异步环境中基于时钟精度差的时钟同步

在异步通信模型的基础上，提出了一个基于时钟精度差的时钟同步策略，并给出了一个完整的系统模型，研究的重点有3个方面：（1）在时钟同步过程中使用单向信息传输策略，可以有效地减少网络负载；（2）客户机使用本地节点的时间信息和来自于参考时钟的时间戳信息构造一个线性数学模型，并获得本地时钟与参考时钟的运行精度差；（3）根据本地节点计算出的时钟精度差，构造一个自适应的容错模型，能够保证当本地节点与参考节点的连接出现故障时，本地的时钟同步系统还能够正常工作。该文不仅给出了一个详细的数学模型，而且还在实际的Internet环境中进行了模拟试验，取得了满意的结论。     

Phase clocks for transient fault repair

Phase clocks are synchronization tools that implement a form of logical time in distributed systems. For systems tolerating transient faults by self-repair of damaged data, phase clocks can enable reasoning about the progress of distributed repair procedures. This paper presents a phase clock algorithm suited to the model of transient memory faults in asynchronous systems with read/write registers. The algorithm is self-stabilizing and guarantees accuracy of phase clocks within O(k) time following an initial state that is.     

基于RTEthernet改进的时钟同步算法

以太网其庞大的网络系统在复杂的环境中存在网络链路延迟,节点时钟的漂移,同步能力差等问题。通过研究RTEthernet协议的起源和工作原理,考虑到影响实时以太网时间同步精密度的时钟拜占庭故障、网络传输延迟和漂移率等三个因素,建立了符合RTEthernet协议的通信模型。对FTA时钟同步算法在故障下时钟同步精密度损失率提升较少的问题进行了研究,引入了滑动窗口技术,提出了容错滑动窗口（Fault-Tolerant Sliding Window, FTSW）算法。容错滑动窗口算法能进一步提高分布式系统在进行时钟同步是对故障节点的容错能力。最后,使用CANoe仿真工具对FTSW算法进行仿真验证, FTSW算法的容错性优于FTA时钟同步算法算法,且在系统（七个节点）中存在两个拜占庭故障的情况下,同步后的精密度损失率降低了7.1%。     

时钟同步的假设检验研究

时钟同步技术是分布式系统中非常活跃的研究领域之一，由于大多数分布式系统实际上是不同步的，因此需要采用容错时钟同步算法确保消息通信的有界延迟，而基于假设检验的时钟同步技术可以避免因错失对两个高概率不同步时钟进行同步调整而造成系统不正常使用的情况。该文讨论了时钟同步的假设检验问题。除了假设检验、两类错误概率，还给出了概率最小时钟偏差、时钟同步概率等概念。在时钟偏差的统计分布特性近似于服务正态分布的假设条件之下，提出了基于非中心t分布的时钟同步假设检验方案。最后，基于服务器和客户端之间双向消息通信传输模式，给出时钟偏差的估计和检验样本。     

基于连续时间戳通信模型的时钟同步

时钟同步是分布式系统、通信领域中的核心技术之一。本论文提出了在异步通信网络环境中实现时钟同步的一种新的方法,即采用连续的时间戳通信模型来构造时钟精度差模型,并根据获得的时钟精度差时钟调整。这种方法不仅能够有效的减少流量对时钟同步的影响,而其也可以进一步提高时钟同步的精度和自适应能力。     

A Time-Optimal Self-Stabilizing Synchronizer Using A Phase Clock

A synchronizer with a phase counter (sometimes called asynchronous phase clock) is an asynchronous distributed algorithm, where each node maintains a local "pulse counter" that simulates the global clock in a synchronous network. In this paper, we present a time-optimal self-stabilizing scheme for such a synchronizer, assuming unbounded counters. We give a simple rule by which each node can compute its pulse number as a function of its neighbors' pulse numbers. We also show that some of the popular correction functions for phase clock synchronization are not self-stabilizing in asynchronous networks. Using our rule, the counters stabilize in time bounded by the diameter of the network, without invoking global operations. We argue that the use of unbounded counters can be justified by the availability of memory for counters that are large enough to be practically unbounded and by the existence of reset protocols that can be used to restart the counters in some rare cases where faults will make this necessary.     

Interval-based Clock Synchronization

In this paper, we develop and analyze a simple interval-based algorithm suitable for fault-tolerant external clock synchronization. Unlike usual internal synchronization approaches, our convergence function-based algorithm provides approximately synchronized clocks maintaining both precision and accuracy w.r.t. external time. This is accomplished by means of a time representation relying on intervals that capture external time, providing accuracy information encoded in interval lengths. The algorithm, which is generic w.r.t. the convergence function and relies on either instantaneous correction or continuous amortization for clock adjustment, is analyzed by utilizing a novel, interval-based framework for establishing worst-case precision and accuracy bounds subject to a fairly detailed system model. Apart from individual clock rate and transmission delay bounds, our system model incorporates non-standard features like clock granularity and broadcast latencies as well. Relying on a suitable notion of internal global time, our analysis unifies treatment of precision and accuracy, ending up in striking conceptual beauty and expressive power.     

