Time Synchronization In Networks Research Articles

APE-Sync: An Adaptive Power Efficient Time Synchronization for Mobile Underwater Sensor Networks

Time synchronization is a cooperative work's foundation among underwater sensor networks' nodes; it takes a crucial role in the application and the study of underwater sensor networks. Several time synchronization protocols have been presented for terrestrial radio networks, but they cannot be instantly utilized to real-time underwater sensor networks due to the slow propagation speed of the underwater acoustic signals, the mobility among sensor nodes, and the energy limitation in the underwater wireless sensor networks. In this paper, we propose an adaptive power-efficient time synchronization for mobile underwater sensor networks, called APE-Sync. The proposed scheme takes into account the time-varying clock skew and combines the Doppler-Enhanced synchronization protocol (DE-Sync) and the Kalman filter tracking the clock skew to achieve time synchronization. The proposed scheme also helps in reducing the energy consumption of the nodes by guaranteeing accurate time synchronization. The simulation results are presented, which show that APE-Sync outperforms existent time synchronization schemes in both power efficiency and precision.

PROVIDING INCREASED REQUIREMENTS FOR TIME SYNCHRONIZATION QUALITY UNDER GNSS VULNERABILITIES

In different industries, it is increasingly necessary to provide consumers with accurate timing and frequency signals.The most common source of accurate time synchronization signals in Ukraine is GPS, but they are vulnerable to a variety of deliberate and unintentional obstacles, which does not ensure high reliability of delivery of accurate time synchronization signals to consumers. One promising solution to the problem is to receive an exact time signal from the national time benchmark. National time benchmarks can be used as a backup source of accurate time synchronization in the implementation of the 5g mobile standard, as only they outside the mobile synchronization network have the accuracy required for 5g The article is devoted to the problems of creating networks in the face of increasing quality requirements to time and frequency synchronization in industries that are critical to time accuracy. The paper presents a comparative analysis of the requirements to time synchronization accuracy in various industries of strategic importance for the country. Ways of delivering accurate time signals from national time standards in the context of GNSS vulnerability are considered, an examples of technical solutions for delivering accurate time signals to remote users based on current ITU-T and IEEE recommendations are provided. These technical solutions can be successfully taken into account for the development and modernization of the Time Synchronization and Reference Frequencies Service. Solutions to the problem of synchronization signals delivery from national frequency and time standards to users, based on modern PTP IEEE 1588-2008 equipment are considered. The main factors that negatively affect the accuracy of the time synchronization networks constructed in accordance with the PTP 1588-2008 protocol are considered, and recommendations are given to minimize their impact.

Photonics-based wideband distributed coherent aperture radar system.

A photonics-based wideband distributed coherent aperture radar (DCAR) system is proposed and experimentally demonstrated. In the proposed system, the central controlling system and several spatially dispersed remote transceivers are connected by the optical fiber-based time synchronization network. In the central controlling system, the optical-carried orthogonal/coherent linear frequency modulated waveforms (LFMWs) are generated by a reconfigurable multi-channel optical arbitrary waveform generator (RMOAWG), and the signal processing for the echo waves is also implemented there. While in the remote transceivers, only the optical/RF and RF/optical conversions are carried out. Benefitting from the use of photonics-based methods, bandwidths of the generated radar signals can be large, improving the detection resolution of the system. Due to the centralized signal generation and processing, the remote transceivers can be simplified, reducing the system complexity. Moreover, the fiber-based distribution ensures low loss, good transportability, and great flexibility. Experimentally, a two-unit DCAR system operating in X-band with a bandwidth of 3 GHz is presented. When full coherence is achieved, signal-to-noise ratio (SNR) gains of 8.3 dB and 8.33 dB are obtained over a single radar for radar 1 and radar 2, respectively. Results are in good agreement with theoretical prediction. Theoretically, with such SNR gain, the range detection precision can be improved to about 2.6 times that of a single radar.

