Cooperative Synchronization in Wireless Networks

Synchronization is a key functionality in wireless network, enabling a wide variety of services. We consider a Bayesian inference framework whereby network nodes can achieve phase and skew synchronization in a fully distributed way. In particular, under the assumption of Gaussian measurement noise, we derive two message passing methods (belief propagation and mean field), analyze their convergence behavior, and perform a qualitative and quantitative comparison with a number of competing algorithms. We also show that both methods can be applied in networks with and without master nodes. Our performance results are complemented by, and compared with, the relevant Bayesian Cram\'er-Rao bounds

On fast and accurate detection of unauthorized wireless access points using clock skews

Journal ArticleWe explore the use of clock skew of a wireless local area network access point (AP) as its fingerprint to detect unauthorized APs quickly and accurately. The main goal behind using clock skews is to overcome one of the major limitations of existing solutions-the inability to effectively detect Medium Access Control (MAC) address spoofing. We calculate the clock skew of an AP from the IEEE 802.11 Time Synchronization Function (TSF) time stamps sent out in the beacon/probe response frames. We use two different methods for this purpose-one based on linear programming and the other based on least-square fit. We supplement these methods with a heuristic for differentiating original packets from those sent by the fake APs. We collect TSF time stamp data from several APs in three different residential settings. Using our measurement data as well as data obtained from a large conference setting, we find that clock skews remain consistent over time for the same AP but vary significantly across APs. Furthermore, we improve the resolution of received time stamp of the frames and show that with this enhancement, our methodology can find clock skews very quickly, using 50-100 packets in most of the cases. We also discuss and quantify the impact of various external factors including temperature variation, virtualization, clock source selection, and NTP synchronization on clock skews. Our results indicate that the use of clock skews appears to be an efficient and robust method for detecting fake APs in wireless local area networks

A Regular Pattern of Timestamps Between Machines with Built-in System Time

This paper studied the effect of 15.6 ms time resolution where the collected timestamps are in a form of parallel dotted lines, instead of one straight line like in classical case. The dotted lines made the clock skew measurement of two devices to become incorrect as the measurement which normally follow the cluster of offsets but now follow the parallel dotted lines. Dotted lines pattern is required in order to understand how to correct the clock skew measurement on data containing dotted lines. To model the dotted lines pattern is through Dotted lines Grouping Method, a tools to find the characteristics of the dotted lines. The dotted lines grouping method was then tested data obtained from wired and wireless communication of two similar devices. The dotted line grouping method results equal maximum number of dot of 10 for both data, which indicated the robustness of the dotted lines grouping method

A NOVEL APPROACH FOR COVERT COMMUNICATION OVER TCP VIA INDUCED CLOCK SKEW

The goal of this thesis is to determine the feasibility and provide a proof of concept for a covert communications channel based on induced clock skew. Transmission Control Protocol (TCP) timestamps provide a means for measuring clock skew between two hosts. By intentionally altering timestamps, a host can induce artificial clock skew as measured by the receiver, thereby providing a means to covertly communicate. A novel scheme for transforming symbols into skew values is developed in this work, along with methods for extraction at the receiver. We tested the proposed scheme in a laboratory network consisting of Dell laptops running Ubuntu 16.04. The results demonstrated a successful implementation of the proposed covert channel with achieved bit rates as high as 33 bits per second under ideal conditions. Forward error correction was also successfully employed in the form of a Reed–Solomon code to mitigate the effects of variation in delay over the Internet.Lieutenant, United States NavyApproved for public release; distribution is unlimited

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 correction in dynamic wireless sensor networks (WSNs). First, the unreliable links are described by stochastic connections, reflecting the characteristic of changing connectivity gleaned from dynamicWSNs. 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 term of converging speed, collision rate, and the robustness of resisting delay, and outperform other existing protocols

Software Defined Radio Implementation of Carrier and Timing Synchronization for Distributed Arrays

The communication range of wireless networks can be greatly improved by using distributed beamforming from a set of independent radio nodes. One of the key challenges in establishing a beamformed communication link from separate radios is achieving carrier frequency and sample timing synchronization. This paper describes an implementation that addresses both carrier frequency and sample timing synchronization simultaneously using RF signaling between designated master and slave nodes. By using a pilot signal transmitted by the master node, each slave estimates and tracks the frequency and timing offset and digitally compensates for them. A real-time implementation of the proposed system was developed in GNU Radio and tested with Ettus USRP N210 software defined radios. The measurements show that the distributed array can reach a residual frequency error of 5 Hz and a residual timing offset of 1/16 the sample duration for 70 percent of the time. This performance enables distributed beamforming for range extension applications.Comment: Submitted to 2019 IEEE Aerospace Conferenc