Timed-pNets: a communication behavioural semantic model for distributed systems

This paper presents an approach to build a communication behavioural semantic model for heterogeneous distributed systems that include synchronous and asynchronous communications. Since each node of such system has its own physical clock, it brings the challenges of correctly specifying the system time constraints. Based on the logical clocks proposed by Lamport, and CCSL proposed by Aoste team in INRIA, as well as pNets from Oasis team in INRIA, we develop timed-pNets to model communication behaviours for distributed systems. Timed-pNets are tree style hierarchical structures. Each node is associated with a timed specification which consists of a set of logical clocks and some relations on clocks. The leaves are represented by timed-pLTSs. Non-leaf nodes (called timed-pNets nodes) are synchronisation devices that synchronize the behaviours of subnets (these subnets can be leaves or non-leaf nodes). Both timed-pLTSs and timed-pNets nodes can be translated to timed specifications. All these notions and methods are illustrated on a simple use-case of car insertion from the area of intelligent transportation systems (ITS). In the end the TimeSquare tool is used to simulate and check the validity of our model.     

无线传感器网络层次参考时间同步算法的研究

无线传感器在网络应用中要求节点间保持时间同步,但现存的经典时间同步算法,因节点的接收时间受时钟偏差和传输延迟的影响,其同步精度不高。为提高网络时间同步精度,均衡节点能耗,提出了一种改进的层次参考时间同步算法（Improved Hierarchy Referencing Time Synchronization,IHRTS）。该算法基于节点在层次结构中唯一物理位置的时间特性,采用贝叶斯估计对节点接收时间进行估算,缩小时间偏差的误差范围,获得比较精确的同步偏移量,从而改善时间同步精度;同时采用无线信道的广播特性与双向同步机制的同步思想,最小化了通信负载,均衡了节点能耗。通过仿真结果表明将贝叶斯估计方法应用到时间同步算法中,在均衡节点能量消耗同时有效地提高了网络同步精度。     

异步环境中基于时钟精度差的时间同步问题研究

随着计算机网络技术和分布式理论的发展，在异步环境中的时间同步问题就显得越来越重要。提出了一种基于时钟精度差的时间同步策略，并给出了相应的证明，同时也讨论了一种基于连续时间变化的领导选择策略，以保证获得正确的标准时钟基。     

矿用分布式采集系统时钟同步方案设计与实现

单一的时钟同步技术由于其精度及应用局限性,无法满足矿用分布式采集系统高精度、高可靠性的时钟同步性能要求。针对上述问题,提出了基于北斗+IEEE 1588V2+本地后备时钟的三级协同时钟同步方案。选用部署在地面的T600-BDGOCXC型北斗授时服务器作为主时钟,为系统提供纳秒级精度的绝对时钟;采用STM32F407+DP83848及PTPd协议栈实现支持IEEE 1588V2协议的采集节点,通过井下工业环网将北斗的绝对时钟同步到各采集节点;本地后备时钟采用STM32F407内部RTC(实时时钟)实现,给各采集节点提供秒级精度的时间戳初值,便于各采集节点用最短时间实现与主时钟的同步。测试结果表明,北斗授时服务器与采集节点通过交换机直连的情况下,1 min后时钟同步精度达162 ns;北斗授时服务器与采集节点通过三级交换机连接的情况下,时钟同步精度为565 ns;在北斗授时服务器失效的情况下,优先级高的采集节点升级为主时钟并为其余采集节点授时,具有较强的可靠性。     

SAE AS6802协议研究及模块化仿真平台设计

分析安全关键领域的时间触发网络协议SAE AS6802与时间同步算法技术,分别对同步流程、时间同步角色、时序保持算法、集中控制算法、时钟纠正算法、同/异步派系检测算法以及通道择优方法等进行研究,归纳时间同步精度的影响因素。在此基础上,设计一种时间同步算法IP核,并构建基于SystemVerilog的模块化仿真验证平台。利用该平台对时间同步算法进行RTL级仿真,结果验证了该时间同步算法的正确性,其同步精度保持在亚微秒级,满足下一代高安全关键领域的应用要求。     

