Distributed Clock Synchronization Research Articles

Unsupervised Deep-Learning for Distributed Clock Synchronization in Wireless Networks

One of the major factors which limits the throughput in wireless communications networks is the accuracy of time synchronization between the nodes in the network. Synchronization methods based on pulse-coupled oscillators (PCOs) have the advantage of simple implementation and achieve high accuracy when the nodes are closely located. However, such schemes tend to have poor synchronization performance for distant nodes, as well as in the presence of clock frequency offsets between the nodes. In this paper we present a novel PCO-based Deep neural network (DDN)-Aided Synchronization Algorithm coined DASA. We design DASA as a novel low-complexity and interpretable architecture by converting classic PCO-based synchronization into <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a trainable discriminative model</i> . To enable DASA to operate in dynamic settings, we propose a novel, unsupervised, distributed, fast <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">online</i> training scheme which is able to train DASA <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">within a few sampling instances, locally</i> , thereby avoiding the need for information exchange between the nodes or for a central node for coordination. DASDA is demonstrated to achieve an improvement by a factor greater than ten compared to the classic reference scheme. Lastly, we propose another novel, distributed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">offline</i> training scheme for DASA, which is demonstrated to offer a tradeoff between performance and simplicity of deployment compared to the online training scheme, yet, at the same time, DASA with offline training still achieves superior performance compared to the classic reference scheme.

Distributed Clock Synchronization Based on Intelligent Clustering in Local Area Industrial IoT Systems

Accurate clock synchronization in the industr-ial-Internet-of-Things systems forms the cornerstone of distributed interaction and coordination among various infrastructures and machines in an industrial environment. However, due to the widespread use of wireless networks in industrial applications, constraints inherent to wireless networks including uncertain propagation delays, random packets losses, and unguaranteed communication resources are unavoidable, leading to dramatically increased clock synchronization error and unreliable or even outdated information. Meanwhile, time information transmissions are vulnerable to suffer from malicious attacks, causing unreliable timestamps and insecure synchronization. In this article, we proposed a distributed clock synchronization protocol based on an intelligent clustering algorithm to achieve accurate, secure, and packet-efficient clock synchronization. The varying rate of skew of every clock is collected and utilized for cluster formation as well as malicious node detection. According to established clusters, various synchronization frequencies are assigned, which can avoid excessive network access contention, reduce overall communication resource consumption, and improve synchronization accuracy. Meanwhile, a two-tier fault detection algorithm consists of outlier detection and second-order regressive model prediction is applied to determine potential malicious nodes. The simulation results demonstrate that the proposed protocol overwhelms simultaneous synchronization protocols in terms of synchronization performance and faulty node detection.

An Application-Level Method of Arbitrary Synchronization Failure Detection in TTEthernet Networks

Precise distributed clock synchronization is important for Time-Triggered Ethernet (TTEthernet) in which fail-arbitrary synchronization failure cannot be treated by existed protocols unless every synchronization node is equipped with a dedicated clock monitoring. A novel method was presented to detect arbitrary synchronization failure by some application-level routines which make distributed decisions mainly by monitoring the protocol control frames (PCFs). An arbitrary failure node can be exactly detected and located by the routine resided on each of the Compression Masters based on the concerned accusation messages sent from Synchronization Masters or Synchronization Clients. The proposed method can make up the weak points on the detection of the arbitrary failure node of the existed fault-tolerant synchronization mechanism and enhance the synchronization node to resist the arbitrary failure for TTEthernet. By our SAL-based model checking, this method had been formally verified to have a fail-arbitrary detection capacity even in a standard integration configuration. Simulations imply that the quality of the whole clock synchronization is effectively improved after the failure node has been isolated.

Metastability-Containing Circuits

In digital circuits, metastability can cause deteriorated signals that neither are logical 0 nor logical 1, breaking the abstraction of Boolean logic. Synchronizers, the only traditional countermeasure, exponentially decrease the odds of maintained metastability over time. We propose a fundamentally different approach: It is possible to deterministically contain metastability by fine-grained logical masking so that it cannot infect the entire circuit. At the heart of our approach lies a time- and value-discrete model for metastability in synchronous clocked digital circuits, in which metastability is propagated in a worst-case fashion. The proposed model permits positive results and passes the test of reproducing Marino's impossibility results. We fully classify which functions can be computed by circuits with standard registers. Regarding masking registers, we show that more functions become computable with each clock cycle, and that masking registers permit exponentially smaller circuits for some tasks. Demonstrating the applicability of our approach, we present the first fault-tolerant distributed clock synchronization algorithm that deterministically guarantees correct behavior in the presence of metastability. As a consequence, clock domains can be synchronized without using synchronizers, enabling metastability-free communication between them.