Distributed Time and Carrier Frequency Synchronization for Dense Wireless Networks

Dense networks call for interconnectivity of a large number of devices, and in this context, the diversity of interconnected devices introduces challenges to be considered for management and coordination of the network. Synchronization of timing (TO) and carrier frequency offsets (CFO) are critical aspects to be considered to guarantee the proper network interconnectivity. This paper addresses a distributed synchronization method for dense (say $\geq$ 50 nodes), compact, and possibly strongly connected wireless network where the internode distance is small and propagation delay of the entire network is not negligible compared to the inverse of signal bandwidth. Signatures are periodically transmitted by every node to mark the start of the payload and to broadcast the synchronization state that is iteratively minimized by all nodes in aligning jointly the TO/CFO skew with the consensus-type algorithms. Here, the same signature is broadcasted by every node and it collides with those of the others to perform as a reference (compound) signature for receiving nodes that embeds the reference TO/CFO of the network for synchronization. In view to make distributed TO/CFO synchronization practicable, this paper aims to: first, compensate the propagation delays between nodes to mitigate their impact on the global TO/CFO synchronization of the network; second, design and validate chirplike signatures based on Zadoff–Chu sequences that enable the estimate of the TO and CFO mismatch on every node and decouple TO and CFO estimates; and finally, evaluate the mean-square convergence conditions for nodes employing realistic and unstable oscillators. Numerical validations are based on TO/CFO synchronization for orthogonal frequency division multiplexing (OFDM) system.

GNSS Time Synchronization in Vehicular Ad-Hoc Networks: Benefits and Feasibility

Time synchronization is critical for the operation of distributed systems in networked environments. It is also demanded in vehicular ad-hoc networks (VANETs), which, as a special type of wireless networks, are becoming increasingly important for emerging cooperative intelligent transport systems. Global navigation satellite system (GNSS) is a proven technology to provide precise timing information in many distributed systems. It is well recognized to be the primary means for vehicle positioning and velocity determination in VANETs. However, GNSS-based time synchronization is not well understood for its role in the coordination of various tasks in VANETs. To address this issue, this paper examines the requirements, potential benefits, and feasibility of GNSS time synchronization in VANETs. The availability of GNSS time synchronization is characterized by almost 100% in our experiments in high-rise urban streets, where the availability of GNSS positioning solutions is only 80%. Experiments are also conducted to test the accuracy of time synchronization with 1-PPS signals output from consumer grade GNSS receivers. They have shown 30-ns synchronization accuracy between two receivers of different models. All these experimental results demonstrate the feasibility of GNSS time synchronization for stringent VANET applications.

High-Reliability Sub-Nanosecond Network Time Synchronization Method Enabled by Double-Frequency Distributed Time Synchronization

Time synchronization is a long-standing challenge in distributed systems like indoor/outdoor positioning, coordinated multi-point in 4G/5G mobile communication, etc., which require nanosecond-level high-reliable time sync networks. At present, a widely adopted solution for time sync networks is the precision time protocol specified in IEEE 1588v2, which can provide sub-microsecond sync accuracy. In addition, a novel network time sync method called distributed time synchronization has also been proposed recently, which can achieve 50-ns-level accuracy and stronger survivability in metro, regional, and backbone networks. However, both of the above methods encounter difficulties in achieving nanosecond sync accuracy and network reliability simultaneously. In this paper, by analyzing the main factors influencing the degradation of time sync accuracy, we reveal that the finite clock resolution is a major barrier to achieving nanosecond-level time synchronization. In order to establish a low-cost, high-reliability, and sub-nanosecond-level time sync network, we propose a novel network time sync method called double-frequency distributed time synchronization (DF- DTS). By selecting two specific and distinct frequencies for the sending and receiving clocks of the nodes/devices being synchronized, the time sync errors induced by the finite clock resolution can be reduced by a statistical approach. We set up a theoretical model for the DF-DTS and analyze the time sync accuracy through both mathematical derivations and network simulations. Furthermore, we propose a failure-restoration mechanism to enhance the reliability of DF-DTS networks by improving the time sync accuracy under failures and reducing the failure recovery time. Finally, we conduct both point- to-point and network time sync experiments to validate the proposed DF-DTS method. The results demonstrate that DF-DTS can achieve sub-nanosecond-level sync accuracy that is 1-2 orders of magnitude higher than the clock resolution in a prototype four-node DF-DTS network. Moreover, a network simulation under failure cases is conducted, and the results show that our method has significant advantages in both time sync accuracy and recovery time in the failure-restoration process compared to IEEE 1588v2.