Clock synchronisation for UWB and DECT communication networks

Synchronisation deals with the distribution of time and/or frequency across a network of nodes dispersed in an area, in order to align their clocks with respect to time and/or frequency. It remains an important requirement in telecommunication networks, especially in Time Division Duplexing (TDD) systems such as Ultra Wideband (UWB) and Digital Enhanced Cordless Telecommunications (DECT) systems. This thesis explores three di erent research areas related to clock synchronisation in communication networks; namely algorithm development and implementation, managing Packet Delay Variation (PDV), and coping with the failure of a master node. The first area proposes a higher-layer synchronisation algorithm in order to meet the specific requirements of a UWB network that is based on the European Computer Manufacturers Association (ECMA) standard. At up to 480 Mbps data rate, UWB is an attractive technology for multimedia streaming. Higher-layer synchronisation is needed in order to facilitate synchronised playback at the receivers and prevent distortion, but no algorithm is de ned in the ECMA-368 standard. In this research area, a higher-layer synchronisation algorithm is developed for an ECMA-368 UWB network. Network simulations and FPGA implementation are used to show that the new algorithm satis es the requirements of the network. The next research area looks at how PDV can be managed when Precision Time Protocol (PTP) is implemented in an existing Ethernet network. Existing literature indicates that the performance of a PDV ltering algorithm usually depends on the delay pro le of the network in which it is applied. In this research area, a new sample-mode PDV filter is proposed which is independent of the shape of the delay profile. Numerical simulations show that the sample-mode filtering algorithm is able to match or out-perform the existing sample minimum, mean, and maximum filters, at differentlevels of network load. Finally, the thesis considers the problem of dealing with master failures in a PTP network for a DECT audio application. It describes the existing master redundancy techniques and shows why they are unsuitable for the specific application. Then a new alternate master cluster technique is proposed along with an alternative BMCA to suit the application under consideration. Network simulations are used to show how this technique leads to a reduction in the total time to recover from a master failure

A Time Synchronization Protocol for TDMA Based Wireless Sensor Networks

학위논문 (석사)-- 서울대학교 대학원 : 전기공학부, 2013. 8. 이정우.There has been much interest in wireless sensor networks recently, due to their diverse range of possible applications. Although there have been much research in MAC layer protocols for wireless sensor networks, these works are mainly focussed on the power savings and efficiencies of the protocols. For sensor networks which are in-situ and do not require much flexibility, such as a battery management system, energy is not always the most important factor, but rather reliability and scalability (where sensing periods are known). As such, a traditional TDMA protocol can be considered as a good option. Time synchronization in wireless sensor networks have also been considered by many academics, but work related to time synchronization in TDMA networks have been much less popular. In this thesis, a time synchronization protocol for TDMA based wireless sensor networks is proposed, Propagating Chain Time Synchronization. Propagating Chain Time Synchronization is a novel protocol for synchronizing TDMA based networks. The scheme achieves improved synchronization errors compared to traditional beacon synchronization methods, through skew correction estimated from chained two-way message exchanges, which employ piggybacking and overhearing.1 Introduction 1 1.1 Wireless Sensor Networks 1 1.1.1 Challenges in Designing Wireless Sensor Networks 2 1.2 Thesis Motivation 7 1.2.1 Wireless Sensor Networks in Battery Management Systems 7 2 Time Synchronization 10 2.1 Overview 10 2.2 Models of Clock Synchronization 11 2.2.1 Typical Synchronization Errors 13 2.3 Related Work 14 2.3.1 Sender-Receiver Synchronization 14 2.3.2 Receiver-Receiver Synchronization 16 2.3.3 Receiver-Only Synchronization 17 2.3.4 Clock Skew Estimation and Correction 18 2.3.5 Clock Synchronization in TDMA Based Networks 19 3 Propagating Chain Time Synchronization for TDMA Based Wireless Sensor Networks 21 3.1 Overview 21 3.2 System Model 21 3.2.1 Basic Assumptions 22 3.2.2 Topology 22 3.2.3 Chained Synchronization 23 3.2.4 Overhearing and Piggybacking 24 3.2.5 Propagating Skew Correction 28 4 Theoretical Error Analysis 31 4.1 System Models 31 4.2 Node Clock Modelling 32 4.3 TSF 34 4.4 Chained Synchronization 36 4.5 Two-Way Message Exchange Synchronization Error 38 5 Simulation 42 5.1 Simulation Parameters 42 5.2 Simulation Results 46 6 Conclusion 52 Bibliography 54Maste

