Average time synchronization in wireless sensor networks by pairwise messages

Most of previous algorithms for time synchronization choose a specific node’s (denoted as a root or leader) local time to be the reference time, which is easily disturbed by many events (e.g. root node’s power down or damage). The Gaussian distribution for the nodes’ local clocks has been reported by a few authors based on laboratory tests, the average of all nodes’ clocks is the best approximation to the ideal time. In this paper, the possibility to realize average time synchronization in wireless sensor networks by pairwise messages exchange is studied, and a simple algorithm (ATSP) is proposed, which synchronizes all the nodes’ clocks to their average. For networks with clock skew, the algorithm compensates the frequencies of nodes to their average also. Using the Lyapunov’s stability theory, convergence analyses and proofs of the algorithm are given. Synchronization error (accuracy) of the algorithm is estimated by using probability theory also, which indicates that the synchronization error of the algorithm is linearly related to the standard deviation of the message delay. Simulations are performed on a 300 nodes network to examine the performance of the algorithm, which verified the theoretical results.     

AUV辅助的水下传感器网络时钟同步算法

针对当前水下传感器网络中的时钟同步难题,设计了一种三元阵被动定位自主水下航行器（ AUV）模型,并基于此模型提出了AUV辅助的时钟同步（ AUV-Sync）算法。该算法通过AUV与节点之间相对运动过程中进行的信息交换来对节点相对距离进行估计,进而基于相对距离计算单向传播时延来降低由于节点移动性所导致的误差。最后,通过两轮加权最小二乘法进行线性回归来估计时钟同步的参数。仿真结果表明：在存在节点漂移的动态水下传感器网络环境中,该算法较其他相关算法有更高的精度。     

可变周期的无线传感器时间同步算法

时间同步技术是无线传感器网络应用中的一项关键技术.介绍了几种无线传感器网络的主流同步算法.通过对比,总结了各种算法的优点和限制条件.最后提出了可变周期的无线传感器时间同步算法,该算法利用改进的参考广播同步协议估计时间偏差和速率偏差,并通过估计出来的偏差值动态调整下次同步的周期,达到减少同步次数,节约能量的目的.     

无线传感器网络TPSN-RBS联合时间同步算法

针对大规模多跳传感器网络节点间所存在的同步误差及其累积误差问题,提出了一种基于加权最小二乘法的TPSN-RBS联合时间同步算法.该算法充分利用可监听到的消息,通过加权最小二乘法估计得到节点逻辑时钟的时间偏移和频率偏移的最优解.用Cramér-Rao下界对本算法进行性能分析,同时与TPSN算法进行仿真对比,结果表明:该算法提高了节点间的同步精度,且在节点密集的大规模无线传感器网络中,在保证较低通信量的同时降低了累积误差.     

基于分簇的高能效无线传感器网络时间同步算法

针对经典时间同步算法应用于无线传感器网络时主要是提高同步精度而忽略网络能耗的问题,提出了一种基于分簇的高能效无线传感器网络时间同步算法.该算法基于分簇的网络拓扑结构,基站与簇首采用改进的双向同步机制传播同步消息包,簇首与簇内节点采用双向同步机制与被动监听相结合的方式完成时间同步,减少了消息包传输的数量,同时设计了同步分组延迟的周期更新因子,进一步减少了报文开销.仿真结果证明:该机制能减少同步报文开销,降低网络能量消耗,保证良好的同步精度.     

Ad Hoc网络时间同步技术研究

通过分析影响时间同步性能的6类网络延时,对几种基本的无线传感器网络时钟同步协议进行了介绍和性能比较,提出了一种基于时延探测机制的AdHoc网络时间同步协议.协议通过构造特定的工作方式及信息格式,将各类时延误差作为一个整体来探测和消除.此外,由于时延探测过程和时间校正过程相互分离,减轻了时延探测过程中对于信息成功交互的时间要求,有利于降低网络的冲突和负担.分析表明,DMC-TS协议实现简单,扩展性好,由于针对所有的延时误差都进行了校正,因此能够提供较好的同步精度.     