Global Synchronization of Pulse-Coupled Oscillators on Trees

Consider a distributed network on a finite simple graph $G=(V,E)$ with diameter $d$ and maximum degree $\Delta$, where each node has a phase oscillator revolving on $S^1=\mathbb{R}/\mathbb{Z}$ with unit speed. Pulse-coupling is a class of distributed time evolution rule for such networked phase oscillators inspired by biological oscillators, which depends only upon event-triggered local pulse communications. In this paper, we propose a novel inhibitory pulse-coupling and prove that arbitrary phase configuration on $G$ synchronizes by time $51d$ if $G$ is a tree and $\Delta\leq3$. We extend this pulse-coupling by letting each oscillator throttle the input according to an auxiliary state variable. We show that the resulting adaptive pulse-coupling synchronizes arbitrary initial configuration on $G$ by time $83d$ if $G$ is a tree. As an application, we obtain a universal randomized distributed clock synchronization algorithm, which uses $O(\log\Delta)$ memory per node and converges on any $G$ with an expected worst case running time of $O(|V|+(d^5+\Delta^2)\log|V|)$.

Synchronization Improvement of Distributed Clocks in EtherCAT Networks

The aim of this letter is to improve the performance of clock synchronization in EtherCAT networks. Although EtherCAT synchronizes the slave clocks to the reference clock using a distributed clock synchronization mechanism, a synchronization error still exists between them. We propose a method to estimate and significantly reduce this error. In addition, we implement the method within the existing synchronization mechanism without adding an excessive computational load. The experimental results for synchronization performance verify that the proposed method improves the accuracy of clock synchronization in EtherCAT networks.

Temperature compensated Kalman distributed clock synchronization

Time synchronization is a fundamental problem in any distributed system. In particular, wireless sensor networks (WSNs) require scalable time synchronization for implementing distributed tasks on multiple sensor nodes. We propose a temperature-compensated Kalman based distributed synchronization protocol (TKDS) using a two-way sender-receiver synchronization scheme, to achieve high synchronization accuracy while modelling the clock skew change based on its physical characteristics. By asynchronously combining estimates from neighbours, TKDS achieves better performance than the spanning tree based protocols in a fully-distributed fashion. The synchronization performance is evaluated numerically and compared with that of other well-known synchronization protocols.

A Distributed Consensus-Based Clock Synchronization Protocol for Wireless Sensor Networks

The birth of computer networks and distributed systems has led to the appearance of the clock synchronization problem. This issue has gained increasing importance with the emergence of new resource constrained networks such as wireless sensor networks. In this paper, we propose a new distributed clock synchronization algorithm, referred to as Weighted Consensus Clock Synchronization (WCCS), whose objective is to achieve a consensus clock among network nodes. In this distributed approach and in contrast to centralized schemes, each node periodically exchanges the local clock reading with its immediate neighbor nodes. Then, each node employs these time informations to calculate its relative offset and skew with respect to its neighbor nodes using a weighted average consensus based technique. The effectiveness of WCCS is proved through both simulations and an experimental study on TelosB mote using TinyOS.

Clock synchronization algorithms for networked systems

Clock synchronization is a classical topic in the field of networked systems. Real-time industrial Ethernet and wireless sensor networks are two typical kinds of networked systems that have important applications in modern industry. In industrial Ethernet, people usually adopt the IEEE 1588 clock synchronization mechanism, and in wireless sensor networks, people often employ a distributed synchronization algorithm due to limited resources, unreliable communication links, and dynamic network topologies. In recent years, along with the development of a theoretical framework of distributed multi-agent consensus, there are an increasing number of scholars in the control community who are shifting their focus to the distributed clock synchronization algorithm and its applications, and there have been key breakthroughs in the algorithm convergence theorems. Owing to their robustness, flexibility, and ease of implementation, distributed consensus clock synchronization protocols have good prospects; however, many challenging scientific problems remain. In this paper, we summarize the latest research trends on hierarchical and distributed clock synchronization algorithms, especially highlighting the representative clock synchronization algorithms based on consensus control that were proposed in the last five years, and discuss the future directions that aim at providing valuable academic references for researchers working in relevant fields.