Bi-iterative algorithm for joint localization and time synchronization in wireless sensor networks

Abstract This paper efficiently solves the problem of joint localization and synchronization in wireless sensor networks by using two-way message exchange mechanism. Employing an affine model of unsynchronized local clocks, we first establish the famous Maximum Likelihood (ML) estimator for joint localization and synchronization, which derives the best consistent solution. Unfortunately, achieving directly the ML estimator is usually difficult because the ML cost functions are highly non-linear and non-convex. In order to effectively obtain the ML estimator, a novel bi-iterative algorithm is proposed by alternately searching the sensor node location and clock parameters (clock skew and clock offset). We prove that the proposed algorithm is convergent, and analyze that its estimation accuracy for the sensor node locations and clock parameters can approximately reach the Cramer–Rao Lower Bound (CRLB) under the mild noise condition. Compared with some previous methods, the proposed bi-iterative algorithm is more computationally efficient, and takes fewer anchor nodes and less communication overhead. Simulation results verify theoretical analysis and show that the proposed algorithm has the better performance than the several existing algorithms.

Mobile beacon-based adaptive time synchronization for wireless sensor networks

Time synchronization of clocks in the sensor nodes for wireless sensor networks (WSNs) is a fundamental technology for most mission-critical applications. Most of past research in time synchronization for WSNs, however, has only focused on achieving some of the goals at a time, such as accuracy, energy consumption, completion time, etc., making these solutions less capable of adapting to different application requirements. In this paper, we propose a new time synchronization algorithm named MBATS (mobile beacon-based adaptive time synchronization) in which a mobile beacon is employed to move or fly over the sensor deployment area to complete time synchronization. Moreover, MBATS is designed so that the number of sensor nodes that are synchronized by one instance of time synchronization from the mobile beacon could vary dynamically to meet application requirements on accuracy, completion time and energy consumption, making the proposed MBATS algorithm highly adaptable to different application requirements. In addition to showing the advantage of the proposed MBATS algorithm on the adaptability of time synchronization as well as on some of the main metrics of synchronization over comparable schemes for WSNs, we also present the results of our study on comparing the performance of letting the mobile beacon traverse along a designing path versus follow a random path. Such a study is important since it would allow us to learn the performance gains that we can expect to achieve with extra control effort spent on designing the path over the effortless random path strategy. Such study could provide us with some clues on how to choose a suitable time synchronization strategy to better meet application requirements, which may not necessarily be the designed path strategy due to the tradeoff between cost and performance gains.

Time synchronization for long-chain-type wireless sensor networks

In view of the requirement on time synchronization for a kind of typical wireless sensor networks with long-chain-type topologies, taking into account the three-dimensional network model currently used by the localization algorithms, one feasible time synchronization algorithm is proposed in this article. Under the three-dimensional topology, nodes in the wireless sensor networks are divided into three kinds: the sensing nodes, the relay nodes, and the measured nodes. By analyzing the distribution characteristics of these three kinds of nodes, with the introduction of the pairwise broadcast synchronization and the series multi-hop synchronization protocol, the estimation of the clock offset and the clock skew of all the nodes in the wireless sensor networks are performed by virtue of the joint maximum likelihood and the least-squares method, thus the time synchronization of the wireless sensor networks with long-chain-type topology is solved. Moreover, the sink node evaluates the network periodically and adjusts the synchronizing cycle based on the difference between the network synchronization error and the given synchronization accuracy. The feasibility and effectiveness of this scheme are analyzed by simulations from the synchronization accuracy, the number of synchronization message, and the synchronizing cycle points of view.