Probabilistic Graphical Models: an Application in Synchronization and Localization

Die Lokalisierung von mobilen Nutzern (MU) in sehr dichten Netzen erfordert häufig die Synchronisierung der Access Points (APs) untereinander. Erstens konzentriert sich diese Arbeit auf die Lösung des Problems der Zeitsynchronisation in 5G-Netzwerken, indem ein hybrider Bayesischer Ansatz für die Schätzung des Taktversatzes und des Versatzes verwendet wird. Wir untersuchen und demonstrieren den beträchtlichen Nutzen der Belief Propagation (BP), die auf factor graphs läuft, um eine präzise netzwerkweite Synchronisation zu erreichen. Darüber hinaus nutzen wir die Vorteile der Bayesischen Rekursiven Filterung (BRF), um den Zeitstempel-Fehler bei der paarweisen Synchronisierung zu verringern. Schließlich zeigen wir die Vorzüge der hybriden Synchronisation auf, indem wir ein großes Netzwerk in gemeinsame und lokale Synchronisationsdomänen unterteilen und so den am besten geeigneten Synchronisationsalgorithmus (BP- oder BRF-basiert) auf jede Domäne anwenden können. Zweitens schlagen wir einen Deep Neural Network (DNN)-gestützten Particle Filter-basierten (DePF)-Ansatz vor, um das gemeinsame MU-Sync&loc-Problem zu lösen. Insbesondere setzt DePF einen asymmetrischen Zeitstempel-Austauschmechanismus zwischen den MUs und den APs ein, der Informationen über den Taktversatz, die Zeitverschiebung der MUs, und die AP-MU Abstand liefert. Zur Schätzung des Ankunftswinkels des empfangenen Synchronisierungspakets nutzt DePF den multiple signal classification Algorithmus, der durch die Channel Impulse Response (CIR) der Synchronisierungspakete gespeist wird. Die CIR wird auch genutzt, um den Verbindungszustand zu bestimmen, d. h. Line-of-Sight (LoS) oder Non-LoS (NLoS). Schließlich nutzt DePF particle Gaussian mixtures, die eine hybride partikelbasierte und parametrische BRF-Fusion der vorgenannten Informationen ermöglichen und die Position und die Taktparameter der MUs gemeinsam schätzen.Mobile User (MU) localization in ultra dense networks often requires, on one hand, the Access Points (APs) to be synchronized among each other, and, on the other hand, the MU-AP synchronization. In this work, we firstly address the former, which eventually provides a basis for the latter, i.e., for the joint MU synchronization and localization (sync&loc). In particular, firstly, this work focuses on tackling the time synchronization problem in 5G networks by adopting a hybrid Bayesian approach for clock offset and skew estimation. Specifically, we investigate and demonstrate the substantial benefit of Belief Propagation (BP) running on Factor Graphs (FGs) in achieving precise network-wide synchronization. Moreover, we take advantage of Bayesian Recursive Filtering (BRF) to mitigate the time-stamping error in pairwise synchronization. Finally, we reveal the merit of hybrid synchronization by dividing a large-scale network into common and local synchronization domains, thereby being able to apply the most suitable synchronization algorithm (BP- or BRF-based) on each domain. Secondly, we propose a Deep Neural Network (DNN)-assisted Particle Filter-based (DePF) approach to address the MU joint sync&loc problem. In particular, DePF deploys an asymmetric time-stamp exchange mechanism between the MUs and the APs, which provides information about the MUs' clock offset, skew, and AP-MU distance. In addition, to estimate the Angle of Arrival (AoA) of the received synchronization packet, DePF draws on the Multiple Signal Classification (MUSIC) algorithm that is fed by the Channel Impulse Response (CIR) experienced by the sync packets. The CIR is also leveraged on to determine the link condition, i.e. Line-of-Sight (LoS) or Non-LoS (NLoS). Finally DePF capitalizes on particle Gaussian mixtures which allow for a hybrid particle-based and parametric BRF fusion of the aforementioned pieces of information and jointly estimate the position and clock parameters of the MUs