Timestamp Exchange Research Articles

A cooperative timestamp-free clock synchronization scheme based on fast unscented Kalman filtering for time-sensitive networking

Clock synchronization, built on the classical two-way message exchange scheme, is the key prerequisite for the normal operation of time-sensitive networking (TSN). In practical TSN, the imperfect oscillator caused by environmental changes leads to clock parameters drift. Moreover, synchronization errors accumulate in multi-hop networks, making it difficult for nodes at the edge of the network to achieve precise synchronization performance. Additionally, in some industrial and vehicular scenarios, the energy consumption and complexity of clock synchronization are important factors that need to be considered. To address these problems, this paper proposes a cooperative synchronization clock offset and clock skew joint tracking algorithm based on fast Unscented Kalman filter (FUKF). To further reduce the computation and energy consumption caused by clock synchronization, we introduce randomized singular value decomposition and timestamp-free exchange. The former uses small sub-matrices approximations to replace extremely high-dimensional matrices, reducing computational time in the update stage of the UKF. The latter reduces energy consumption by setting response intervals at the receiving end, eliminating the need for timestamp exchange during the synchronization process. Therefore, this algorithm can achieve long-term synchronization without requiring excessive computational and communication overhead. The results show that the proposed method, while maintaining accuracy unchanged, reduced the running time by 20% to 90% as the number of observations increased, thus verifying the effectiveness of the algorithm.

Synchronizing smart city nodes using Skew Integrated Timestamp (SIT).

Time synchronization among smart city nodes is critical for proper functioning and coordinating various smart city systems and applications. It ensures that different devices and systems in the smart city network are synchronized and all the data generated by these devices is consistent and accurate. Synchronization methods in smart cities use multiple timestamp exchanges for time skew correction. The Skew Integrated Timestamp (SIT) proposed here uses a timestamp, which has time skew calculated from the physical layer and uses just one timestamp to synchronize. The result from the experiment suggests that SIT can be used in place of multiple timestamp exchanges, which saves computational resources and energy.

Fast Kalman filter synchronization algorithm for cooperative synchronization in time sensitive networks

Clock synchronization is crucial in time-sensitive networks (TSN) to ensure the normal operation of devices. However, traditional synchronization techniques face challenges such as the accumulation of synchronization errors over multiple hops and slow synchronization speed, which hinder the development of large-scale and multi-hop TSN. In this paper, we propose a cooperative synchronization scheme based on two-way timestamp exchange to reduce error accumulation and enable synchronization information exchange between any node in the network and the reference node. Moreover, we develop a fast Kalman filter (FKF) algorithm to estimate clock offset and clock skew. To reduce time complexity, we replace the calculation of gain matrices with extremely high dimension with a small matrix based on the randomization matrix approximation in the updating phase of the Kalman Filter. Simulation results show that the proposed method significantly reduces synchronization complexity without sacrificing clock synchronization accuracy, making it highly promising for future TSN deployments.

Intelligent and Low Overhead Network Synchronization for Large-Scale Industrial IoT Systems in the 6G Era

Holistic temporal coherence among distributed industrial infrastructures enabled by accurate network synchronization is essential for achieving tight orchestration of large-scale industrial Internet of Things (IIoT) systems. However, the low efficiency and situation agnosticism of conventional synchronization techniques using an inflexible "observing-andcalibrating" approach for clock offset correction are inevitably hindering the performance of many IIoT applications with increasing scale and heterogeneity. In this article, we provide an in-depth analysis of the challenges associated with conventional synchronization schemes over large-scale IIoT systems and then present three promising research directions for achieving intelligent and low overhead IIoT synchronization. Particularly, we first propose a new modelbased network synchronization scheme that can proactively enable low overhead clock calibration by leveraging the inherent characteristics of heterogeneous clocks to avoid frequent timestamp exchange. We then design an intelligent device clustering mechanism with a specifically designed synchronization strategy for each group of IIoT devices by jointly exploiting their distinctive synchronization requirements and multi-dimensional device attributes. To leverage the unique characteristics of each oscillator, we analyze historical timestamps to reject unreliable timestamps and identify unreliable devices. Finally, we envision an edge-cloud collaborative network synchronization paradigm to implement the proposed schemes and demonstrate their efficacy in large-scale IIoT systems.

