and is robust to changes under the condition of node failures and new nodes. And through the design of different control strategies and performance index functions, the method can satisfy various requirements of the synchronization precision, convergence speed, energy consumption and the computational complexity, and so on. Finally, the simulations show that the synchronization accuracy of the proposed method is higher than that of the existing protocol, and the former convergence speed of the synchronization error is faster.

Sensor networks are multi hop wireless networks formed by a large number of resource-constrained sensor nodes. The events detected by the sensor generate a stream of data. Centralized join query processing algorithm incur more communication overhead due to frequent exchange of data between the sink and the sensor nodes. To query or access data generated by the sensor nodes, the sensor network can be viewed as a distributed database. Communication-efficient implementation for join of multiple data streams in a sensor network is particularly challenging due to unique characteristics of the sensor networks such as limited memory and battery energy on individual nodes. Hence the design of Distributed Nested Loop Join Processing (DNLJP) algorithm has been proposed in this paper. The proposed scheme groups sensors based on geographic locations to form a cluster and perform the query processing in a distributed manner over the data collected across different regions. DNLJP also optimizes the query based on the desired optimization criteria like query cost, query execution time etc., and applies the corresponding query processing technique to achieve the desired result. Analysis shows that the communication overhead of the proposed distributed algorithm is reasonably low and the efficiency of the query processing is considerably improved over a wide range of query.

Wireless sensor and actuator networks (WSANs) are heterogeneous networks composed of many different nodes that can cooperatively sense the environment, determine an appropriate action to take, then change the environment’s state after acting on it. As a natural extension of wireless sensor networks (WSNs), WSANs inherit from them a variety of research challenges and bring forth many new ones. These challenges are related to dealing with imprecise and vague information, solving complicated optimization problems or collecting and processing data from multiple sources. Computational intelligence (CI) is an overarching term denoting a conglomerate of biologically and linguistically inspired techniques that provide robust solutions to NP-hard problems, reason in imprecise terms and yield high-quality yet computationally tractable approximate solutions to real-world problems. Many researchers have consequently turned to CI in hope of finding answers to a plethora of WSAN-related challenges. This paper reviews the application of several methodologies under the CI umbrella to the WSAN field. We describe and categorize existing works leaning on fuzzy systems , neural networks , evolutionary computation , swarm intelligence , learning systems , and their hybridizations to well-known or emerging WSAN problems along five major axes: 1) actuation; 2) communication; 3) sink mobility; 4) topology control; and 5) localization. The survey offers informative discussions to help reason through all the studies under consideration. Finally, we point to future research avenues by: 1) suggesting suitable CI techniques to specific problems; 2) borrowing concepts from WSNs that have yet to be applied to WSANs; or 3) describing the shortcomings of current methods in order to spark interest on the development of more refined models.