Fully distributed clock synchronization in wireless sensor networks under exponential delays

In this paper, we study the global clock synchronization problem for wireless sensor networks under unknown exponential delays in the two-way message exchange mechanism. The joint maximum likelihood estimator of clock offsets, clock skews and fixed delays for the network is formulated as a linear programming (LP) problem. Then, based on the alternating direction method of multipliers (ADMM), we propose a fully distributed synchronization algorithm which has low communication overhead and computation cost. Simulation results show that the proposed algorithm achieves better accuracy than consensus algorithm and the distributed least squares algorithm, and can always converge to the centralized optimal solution.

SAGE: Semantic Annotation of Georeferenced Environments

The emergence of portable 3D mapping systems are revolutionizing the way we generate digital 3D models of environments. These systems are human-centric and require the user to hold or carry the device while continuously walking and mapping an environment. In this paper, we adapt this unique coexistence of man and machines to propose SAGE (Semantic Annotation of Georeferenced Environments). SAGE consists of a portable 3D mobile mapping system and a smartphone that enables the user to assign semantic content to georeferenced 3D point clouds while scanning a scene. The proposed system contains several components including touchless speech acquisition, background noise adaptation, real time audio and vibrotactile feedback, automatic speech recognition, distributed clock synchronization, 3D annotation localization, user interaction, and interactive visualization. The most crucial advantage of SAGE technology is that it can be used to infer dynamic activities within an environment. Such activities are difficult to be identified with existing post-processing semantic annotation techniques. The capability of SAGE leads to many promising applications such as intelligent scene classification, place recognition and navigational aid tasks. We conduct several experiments to demonstrate the effectiveness of the proposed system.

Event-Based Distributed Clock Synchronization for Wireless Sensor Networks

In wireless sensor networks, a high level of accuracy is required in time synchronization to maintain time consistency of sensed data and to reduce idle listening times. In this technical note, we propose two fully distributed protocols for time synchronization. They are based on algorithms of multi-agent consensus and hence do not require any leader node. Further, they employ an event-based scheme to enhance communication efficiency. We analyze their convergence properties and show that clock synchronization can be achieved with less communication at guaranteed precision.

A group neighborhood average clock synchronization protocol for wireless sensor networks.

Clock synchronization is a very important issue for the applications of wireless sensor networks. The sensors need to keep a strict clock so that users can know exactly what happens in the monitoring area at the same time. This paper proposes a novel internal distributed clock synchronization solution using group neighborhood average. Each sensor node collects the offset and skew rate of the neighbors. Group averaging of offset and skew rate value are calculated instead of conventional point-to-point averaging method. The sensor node then returns compensated value back to the neighbors. The propagation delay is considered and compensated. The analytical analysis of offset and skew compensation is presented. Simulation results validate the effectiveness of the protocol and reveal that the protocol allows sensor networks to quickly establish a consensus clock and maintain a small deviation from the consensus clock.

Stability and Performance Analysis of Clock Synchronization in FlexRay

This paper presents a case study on the performance of a distributed clock synchronization algorithm used in FlexRay, a communication protocol designed to meet the requirements of dependable, fault-tolerant real-time applications. The FlexRay industry consortium drives forward the standardization of a fault-tolerant communication system for advanced automotive applications. In this case study we will analyze two different configurations for typical automotive applications by means of simulation. The focus of the simulation experiments is the assessment of performance and stability of the FlexRay clock synchronization algorithm in the presence of varying clock drift rates. For the analysis we will use SIDERA, a simulation model for time-triggered distributed systems

Iterative quantum algorithm for distributed clock synchronization

Clock synchronization is a well-studied problem with many practical and scientific applications. We propose an arbitrary accuracy iterative quantum algorithm for distributed clock synchronization using only three qubits. The n bits of the time difference Δ between two spatially separated clocks can be deterministically extracted by communicating only O(n) messages and executing the quantum iteration process n times based on the classical feedback and measurement operations. Finally, we also give the algorithm using only two qubits and discuss the success probability of the algorithm.