IHorology: Lowering the Barrier to Microsecond-Level Internet Time

High accuracy, synchronized clocks are essential to a growing number of Internet applications. Standard protocols and their associated server infrastructure typically enable client clocks to synchronize to the order of tens of milliseconds. We address one of the key challenges to high precision Internet timekeeping – the intrinsic contribution to clock error of underlying path asymmetry between client and time server, a fundamental barrier to microsecond level accuracy. We first exploit results of a unique measurement study to reliably quantify asymmetry by taking routing changes into account for the first time, and then to infer the impacts on timing. We then describe three approaches to addressing the path asymmetry problem: LBBE, SBBE and K-SBBE, each based on timestamp exchange with multiple servers, with the goal of tightening bounds on asymmetry for each client. We explore their capabilities and limitations through simulation and model-based argument. We show that substantial improvements are possible, and discuss whether, and how, the goal of microsecond accuracy might be attained.

DNN-Assisted Particle-Based Bayesian Joint Synchronization and Localization

In this work, we propose a Deep neural network-assisted Particle Filter-based (DePF) approach to address the Mobile User (MU) joint synchronization and localization (sync&loc) problem in ultra-dense networks. In particular, DePF deploys an asymmetric time-stamp exchange mechanism between the MUs and the Access Points (APs), which, traditionally, provides us with information about the MUs’ clock offset and skew. However, information about the distance between an AP and an MU is also intrinsic to the propagation delay experienced by the exchanged time-stamps. In addition, to estimate the angle of arrival of the received synchronization packets, DePF draws on the multiple signal classification algorithm that is fed with the Channel Impulse Response (CIR) experienced by the sync packets. The CIR is also leveraged to determine the link condition, i.e. Line-of-Sight (LoS) or Non-LoS. Finally, to perform joint sync&loc, DePF capitalizes on particle Gaussian mixtures that allow for a hybrid particle-based and parametric Bayesian Recursive Filtering (BRF) fusion of the aforementioned pieces of information and, thus, jointly estimates the position and clock parameters of the MUs. The simulation results verify the superiority of the proposed algorithm over the state-of-the-art schemes, especially that of the extended Kalman filter- and linearized BRF-based joint sync&loc. In particular, only drawing on the synchronization time-stamp exchange and CIRs from a single AP, for 90% of the cases, the absolute position and clock offset estimation error remain below 1 meter and 2 nanoseconds, respectively.

Energy Stimulated Time Synchronization for Energy Harvesting Wireless Networks

In this work, we develop a new energy-preserving time synchronization method, called the energy stimulated time sync (ESTS), for ultra-low-power wireless networks with energy harvesting capabilities. Compared with existing methods, ESTS does not rely on timestamp exchange but uses short energy tones to synchronize the nodes within the receiving range. ESTS is a centralized synchronization method based on a fully-passive wake-up radio (WuR) circuit, which can provide a variety of synchronization accuracy to meet different application requirements while consuming much lower energy than timestamp-based synchronization methods. We have implemented ESTS on Microchip ATmega256RFR2 system-on-chip (SoC). According to experiment results, ESTS can achieve an average of 5 ms and 70 <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>s time precision for rough synchronization and fine synchronization, respectively. Moreover, the fully-passive WuR circuit consumes zero energy in the idle status. In the entire synchronization process, the rough synchronization only needs to be performed once, and the node consumes as low as 4 <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>J of energy for each fine synchronization, making ESTS a promising solution for ultra-low-power wireless networks.

Digital-Twin-Enabled Intelligent Distributed Clock Synchronization in Industrial IoT Systems

Tight cooperation among distributively connected equipment and infrastructures of an Industrial-Internet-of-Things (IIoT) system hinges on low latency data exchange and accurate time synchronization within sophisticated networks. However, the temperature-induced clock drift in connected industry facilities constitutes a fundamental challenge for conventional synchronization techniques due to dynamic industrial environments. Furthermore, the variation of packet delivery latency in IIoT networks hinders the reliability of time information exchange, leading to deteriorated clock synchronization performance in terms of synchronization accuracy and network resource consumption. In this article, a digital-twin-enabled model-based scheme is proposed to achieve an intelligent clock synchronization for reducing resource consumption associated with distributed synchronization in fast-changing IIoT environments. By leveraging the digital-twin-enabled clock models at remote locations, required interactions among distributed IIoT facilities to achieve synchronization is dramatically reduced. The virtual clock modeling in advance of the clock calibrations helps to characterize each clock so that its behavior under dynamic operating environments is predictable, which is beneficial to avoiding excessive synchronization-related timestamp exchange. An edge-cloud collaborative architecture is also developed to enhance the overall system efficiency during the development of remote digital-twin models. Simulation results demonstrate that the proposed scheme can create an accurate virtual model remotely for each local clock according to the information gathered. Meanwhile, a significant enhancement on the clock accuracy is accomplished with dramatically reduced communication resource consumption in networks with different packet delay variations.

