Category Archives: Communication Networks

Interesting review of time delay measurement in one-way messages in networks at the application level

P. Ferrari, A. Flammini, E. Sisinni, S. Rinaldi, D. Brandão and M. S. Rocha, Delay Estimation of Industrial IoT Applications Based on Messaging Protocols, IEEE Transactions on Instrumentation and Measurement, vol. 67, no. 9, pp. 2188-2199, DOI: 10.1109/TIM.2018.2813798.

Information and operational technologies merge into the so-called industrial Internet of Things, which is one of the basic pillars of the Industry 4.0 paradigm. Roughly speaking, yet-to-come services will be offered in the automation scenario by industrial devices having an internet connection for sharing data in the cloud. Currently, most efforts are in the development of protocols able to ensure horizontal interoperability among heterogeneous applications. Consequently, poor attention is devoted to time-related performance. In this paper, a new, full software, platform-independent approach is proposed for experimentally evaluating the delay in transferring information across local and intercontinental routes by applications leveraging on messaging middleware. The application is realized using the node-RED web-based framework, due to its availability on different platforms; the widely accepted message queue telemetry transport protocol has been chosen thanks to its low overhead and complexity. For sake of completeness, five different, private and public, brokers are used. The adopted industrial-grade hardware, complemented by global positioning system time reference, permits an overall synchronization and timestamping accuracy of a few milliseconds. The vast measurement campaign highlighted that, generally, quality of service (QoS) type 1 offers low end-to-end delay (average value less than 0.5 s) with reduced variability (0.1 s). However, the maximum end-to-end one-way delay ranges from 1 s for QoS 0 to less than 1.5 s for fully acknowledged QoS 2.

A new algorithm that provably converges to a global clock consensus in a network

Miloš S. Stanković, Srdjan S. Stanković, Karl Henrik Johansson, Distributed time synchronization for networks with random delays and measurement noise, Automatica, Volume 93, 2018, Pages 126-137 DOI: 10.1016/j.automatica.2018.03.054.

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.

On the effects of delays in the stability of a network controlled plant due to both clocks not being synchronized

K. Okano, M. Wakaiki, G. Yang and J. P. Hespanha, Stabilization of Networked Control Systems Under Clock Offsets and Quantization, IEEE Transactions on Automatic Control, vol. 63, no. 6, pp. 1708-1723 DOI: 10.1109/TAC.2017.2753938.

This paper studies the impact of clock mismatches and quantization on networked control systems. We consider a scenario where the plant’s state is measured by a sensor that communicates with the controller through a network. Variable communication delays and clock jitter do not permit a perfect synchronization between the clocks of the sensor and controller. We investigate limitations on the clock offset tolerable for stabilization of the feedback system. For a process with a scalar-valued state, we show that there exists a tight bound on the offset above which the closed-loop system cannot be stabilized with any causal controllers. For higher dimensional plants, if the plant has two distinct poles, then the effect of clock mismatches can be canceled with a finite number of measurements, and hence there is no fundamental limitation. We also consider the case where the measurements are subject to quantization in addition to clock mismatches. For first-order plants, we present necessary conditions and sufficient conditions for stabilizability, which show that a larger clock offset requires a finer quantization.

A novel fast algorithm for clock synchronization in a wireless network, with a nice introduction but assuming negligible communication times and thus not directly applicable in teleoperation

Kan Xie, Qianqian Cai, Minyue Fu, A fast clock synchronization algorithm for wireless sensor networks, Automatica, Volume 92, 2018, Pages 133-142, DOI: 10.1016/j.automatica.2018.03.004.

This paper proposes a novel clock synchronization algorithm for wireless sensor networks (WSNs). The algorithm is derived using a fast finite-time average consensus idea, and is fully distributed, meaning that each node relies only on its local clock readings and reading announcements from its neighbours. For networks with an acyclic graph, the algorithm converges in only d iterations for clock rate synchronization and another d iterations for clock offset synchronization, where d is the graph diameter. The algorithm enjoys low computational and communicational complexities and robustness against transmission adversaries. Each node can execute the algorithm asynchronously without the need for global coordination. Due to its fast convergence, the algorithm is most suitable for large-scale WSNs. For WSNs with a cyclic graph, a fast distributed depth-first-search (DFS) algorithm can be applied first to form a spanning tree before applying the proposed synchronization algorithm.

Optimal routing in communication networks with probabilistic models of delays that are acquired on-line

M. S. Talebi, Z. Zou, R. Combes, A. Proutiere and M. Johansson, Stochastic Online Shortest Path Routing: The Value of Feedback, IEEE Transactions on Automatic Control, vol. 63, no. 4, pp. 915-930, DOI: 10.1109/TAC.2017.2747409.

This paper studies online shortest path routing over multihop networks. Link costs or delays are time varying and modeled by independent and identically distributed random processes, whose parameters are initially unknown. The parameters, and hence the optimal path, can only be estimated by routing packets through the network and observing the realized delays. Our aim is to find a routing policy that minimizes the regret (the cumulative difference of expected delay) between the path chosen by the policy and the unknown optimal path. We formulate the problem as a combinatorial bandit optimization problem and consider several scenarios that differ in where routing decisions are made and in the information available when making the decisions. For each scenario, we derive a tight asymptotic lower bound on the regret that has to be satisfied by any online routing policy. Three algorithms, with a tradeoff between computational complexity and performance, are proposed. The regret upper bounds of these algorithms improve over those of the existing algorithms. We also assess numerically the performance of the proposed algorithms and compare it to that of existing algorithms.