无线传感器网络中多跳时间同步算法的研究

提出了一种基于簇型的多跳时间同步算法——CBTS。利用Leach算法将网络划分成不同的簇,在此基础上把节点间的时间同步分为水平同步和垂直同步两个阶段来完成。在水平同步阶段,通过构建基站与簇头节点的层次拓扑结构,采用双向消息交换同步机制来完成簇头节点与基站的时间同步。在垂直同步阶段,采用双向消息交换和参考广播相结合的同步机制,来完成簇头节点和簇成员之间的时间同步,并利用最小方差线性拟合的方法估计了节点的时钟偏差,提高了时钟同步的精度,最终实现了整个网络节点的时间同步。经过仿真测试,证明该算法具有较低的消息交换开销和不错的同步精度。     

基于簇的能量均衡无线传感器网络时间同步算法

针对现有时间同步算法应用于多跳无线传感器网络时存在的误差累计和能耗不均衡问题,提出一种基于簇的能量均衡时间同步算法。该算法基于簇状分层的网络拓扑,簇首之间采用双向监听机制代替双向交换机制,以减小通信开销和发送时延带来的同步误差;簇成员节点利用双向交换和单向广播相结合的机制与簇首同步,并通过最优剩余能量选取回应节点,均衡簇内节点能耗。对提出的新方法和传统的同步算法在精度和能耗方面进行理论分析和仿真验证,结果表明,该算法在保证较高同步精度的前提下可以减小通信开销,均衡网内节点能量消耗,延长网络生命周期。     

An accurate on-demand time synchronization protocol for wireless sensor networks

Time synchronization is a critical component in any wireless sensor network (WSN). In terms of energy consumption, on-demand time synchronization is better than continuous synchronization. However, currently existing on-demand time synchronization protocols have a very low accuracy and very strong spatial accumulative effect. These features are not suitable for several types of WSN applications, such as applications with stringent temporal requirements, or applications that have a large spatial region of interest. In this paper, we propose an on-demand time synchronization protocol, named AOTSP (Accurate On-demand Time Synchronization Protocol), which differs from other protocols of the same category by having the following advantages, as shown in our theoretical analysis and simulation results: (1) weak spatial accumulative effect; (2) fairly low communication cost; (3) low computational complexity; (4) high accuracy; (5) high scalability. Such features make AOTSP a suitable time synchronization protocol for a broad range of WSN applications.     

Projective synchronization for fractional-order memristor-based neural networks with time delays

In this paper, the global projective synchronization for fractional-order memristor-based neural networks with multiple time delays is investigated via combining open loop control with the time-delayed feedback control. A comparison theorem for a class of fractional-order systems with multiple time delays is proposed. Based on the given comparison theorem and Lyapunov method, the synchronization conditions are derived under the framework of Filippov solution and differential inclusion theory. Several feedback control strategies are given to ensure the realization of complete synchronization, anti-synchronization and the stabilization for the fractional-order memristor-based neural networks with time delays. Finally, a numerical example is given to illustrate the effectiveness of the theoretical results.

     

WSN中时间同步技术发展研究

针对无线传感器网络WSN重要技术之一的时间同步技术研究了几种时间同步技术的实现过程。典型同步技术应用于大规模无线传感器网络时鲁棒性和可扩展性受到限制,在此基础上重点研究新的同步技术——协作同步和萤火虫同步。协作同步解决了多跳误差累积的情况;萤火虫同步机制简单,平等对待各个节点,可扩展性和鲁棒性优越。通过对国内外学者的研究成果进行探讨总结,指出了未来时间同步技术的研究重点和方向。     

基于GPS的无线传感器网络时间同步技术

在无线传感器网络测试系统中,为了对测试对象进行准确的测试,时间同步技术至关重要。根据GPS授时原理,提出了利用GPS秒脉冲对同步控制系统进行触发,并无线广播同步触发信号,同时,各传感器节点通过接收同步触发信号,进行时钟同步触发、时钟校准,以及对测试事件触发的同步采集。通过验证,各传感器节点之间的事件同步触发误差为1μs左右,并具有很高的可靠性,可广泛应用于航空、航海等工程测试领域。     

基于生成树的无线传感器网络时钟同步算法

考虑到无线传感器网络时钟同步多跳误差累积问题,提出一种基于动态生成树的全网时钟同步算法,只需由根节点开始沿树边广播一次同步消息,全网待同步节点即可采用时钟偏差补偿,并结合贝叶斯后验估计算法对时钟进行准确估计。一个同步轮次内每个节点至多广播两次消息即可完成全网同步。使用OMNeT++软件进行仿真实验表明算法有效降低了多跳累积误差,提高了同步精度,且具有较快的收敛速度和较低的开销。     