Timestamp-Free Clock Syntonization for IoT Using Carrier Frequency Offset

System-level timing fluctuations caused by unstable low-cost clocks and end-to-end communication delays are the main sources of uncertainties in existing synchronization mechanisms that rely on timestamp exchanges. This paper introduces a timestamp-free clock syntonization approach, carrier frequency offset (CFO)-assisted syntonization (CFOSynt), to estimate the clock skew between a pair of nodes by utilizing carrier frequency offset. To enable CFOSynt, we leverage the fact that RF oscillators in the radio can be used as the reference to calibrate the system clock oscillators, and the pairwise RF clock information is carried in the transmission carrier frequency. By incorporating CFO and capturing the clock frequency relationship in system clock skew estimation, CFOSynt can eliminate the need for timestamping and the impact of delay uncertainties. To validate the design, CFOSynt is implemented on two common off-the-shelf (COTS) IoT platforms with access to the CFO estimation from the radio chip. Extensive experiments are conducted to evaluate CFOSynt, and CFOSynt can estimate the clock skew of 32 kHz low-cost electronic oscillators with a mean error of -2.46 Hz. In comparison with timestamp-based approaches, CFOSynt achieves up to 70-90% improvement in skew estimation error and shows significant reliability when low-cost oscillators are used.

Energy efficient cross layer time synchronisation in cognitive radio networks

Time synchronization is a vital concern for any Cognitive Radio Network (CRN) to perform dynamic spectrum management. Each Cognitive Radio (CR) node has to be environment aware and self adaptive and must have the ability to switch between multiple modulation schemes and frequencies. Achieving same notion of time within these CR nodes is essential to fulfill the requirements for simultaneous quiet periods for spectrum sensing. Current application layer time synchronization protocols require multiple timestamp exchanges to estimate skew between the clocks of CRN nodes. The proposed symbol timing recovery method already estimates the skew of hardware clock at the physical layer and use it for skew correction of application layer clock of each node. The heart of application layer clock is the hardware clock and hence application layer clock skew will be same as of physical layer and can be corrected from symbol timing recovery process. So one timestamp is enough to synchronize two CRN nodes. This conserves the energy utilized by application layer protocol and makes a CRN energy efficient and can achieve time synchronization in short span.

Synchronous Rendezvous Based on Cluster in Low-Duty-Cycle Wireless Sensor Network

Abstract This paper presents SRBC, a new design for synchronous rendezvous in low-duty-cycle Wireless Sensor Network (WSN).The main idea of SRBC is utilizing the cluster, which includes a part of the sensor nodes in the WSN. Each node in a cluster alters its clock drift as well as its skew towards the central node of the cluster through exchange of their clock information with normal communication. Then it reduces the overhead of the process usual for the traditional time-stamp exchange. In different clusters, the border nodes exchange the relative clock drift as well as the skew to improve the performance during the synchronous rendezvous. Results show that the design of SRBC is practical and effective.

Node Synchronization Across Two-layered Heterogeneous Clustered Wireless Sensor Network

Time synchronization is a mandatory feature needed for Wireless Sensor Network to operate consistently and to be capable to chronologically link to global time. Time synchronization is important when the sensor nodes employ TDMA [9] based medium access protocols and when sensor nodes want to operate on some time managed schedule as well. Time stamping is one of the most widely used approaches, because of simple implementation and of being quite precise. Based on the time stamp exchange approach we provide network wide synchronization that employs neighbouring node information to determine if synchronization should be continued on the same level or in the second level of nodes.

Voltage-Aware Time Synchronization for Wireless Sensor Networks

The low-cost crystal oscillators in wireless sensor networks are prone to be affected by their working conditions such as voltage, temperature, and humidity. Such effect is often ignored by existing time synchronization solutions that typically assume the frequency error of a given node to be constant and hence adopt frequent timestamp exchanges, resulting in high energy consumptions. We propose a novel voltage-aware time synchronization (VATS) scheme that is inspired by the fact that the clock skew is highly correlated to voltage supplies. VATS features a two-phase process: (i) it first estimates the clock skew and updates the frequency error autonomously based on the local voltage level; (ii) it then adjusts the resynchronization intervals dynamically according to a given synchronization error controlling factor and the synchronization error accumulating rate to balance the calibration accuracy and cost. Since VATS leverages voltage measurements to assist clock skew estimation, it does not require frequent timestamp exchanges as in traditional schemes. Extensive simulation results illustrate the superior performance of the proposed method in terms of calculation accuracy, robustness, and reduced timestamp exchanges for energy saving.