Node-Identification-Based Secure Time Synchronization in Industrial Wireless Sensor Networks.

Time synchronization is critical for wireless sensors networks in industrial automation, e.g., event detection and process control of industrial plants and equipment need a common time reference. However, cyber-physical attacks are enormous threats causing synchronization protocols to fail. This paper studies the algorithm design and analysis in secure time synchronization for resource-constrained industrial wireless sensor networks under Sybil attacks, which cannot be well addressed by existing methods. A node-identification-based secure time synchronization (NiSTS) protocol is proposed. The main idea of this protocol is to utilize the timestamp correlation among different nodes and the uniqueness of a node’s clock skew to detect invalid information rather than isolating suspicious nodes. In the detection process, each node takes the relative skew with respect to its public neighbor as the basis to determine whether the information is reliable and to filter invalid information. The information filtering mechanism renders NiSTS resistant to Sybil attacks and message manipulation attacks. As a completely distributed protocol, NiSTS is not sensitive to the number of Sybil attackers. Extensive simulations were conducted to demonstrate the efficiency of NiSTS and compare it with existing protocols.

Time Synchronization Network for EAST Poloidal Field Power Supply Control System Based on IEEE 1588

This paper clearly demonstrates the architecture of time synchronization network of experimental advanced superconducting tokamak (EAST) poloidal field (PF) power supply control system and analyzes characteristics of the IEEE 1588 in detail at the same time. IEEE 1588 has unique advantages for this paper to utilize as protocol in order to improve the accuracy of the time synchronization network (TSN). Consequently, we design a time synchronization system based on PCI eXtensions for Instrumentation-6683H. This paper utilizes one master node to synchronize all slave nodes on the time network in each second. The result shows that the synchronization accuracy can be achieved within 50 ns. According to results of experiments, this system can perfectly meet the requirements of EAST PF power supply control system.

Distributing Deterministic, Accurate Time for Tightly Coordinated Network and Software Applications: IEEE 802.1AS, the TSN profile of PTP

Real-time cooperation among agents requires sharing of certain temporal information. That information might the form of take action X now! or it might better be achieved in some cases by establishing an a priori agreement by which each action should be taken and when, together with a shared understanding of what time it is. Distributed applications increasingly rely on this latter synchronized time approach rather than explicit right now! communication for cooperative sense and control tasks. Such a capability is foundational to the field of cyber physical systems (CPS) where computation and communication interact with the physical world in real time. While multiple historical approaches exist for time-synchronized (and timely) communication among sensing, computing, and actuating devices, the addition of robust temporal performance characteristics to standard Ethernet through the work of the Time Sensitive Networking (TSN) Task Group of IEEE 802.1 is bringing together the benefits of standard IT networking with excellent worst case latency and synchronization performance that was previously available only to these specialized and sometimes proprietary networks. Such advances will be important in next-generation applications as diverse as CPS/real-time distributed control, smart factories, sensor fusion, autonomous vehicles, and even cloud applications [1]. As we show, precision network time synchronization can dramatically enhance the operation of these applications and is a requirement of certain new network latency enhancements as well.

Delayed Feedback Controller based Finite Time Synchronization of Discontinuous Neural Networks with Mixed Time-Varying Delays

This paper addresses finite-time synchronization of artificial neural networks with discrete and distributed time-varying delays as well as discontinuous neuron activation functions which may be unbounded or non-monotonic. Under the framework of Filippov solution, delay feedback controller is studied by constructing nonsmooth Lyapunov function and differential inclusions theory based on finite time convergence theorem. Several effective new criteria are derived. Moreover, the estimation of settling time is established. The obtained results ensure that the synchronization in finite time is achieved. Finally, simulation results are presented to demonstrate the analytical results.