水声传感器网络中成簇时间同步协议研究

针对水声通信传播延迟长和节点移动性强的问题, 提出一种水声传感器网络中成簇时间同步协议。该协议将时间同步分为簇间同步和簇内同步两个过程, 并且通过簇间同步和簇内同步的局部并发执行减少同步过程中产生的报文数量。此外, 该协议在计算时钟频偏和相偏的过程中通过区分因为节点移动性造成的下游链路和上游链路传输延迟的不同来提高同步精度。仿真结果表明, 与其他同步协议相比, 成簇时间同步协议可以在降低同步误差的同时, 有效减少网络中同步报文的数量, 从而达到降低能量消耗的目的。     

An application adaptation layer for wireless sensor networks

In wireless sensor networks, poor performance or unexpected behavior may be experienced for several reasons, such as trivial deterioration of sensing hardware, unsatisfactory implementation of application logic, or mutated network conditions. This leads to the necessity of changing the application behavior after the network has been deployed. Such flexibility is still an open issue as it can be achieved either at the expense of significant energy consumption or through software complexity. This paper describes an approach to adapt the behavior of running applications by intercepting the calls made to the operating system services and changing their effects at run-time. Customization is obtained through small fragments of interpreted bytecode, called adaptlets, injected into the network by the base station. Differently from other approaches, where the entire application is interpreted, adaptlets are tied only to specific services, while the bulk of the application is still written in native code. This makes our system able to preserve the compactness and efficiency of native code and to have little impact on the overall application performance. Also, applications must not be rewritten because the operating system interfaces are unaffected. The adaptation layer has been implemented in the context of TinyOS using an instruction set inspired to the Java bytecode. Examples that illustrate the programming of the adaptation layer are presented together with their experimental validation.     

一种异构传感器网络时间同步的安全性研究

分布式时间同步技术是无线传感器网络（WSNs）应用中的一项重要支撑技术,保证时间同步的安全性至关重要。针对敌对异构网络环境,通过利用存储有高能量的高级传感器,提出了一种高效的异构WSNs时间同步安全方案。Tmote传感器节点实验表明：该方案比现有的同步方案更能有效提高同步精度,减少通信开销和抵御网络的多种攻击。     

无线传感器网络时间同步算法的安全性研究

时间同步是无线传感器网络的重要支撑技术之一,保证时间同步安全性对无线传感器网络的应用至关重要.根据现有的三类同步机制的典型算法的原理分析其安全性,然后对目前的安全措施进行了分析总结.     

Toward secure and scalable time synchronization in ad hoc networks

Time synchronization is crucial in ad hoc networks. Due to the infrastructure-less and dynamic nature, time synchronization in such environments is vulnerable to various attacks. Moreover, time synchronization protocols such as IEEE 802.11 TSF (Timing Synchronization Function) often suffer from scalability problem.In this paper, we address the security and the scalability problems of time synchronization protocols in ad hoc networks. We propose a novel suite of time synchronization mechanisms for ad hoc networks based on symmetric cryptography. For single-hop ad hoc networks, we propose SSTSP, a scalable and secure time synchronization procedure based on one-way Hash chain, a lightweight mechanism to ensure the authenticity and the integrity of synchronization beacons. The “clock drift check” is proposed to counter replay/delay attacks. We then extend our efforts to the multi-hop case. We propose MSTSP, a secure and scalable time synchronization mechanism based on SSTSP for multi-hop ad hoc networks. In MSTSP, the multi-hop network is automatically divided into single-hop clusters. The secure intra-cluster synchronization is achieved by SSTSP and the secure inter-cluster synchronization is achieved by exchanging synchronization beacons among cluster reference nodes via bridge nodes.The proposed synchronization mechanisms are fully distributed without a global synchronization leader. We further perform analytical studies and simulations on the proposed approaches. The results show that SSTSP can synchronize single-hop networks with the maximum synchronization error under 20 μs and MSTSP 55–85 μs for multi-hop networks, which are, to the best of our knowledge, among the best results of currently proposed solutions for single-hop and multi-hop ad hoc networks. Meanwhile, our approaches can maintain the network synchronized even in hostile environments.     

On the minimal firing time of the firing squad synchronization problem for polyautomata networks

For each class Γ of polyautomata networks (networks composed of copies of a same finite automaton and connections from output terminals to input terminals of the copies), the firing squad synchronization problem (fssp) for Γ is naturally defined. Supposed that the fssp for Γ has a solution. For each network N in Γ, the minimal firing time of N is also naturally defined (the minimum value of the firing time of N when all the solutions of the fssp for Γ are considered). We proved that the minimal firing time of N does not decrease even if we permit the use of infinite state automata as component automata of networks. We also give one characterization of the minimal firing time of N. For many classes Γ, this characterization gives algorithms to calculate the minimal firing time of N for each given N.