基于推后补偿的时钟同步算法研究

时钟同步精度是网络时间同步的重要指标。目前网络节点的增多及涉及行业领域的扩大,给时钟同步精度提出了更高的要求,因此需要研究新的时钟同步算法提高时钟同步精度。本文以网络时钟同步的单向比较法为研究对象,针对其在时钟同步精度上的不足,提出了一种新的时钟同步算法,并给出了此算法在PPS信号同步中的应用,实验结果表明所提出的时钟同步算法提高了PPS信号的传递精度。该算法简单明了,易于工程实现。     

无线传感器网络环境下基于卡尔曼滤波的PTP协议

作为分布式系统的重要组成部分,精确时间同步是对时间敏感的工业无线网络的核心技术.基于时间信息包交换的IEEE 1588精确时间同步协议（PTP）主要针对有线网络提出,其同步精度受制于时间戳的精度和传输延迟抖动.在无线传感网中,节点难以获取精确时钟戳,同时由于信道共享、包冲突和信道衰落,无线网络的传输延迟抖动非常明显. 研究了无线网络中PTP的性能与时间戳精度之间的关系,提出了一个自回归模型来描述时钟漂移,将PTP中的包交换过程抽象为一组状态空间方程,将延迟抖动等作为观测噪音,从而利用卡尔曼滤波器予以滤除.仿真结果表明,在不同时间戳精度和延迟抖动下,卡尔曼滤波能有效改善时钟误差和稳定性.     

BOND:一种双向的单路网络延时测量方法

用软件的方法进行单路网络延时的精确测量，需要解决两个端系统的时钟异步问题．现有的测量算法(如Ping和LPA)在测量精度和实时性方面存在着明显的不足，针对这些问题，本文提出一种双向的单路网络延时测量方法，新的延时测量算法对LPA算法进行了改进．实验结果表明，双向单路网络延时测量方法是一种十分有效的延时测量算法，特别适合于工程应用。     

Average TimeSynch: A consensus-based protocol for clock synchronization in wireless sensor networks

This paper describes a new consensus-based protocol, referred to as Average TimeSync (ATS), for synchronizing the clocks of a wireless sensor network. This algorithm is based on a cascade of two consensus algorithms, whose main task is to average local information. The proposed algorithm has the advantage of being totally distributed, asynchronous, robust to packet drop and sensor node failure, and it is adaptive to time-varying clock drifts and changes of the communication topology. In particular, a rigorous proof of convergence to global synchronization is provided in the absence of process and measurement noise and of communication delay. Moreover, its effectiveness is shown through a number of experiments performed on a real wireless sensor network.     

基于卡尔曼滤波估计的一致性时钟同步算法

针对无线传感器网络（WSN）的众多应用都需要依赖时钟同步的节点协同完成,而由于节点的晶体震荡器受自身以及外界环境的影响,使得节点时钟偏斜和时钟偏移两个参数发生变化导致时钟不同步问题,提出了基于分布式卡尔曼滤波估计的一致性补偿时钟同步算法DKFCC。该算法首先利用双向信息交换机制以及分布式卡尔曼滤波实现时钟偏斜和偏移两个参数的最优估计,然后基于时钟参数的最优估计值采用一致性补偿方法实现节点的时钟同步。实验结果表明：在100个节点随机部署的WSN中,采用虚拟全局一致性方式的DKFCC同步算法比异步一致性同步（AC）算法的同步均方根误差（SRAMSE）值降低了约95%,具有较高的同步精度;同时,所提出算法从时钟参数层面实现同步,无需频繁地进行时钟同步操作,相比AC算法更节能。     

Scaled consensus for asynchronous high‐order discrete‐time multiagent systems

This paper investigates the distributed scaled consensus problem of multiple agents with high‐order dynamics under the asynchronous setting, where each agent measures the neighbors' information at certain discrete time instants according to its own clock rather than the whole discrete process and all agents' clocks are independent of each other. Assume that the communication topology can be arbitrarily switched and the information transfer between agents has a time‐varying delay. Under the designed asynchronous distributed control protocol, it is shown that the agents with the same scale will reach a common final state, while the agents with different scales will reach different final states. Moreover, an effective parameters selection strategy is presented for a large number of gain parameters in high‐order multiagent systems based on novel model transformation techniques. Simulation examples are provided to demonstrate the high‐order scaled consensus performances for the agents in the presence of asynchronous setting.     

一种线型结构无线传感器网络的时间同步算法

针对煤矿井下液压支架模糊控制监测系统网络拓扑,提出了一种适用于线型结构无线传感器网络的时间同步算法。该算法同步过程分为簇间同步和簇内同步,簇间同步采用双向同步机制修正时间偏移的方法减少同步误差,簇内同步通过最小平方线性拟合方法构造逻辑时钟函数,从而得到簇内任一节点与簇首节点的频偏和相偏估计值,以提高同步精度。仿真结果表明,该算法能有效减少时间同步误差和能量消耗。     

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。