Efficient Clock Synchronization in Wireless Sensor Networks

Time synchronization is indispensable for convenient network management, device monitoring, security, and other fundamental operations in industrial wireless sensor networks (IWSNs). Over the past few decades, a wide variety of highly accurate clock synchronization protocols have been investigated by employing powerful statistical signal processing techniques. However, most two-way exchange estimation schemes do not readjust the node's local clock upon every resynchronization before the clock parameters are estimated. And it may not be appropriate in IWSNs where time synchronization is consistently required. Based on the two-way message exchange mechanism, this paper investigates the clock synchronization schemes of active node and overhearing node with immediate clock readjustment. The maximum-likelihood estimators of the clock skew and the corresponding Cramer–Rao lower bounds are derived assuming Gaussian delays. Simulation and experimental results validate the performance of the proposed estimators.

Clock synchronization is a mechanism for providing a standard time to various devices across a network. This monograph provides a comprehensive overview of recent developments for clock synchronization protocols built on two-way message exchanges. Several clock synchronization protocols are available in the literature for distributing time from high-cost, high-stability clocks (termed masters) to low-cost, low-stability clocks (termed slaves) via an interconnecting network. A number of clock synchronization protocols are built on two-way message exchanges. These include the timing protocol for sensor networks (TPSN), lightweight time synchronization (LTS) protocol, tiny-sync and mini-sync, network time protocol (NTP) and the IEEE 1588 precision time protocol (PTP). The messages traveling between the master and slave nodes can encounter several intermediate switches and routers, accumulating delays at each node. The main factors contributing to the overall delay are the fixed propagation and processing delays at the intermediate nodes along the network path between the master and slave, as well as the stochastic queuing delays at each such node. Popular probability density function (pdf) models for modeling the stochastic delays include Gaussian, exponential, gamma, Weibull, and log-normal. Although these pdf models for the stochastic queuing delays apply to several scenarios, they might not be suitable in specific scenarios such as cellular base station synchronization using mobile backhaul networks and IEEE 1588 in 4G Long Term Evolution (LTE) networks. Further, there could be possibly unknown asymmetries between the fixed path delays in the forward master-to-slave path and the reverse slave-to-master path. These unknown asymmetries could arise from various sources, including delay attacks or incorrect modeling. In this monograph, we present recent developments for clock synchronization protocols built on the two-way message exchange. After an introduction to the basic concepts and mathematical models, the optimum estimators are presented for estimating the clock skew and offset that are applicable for any pdf model of the stochastic delays. Robust algorithms that can also handle unknown path asymmetries are presented next. The focus is on techniques that consider practical, relevant measurement models in order to guide the reader from physical observations to the actual synchronization of the clocks at the slave and master.

One-way text messaging to re-engage people in HIV care has shown promise, but little is known about two-way messaging where the recipient is asked to respond. We evaluated a two-way text messaging intervention, informed by behavioural science, to increase re-engagement in care following missed antiretroviral therapy (ART) visits. We conducted an individual-level randomised controlled trial between February and March 2023 in Capricorn District, South Africa. Adults aged ≥18 years who had missed an ART visit by >28 days were randomised to (1) a standard one-way text message, or (2) behaviourally informed two-way text messages. Two-way messages asked recipients to reply by selecting from a list of reasons for their missed visit. Those who responded received another text message tailored to their response and designed using behavioural economics principles (affect heuristic, availability heuristic, loss aversion, social norms, providing slack, expanding choice). The primary outcome was return to care within 45 days, analysed as 1) intention to treat, and 2) restricted to successful message delivery. 3,695 participants were randomised: 1,845 to the one-way message group and 1,850 to the two-way message group. 27.9% (515/1845) of participants sent a one-way message and 27.2% (503/1850) sent a two-way message returned for an ART visit within 45 days (proportion difference:-0.7%, p-value: 0.622). In an analysis restricted to participants whose text message was delivered, 28.3% (310/1094) in the one-way message group compared to 28.3% (304/1076) in the two-way message group returned to care (proportion difference:-0.09%, p-value: 0.966). 19.5% (210/1076) responded to the two-way message. The two most commonly reported reasons for missed appointments were being out of town (41.0%) and still having medication (31.0%.). Among those who responded, 27.1% (95%CI: 21.3-33.7) returned for an ART visit. Clients ≥50 years were less likely to respond to the two-way text message, (AOR 0.4; 95%CI: 0.2-0.9). Behaviourally informed two-way text messages did not improve return to care over one-way messages. However, they elicited reasons for disengagement, which could inform future outreach for missed visits. Additional research is needed on the mode, content and timing of two-way messages intended to increase return to care. PACTR202202748760768 & DOH-27-042022-6703.