AUV-Aided Joint Localization and Time Synchronization for Underwater Acoustic Sensor Networks

For the purpose of localization and time synchronization of underwater sensor networks, buoys are generally distributed on the sea surface of the area of interest, serving as fixed anchors. However, this method is not economical and has poor scalability. An alternative is to employ an autonomous underwater vehicle (AUV) as a mobile anchor. By receiving the periodical broadcast signals from the AUV, any sensor in the communication range can measure time of arrival of received packets and obtain a series of nonlinear equations. In this letter, we proposed an efficient linear algorithm to solve the nonlinear equations, and gave closed-form positioning and synchronization error analysis. Besides, we show that the proposed method can approach the Cramér-Rao lower bound by both theoretical analysis and simulation.

Distributed time synchronization for networks with random delays and measurement noise

In this paper a new distributed asynchronous algorithm is proposed for time synchronization in networks with random communication delays, measurement noise and communication dropouts. Three different types of the drift correction algorithm are introduced, based on different kinds of local time increments. Under nonrestrictive conditions concerning network properties, it is proved that all the algorithm types provide convergence in the mean square sense and with probability one (w.p.1) of the corrected drifts of all the nodes to the same value (consensus). An estimate of the convergence rate of these algorithms is derived. For offset correction, a new algorithm is proposed containing a compensation parameter coping with the influence of random delays and special terms taking care of the influence of both linearly increasing time and drift correction. It is proved that the corrected offsets of all the nodes converge in the mean square sense and w.p.1. An efficient offset correction algorithm based on consensus on local compensation parameters is also proposed. It is shown that the overall time synchronization algorithm can also be implemented as a flooding algorithm with one reference node. It is proved that it is possible to achieve bounded error between local corrected clocks in the mean square sense and w.p.1. Simulation results provide an additional practical insight into the algorithm properties and show its advantage over the existing methods.

FADS: Fast Scheduling and Accurate Drift Compensation for Time Synchronization of Wireless Sensor Networks

Most of wireless sensor networks require time synchronization to wake up sensor nodes at scheduled time. It helps accomplish coordinated communications among the nodes to save energy. Wireless sensor nodes are often equipped with a low-cost clock oscillator with large clock drift. Thus, it is important to correct the long-term synchronization error caused by clock drift. This paper introduces a low power protocol for long-term synchronization called fast scheduling and accurate drift compensation for time synchronization (FADS). It chooses a minimal sequence of synchronization steps by estimating the density of the neighbor nodes. FADS achieves low power synchronization by reducing the number of message exchanges and increasing the synchronization interval (the period in which each node needs to perform synchronization). To evaluate the performance of FADS, we implemented it in real wireless networks with sensor hardware and conducted extensive tests. This paper demonstrates that FADS achieves substantially fewer messages and less delays in the scheduling phase than a previous technique, Density Table-based Synchronization (DTSync). Simulation results show that FADS reduces the number of messages by 50 times and reduces the delay by 4 times compared with DTSync for large networks. In addition, experiments with actual sensor devices demonstrate that it allows significantly less frequent synchronization 195 times improvement compared with DTSync.

Randomized and Efficient Time Synchronization in Dynamic Wireless Sensor Networks: A Gossip‐Consensus‐Based Approach

This paper proposes novel randomized gossip‐consensus‐based sync (RGCS) algorithms to realize efficient time calibration in dynamic wireless sensor networks (WSNs). First, the unreliable links are described by stochastic connections, reflecting the characteristic of changing connectivity gleaned from dynamic WSNs. Secondly, based on the mutual drift estimation, each pair of activated nodes fully adjusts clock rate and offset to achieve network‐wide time synchronization by drawing upon the gossip consensus approach. The converge‐to‐max criterion is introduced to achieve a much faster convergence speed. The theoretical results on the probabilistic synchronization performance of the RGCS are presented. Thirdly, a Revised‐RGCS is developed to counteract the negative impact of bounded delays, because the uncertain delays are always present in practice and would lead to a large deterioration of algorithm performances. Finally, extensive simulations are performed on the MATLAB and OMNeT++ platform for performance evaluation. Simulation results demonstrate that the proposed algorithms are not only efficient for synchronization issues required for dynamic topology changes but also give a better performance in terms of converging speed, collision rate, and the robustness of resisting delay, and outperform other existing protocols.