Resiliency of distributed clock synchronization networks

Clock synchronization refers to techniques and protocols used to maintain mutually consistent time-of-day clocks in a coordinated network of computers. A (clock) synchronization network is an interconnection of computers to implement a particular clock synchronization solution. To prevent clock-dependency loops, most synchronization networks use a stratified approach which is essentially a tree structure with a Primary Reference Clock (at "stratum- 0"). A node at stratum-i+1 exchanges synchronization messages with its parent node at stratum-i and also with some other nodes at same or other level. The purpose of this redundancy is two fold: (i) to calculate smoother steering rate adjustment, (ii) to maintain connectivity in the event of a failure. We provide an analytical framework to evaluate the performance of different approaches for resilient synchronization networks. To evaluate resiliency of synchronization networks, we characterize failure recovery metrics like connectivity and failure detection delay in terms of parameters related to network topology and failure recovery solutions.

Evaluation of Effective Resistances in Pseudo-Distance-Regular Resistor Networks

The effective resistance or two-point resistance between two nodes of a resistor network is the potential difference that appears across them when a unit current source is applied between the nodes as terminals. This concept arises in problems which deal with graphs as electrical networks including random walks, distributed detection and estimation, sensor networks, distributed clock synchronization, collaborative filtering, clustering algorithms and etc. In the previous paper (Jafarizadeh et al. in J. Math. Phys. 50:023302, 2009) a recursive formula for evaluation of effective resistances on the so-called distance-regular networks was given based on the Christoffel-Darboux identity. In this paper, we consider more general networks called pseudo-distance-regular networks or QD type networks, where we use the stratification of these networks and show that the effective resistances between a given node, say α, and all of the nodes β belonging to the same stratum with respect to α, are the same. Then, based on the spectral techniques, for those α,β’s which satisfy \(L^{-1}_{\alpha\alpha}=L^{-1}_{\beta\beta}\) (L−1 is the pseudo-inverse of the Laplacian of the network), an analytical formula for effective resistances \(R_{\alpha\beta^{(m)}}\) (the equivalent resistance between terminals α and β, so that β belongs to the m-th stratum with respect to α) is given in terms of the first and second orthogonal polynomials associated with the network. From the fact that in distance-regular networks, \(L^{-1}_{\alpha\alpha}=L^{-1}_{\beta\beta}\) is satisfied for all nodes α,β of the network, the effective resistances \(R_{\alpha\beta^{(m)}}\) for m=1,2,…,d (d is diameter of the network which is the same as the number of strata) are calculated directly, by using the given formula.

A PI consensus controller with gossip communication for clock synchronization in wireless sensors networks

In this paper a distributed clock synchronization algorithm is proposed. The algorithm requires gossip asynchronous communication between the nodes of the network, and because of its proportional-integral (PI) structure it is able to compensate both initial offsets and different clock speeds. Convergence of the algorithm is proved and analysed with respect to the controller parameter, while scalability issues are addressed by simulations.

A PI Consensus Controller for Networked Clocks Synchronization

In this paper we propose a novel distributed clock synchronization protocol for networks of clocks which have different initial offsets and internal clock speeds. The algorithm is based on an PI-like consensus protocol where the proportional (P) part compensates the different clock speeds while the integral part (I) eliminates the different clock offsets. This synchronization protocol is formally studied in its synchronous implementation and we provide both convergence guarantees as well optimal design using standard optimization tools when the underlaying communication graph is known. We also show how this protocol can be readily used to study the effect of noise and external disturbances on the steady-state performance. Finally, some simulations are presented.

Gradient clock synchronization

We introduce the distributed gradient clock synchronization problem. As in traditional distributed clock synchronization, we consider a network of nodes equipped with hardware clocks with bounded drift. Nodes compute logical clock values based on their hardware clocks and message exchanges, and the goal is to synchronize the nodes' logical clocks as closely as possible, while satisfying certain validity conditions. The new feature of gradient clock synchronization (GCS for short) is to require that the skew between any two nodes' logical clocks be bounded by a nondecreasing function of the uncertainty in message delay (call this the distance) between the two nodes, and other network parameters. That is, we require nearby nodes to be closely synchronized, and allow faraway nodes to be more loosely synchronized. We contrast GCS with traditional clock synchronization, and discuss several practical motivations for GCS, mostly arising in sensor and ad-hoc networks. Our main result is that the worst case clock skew between two nodes at distance d or less from each other is Ω (d+log D/log log D), where D is the diameter1 of the network. This means that clock synchronization is not a local property, in the sense that the clock skew between two nodes depends not only on the distance between the nodes, but also on the size of the network. Our lower bound implies, for example, that the TDMA protocol with a fixed slot granularity will fail as the network grows, even if the maximum degree of each node stays constant.