Joint Ranging and Synchronization for an Anchorless Network of Mobile Nodes

Synchronization and localization are critical challenges for the coherent functioning of a wireless network, which are conventionally solved independently. Recently, various estimators have been proposed for pairwise synchronization between immobile nodes, based on time stamp exchanges via two-way communication. In this paper, we consider a network of mobile nodes for which a novel joint time-range model is presented, treating both unsynchronized clocks and the pairwise distances as a polynomial functions of true time. For a pair of nodes, a least squares solution is proposed for estimating the pairwise range parameters between the nodes, in addition to estimating the clock offsets and clock skews. Extending these pairwise solutions to network-wide ranging and clock synchronization, we present a central data fusion based global least squares algorithm. A unique solution is nonexistent without a constraint on the cost function e.g., a clock reference node. Ergo, a constrained framework is proposed and a new Constrained Cramér-Rao Bound (CCRB) is derived for the joint time-range model. In addition, to alleviate the need for a single clock reference, various clock constraints are presented and their benefits are investigated using the proposed solutions. Simulations are conducted and the algorithms are shown to approach the theoretical limits.

Secure Multipath Routing Protocol for Certificate Exchange in Mobile Ad Hoc Network

In Mobile Ad Hoc Networks (MANET), the certificate exchange technique helps the nodes to authenticate themselves with the members in the network before they get joined and starts accessing the network resources. The paths we use for certificate exchange should be secured and reliable. In MANET, the existing routing protocols do not provide security so that an active attacker can alter the control packets which can compromise the routing protocol’s integrity. Also the replay attack occurs when an attacking node listens to packets and then re-broadcast the same packets. In order to overcome these issues, in this paper, we propose a secure multipath routing protocol for certificate exchange which is based on optimized link state routing (OLSR) in MANET. Our proposed approach employs the neighborhood authentication technique that provides integrity. In this technique, the control message such as HELLO and topology control messages are used to construct the routing along with list of shareholder nodes. These control messages assist in attaining and distributing topology information and the generated signatures validates the integrity. Also in order to prevent the replay attacks, the timestamp exchange mechanism is employed. By simulation results we show that the proposed approach offers more security in MANET.

Fast distributed multi-hop relative time synchronization protocol and estimators for wireless sensor networks

The challenging problem of time synchronization in wireless sensor networks is considered in this paper, where a new distributed protocol is proposed for both local and multi-hop synchronization. The receiver-to-receiver paradigm is used, which has the advantage of reducing the time-critical-path and thus improving the accuracy compared to common sender-to-receiver protocols. The protocol is fully distributed and does not rely on any fixed reference. The role of the reference is divided amongst all nodes, while timestamp exchange is integrated with synchronization signals (beacons). This enables fast acquisition of timestamps that are used as samples to estimate relative synchronization parameters. An appropriate model is used to derive maximum likelihood estimators (MLE) and the Cramer-Rao lower bounds (CRLB) for both the offset-only, and the joint offset/skew estimation. The model permits to directly estimating relative parameters without using or referring to a reference’ clock. The proposed protocol is extended to multi-hop environment, where local synchronization is performed proactively and the resulted estimates are transferred to the intermediate/end-point nodes on-demand, i.e. as soon as a multi-hop communication that needs synchronization is initiated. On-demand synchronization is targeted for multi-hop synchronization instead of the always-on global synchronization model, which avoids periodic and continuous propagation of synchronization signals beyond a single-hop. Extension of local MLE estimators is proposed to derive relative multi-hop estimators. The protocol is compared by simulation to some state-of-the-art protocols, and results show much faster convergence of the proposed protocol. The difference has been on the order of more than twice compared to CS-MNS, more than ten times compared to RBS, and more than twenty times compared to TPSN. Results also show scalability of the proposed protocol concerning the multi-hop synchronization. The error does not exceed few microseconds for as much as 10 hops in R4Syn, while in CS-MNS, and TPSN, it reaches few tens of microseconds. Implementation and tests of the protocol on real sensor motes confirm microsecond level precision even in multi-hop scenarios, and high stability (long lifetime) of the skew/offset model.