Mapping the wifi signal for robot localization both precisely and accurately through a complex model of the signal

Renato Miyagusuku, Atsushi Yamashita, Hajime Asama, Precise and accurate wireless signal strength mappings using Gaussian processes and path loss models, Robotics and Autonomous Systems, Volume 103, 2018, Pages 134-150, DOI: 10.1016/j.robot.2018.02.011.

In this work, we present a new modeling approach that generates precise (low variance) and accurate (low mean error) wireless signal strength mappings. In robot localization, these mappings are used to compute the likelihood of locations conditioned to new sensor measurements. Therefore, both mean and variance predictions are required. Gaussian processes have been successfully used for learning highly accurate mappings. However, they generalize poorly at locations far from their training inputs, making those predictions have high variance (low precision). In this work, we address this issue by incorporating path loss models, which are parametric functions that although lacking in accuracy, generalize well. Path loss models are used together with Gaussian processes to compute mean predictions and most importantly, to bound Gaussian processes’ predicted variances. Through extensive testing done with our open source framework, we demonstrate the ability of our approach to generating precise and accurate mappings, and the increased localization accuracy of Monte Carlo localization algorithms when using them; with all our datasets and software been made readily available online for the community.

A gentle tutorial on Industrial Ethernet

K. Langlois et al., EtherCAT Tutorial: An Introduction for Real-Time Hardware Communication on Windows [Tutorial], IEEE Robotics & Automation Magazine, vol. 25, no. 1, pp. 22-122, DOI: 10.1109/MRA.2017.2787224.

Setting up real-time hardware communication for applications such as precise motion control can be time consuming and confusing. Therefore, this tutorial introduces the deployment of an Ethernet for control automation technology (EtherCAT) protocol. We situate EtherCAT, briefly discuss the origins and working principles, and mention advantages over other widely used protocols. Additionally, the main objectives of the tutorial and the required software to complete it are presented. Online supplements are included, explaining all steps to run a Simulink model in real time on a Windows machine within a few hours.

A framework for the performance analysis of collaborative network clock synchronization

Y. Xiong, N. Wu, Y. Shen and M. Z. Win, Cooperative Network Synchronization: Asymptotic Analysis, IEEE Transactions on Signal Processing, vol. 66, no. 3, pp. 757-772, DOI: 10.1109/TSP.2017.2759098.

Accurate clock synchronization is required for collaborative operations among nodes across wireless networks. Compared with traditional layer-by-layer methods, cooperative network synchronization techniques lead to significant improvement in performance, efficiency, and robustness. This paper develops a framework for the performance analysis of cooperative network synchronization. We introduce the concepts of cooperative dilution intensity (CDI) and relative CDI to characterize the interaction between agents, which can be interpreted as properties of a random walk over the network. Our approach enables us to derive closed-form asymptotic expressions of performance limits, relating them to the quality of observations as well as the network topology.

Simultaneous localization and clock synchronization (apparently only offsets are estimated) in wireless networks

Y. Liu, Y. Shen, D. Guo and M. Z. Win, Network Localization and Synchronization Using Full-Duplex Radios, IEEE Transactions on Signal Processing, vol. 66, no. 3, pp. 714-728, DOI: 10.1109/TSP.2017.2770090.

Both localization and synchronization of mobile nodes are important for wireless networks. In this paper, we propose new methods for network localization and synchronization (NLS) using full-duplex radios through only two frames of transmission. Specifically, all nodes simultaneously transmit their signature signals in the first frame, while receiving others’ signals via full-duplex radios. In the second frame, nodes transmit either scrambled versions of their received signals in the first frame or a digital packet of the channel parameter estimates of the received signals. We develop distributed algorithms to estimate the arrival times of different components in the received signals. These arrival times are then used to determine the local network geometry and clock offsets. The Cramér-Rao lower bounds for internode distances and clock offsets are derived, and the former can be translated into error bounds of the node positions. Compared with conventional frequency division duplex or time-division duplex, we demonstrate the high efficiency of NLS using full-duplex radios, revealing its potential beyond data communications in future wireless networks.

A novel method for hard real-time communications using the physical layer of Ethernet and a variation of TDMA

Andrzej Przybył, Hard real-time communication solution for mechatronic systems, Robotics and Computer-Integrated Manufacturing, Volume 49, 2018, Pages 309-316, DOI: 10.1016/j.rcim.2017.08.001.

The paper proposes a method to build a highly efficient real-time communication solution for mechatronic systems. The method is based on the Ethernet physical layer (PHY) and on field programmable gate array (FPGA) technology and offers a better performance when compared to commercially available communication solutions. Although it is not directly compatible with the OSI/ISO model of TCP/IP protocol, vertical integration is done with a gateway. This provides simplicity and safety. Moreover, the use of the FPGA allows for integrating the communication solution with the user algorithm of particular distributed device inside a single chip. Therefore, the proposed solution is efficient and highly integrated.