Security Vulnerabilities and Countermeasures for Time Synchronization in TSCH Networks

Time‐slotted channel hopping (TSCH), which can enable highly reliable and low‐power wireless mesh networks, is the cornerstone of current industrial wireless standards. In a TSCH network, all nodes must maintain high‐precision synchronization. If an adversary launches a time‐synchronization attack on a TSCH network, the entire network communication system can be paralyzed. Thus, time‐synchronization security is a key problem in this network. In this article, time synchronization is divided into single‐hop pairwise, clusterwise, and three‐level multihop according to the network scope. We deeply analyze their security vulnerabilities due to the TSCH technology itself and its high‐precision synchronization requirements and identify the specific attacks; then, we propose corresponding security countermeasures. Finally, we built a test bed using 16 OpenMoteSTM nodes and the OpenWSN software to evaluate the performance of the proposed scheme. The experimental results showed that serious security vulnerabilities exist in time‐synchronization protocols, and the proposed countermeasures can successfully defend against the attacks.

A hierarchical time synchronization algorithm for WSN

Wireless sensor networks generally require large-scale deployment. Each node in the network is solely responsible for data sensing in a certain area. In order to determine the sequence in which sensing data is generated, time synchronization between nodes is necessary. Due to the delay of multi-hop transmission and the influence of the frequency shift of the node crystal frequencies, the error of time synchronization between the nodes is larger. In order to reduce the error of time synchronization, it is common practice to increase the number of synchronizations. But this method will cause the node energy consumption to increase, affecting the network survival time. In order to solve this problem, a layered wireless sensor network time synchronization algorithm is proposed with reference to the unidirectional broadcast mechanism and bidirectional pairing mechanism. The algorithm uses a hierarchical strategy for time synchronization, while introducing a synchronization error compensation mechanism and clock compensation mechanism. Under the premise of ensuring the accuracy of time synchronization, the algorithm reduces the energy consumption of nodes and prolongs the network lifetime.

Fast Parallel Lightpath Re-Optimization for Space-Division Multiplexing Optical Networks Based on Time Synchronization

In recent years, driven by the rapid growth of global Internet traffic, optical networks are evolving to provide higher capacities. To enlarge transmission capacity, researchers focused on the dimension of space and proposed the concept of space-division multiplexing (SDM) by introducing multicore fiber and few-mode fiber into optical networks. In SDM networks, one of the main challenges is the crosstalk among SDM (core and mode) channels, which degrades signal quality. This degradation has a serious impact on routing and resource allocation. Previous studies have mostly been confined to the routing algorithms for crosstalk reduction but few focus on the re-optimization mechanisms in SDM networks, defined as re-routing the existing lightpaths to minimize the overall crosstalk without traffic disruption. In this paper, by analyzing the re-routing signaling processes of two straightforward serial re-optimization mechanisms, we find that the signaling interaction between node controllers and serial re-routing execution leads to long re-optimization times, which makes lightpath re-optimization impossible for practical use. In order to achieve fast lightpath re-optimization for crosstalk reduction in SDM networks, we propose a novel software-defined networking (SDN) based parallel lightpath re-optimization mechanism, enabled by high-precision time synchronization. The basic idea of our proposed parallel lightpath re-optimization mechanism is to make all the nodes execute the switching operations co-ordinately with the help of network time synchronization. Besides, we propose two key algorithms to achieve parallel re-optimization: a simulated annealing algorithm to determine the re-routing sequence of the lightpaths and a time arrangement algorithm to determine the exact time points of the switching operations at each node. Finally, we conduct a prototype experiment and large-scale network simulations to evaluate the performance of the proposed mechanism. The results demonstrate that our parallel re-optimization mechanism can reduce the overall crosstalk in a much shorter time by up to three orders of magnitude compared with the conventional serial mechanisms and thus is more practical for real applications.