Clock synchronization plays a crucial role in Wireless Sensor Networks (WSNs). Assuming that there is no clock skew between sensor nodes, the Maximum Likelihood Estimate (MLE) of clock offset was derived by [1] for clock synchronization protocols assuming exponential random delays and a two-way message exchange mechanism as in TPSN (Timing-sync Protocol for Sensor Networks [2]) or NTP (Network Time Protocol). The MLE is appropriate for the case that the random delays in WSNs are exponentially distributed. However, the performance of the MLE is deteriorated considerably in the case that the underlying distribution is contaminated by or mixed with other distributions. Hence, a robust estimator is needed. In this paper, clock offset estimators based on the robust M-estimation method are proposed and it is shown that the proposed estimators present excellent performance in the mean squared error (MSE) sense under the condition that the underlying distribution is mixed with other distribution.

For many applications, distributed networks require the local clocks of the constituent nodes to run close to an agreed upon notion of time. Most of the widely used clock synchronization algorithms in such systems employ the sender-receiver protocol based on a two-way timing message exchange paradigm. Maximum likelihood estimator (MLE) of the clock offset based on the timing message exchanges between two clocks was derived in D. R. Jeske, On maximum likelihood estimation of clock offset[IEEE Trans. Commun., vol. 53, pp. 53-54, Jan. 2005], when the fixed delays are symmetric and the variable delays in each direction assume an exponential distribution with an unknown mean. Herein, the best linear unbiased estimate using order statistics (BLUE-OS) of the clock offset between two nodes is derived assuming both symmetric and asymmetric exponential network delays, respectively. The Rao-Blackwell-Lehmann-Scheffe? theorem is then exploited to obtain the minimum variance unbiased estimate (MVUE) for the clock offset which it is shown to coincide with the BLUE-OS. In addition, it is found that the MVUE of the clock offset in the presence of symmetric network delays also coincides with the MLE. Finally, in the presence of asymmetric network delays, although the MLE is biased, it is shown to achieve lesser mean-square error (MSE) than the MVUE in the region around the point where the bidirectional network link delays are symmetric and hence its merit as the most versatile estimator is fairly justified.

A neural approach to synchronization in wireless networks with heterogeneous sources of noise

Nanonetworks are expected to expand the capabilities of individual nanomachines by allowing them to cooperate and share information by molecular communication. The information molecules are released by the transmitter nanomachine and diffuse across the aqueous channel as a Brownian motion holding the feature of a strong random movement with a large propagation delay. In order to ensure an effective real-time cooperation, it is necessary to keep the clock synchronized among the nanomachines in the nanonetwork. This paper proposes a model on a two-way message exchange mechanism with the molecular propagation delay based on the inverse Gaussian distribution. The clock offset and clock skew are estimated by the maximum-likelihood estimation. Simulation results by MATLAB show that the mean square errors of the estimated clock offsets and clock skews can be reduced and converge with a number of rounds of message exchanges. The comparison of the proposed scheme with a clock synchronization method based on symmetrical propagation delay demonstrates that our proposed scheme can achieve better performance in terms of accuracy.

In wireless sensor networks, energy efficiency is an important standard for evaluating the effectiveness of the time synchronization protocols. Hence, energy should be conserved as much as possible provided that algorithm can achieve a certain accuracy. In this paper, we propose an energy efficient time synchronization algorithm in which each layer of the network can be synchronized merely by receiving broadcast messages rather than relying on two-way message exchange. Simulation results show that our improved strategy reduced the energy consumption and prolonged the lifetime of network compared with TPSN.

Radio-based two-way message exchange is an important time synchronization method. However, the accuracy of synchronization and ranging decreases when the clock is not ideal. In this study, we considered a time-varying accumulated clock offset, and proposed a Doppler-shift assisted joint time synchronization and ranging method based on Extended Kalman Filter (EKF). The accumulated clock offset and local Doppler shift were modeled; time stamps and Doppler shift information were exchanged during synchronization; the joint estimation of clock and distance parameters were accomplished through the EKF. Simulation experiments showed that the proposed method can further improve the synchronization and ranging accuracy as well as reduce the convergence time compared with the recently proposed extended time-varying Kalman filter method.

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

Fog-based Vehicular ad-hoc networks (VANETs) are new kinds of network that combine fog computing and mobile ad-hoc networks appeared in recent years, which can be used in monitoring traffic, smart driving, safe driving, and so on. Fog-based VANET is composed of a large number of dynamic nodes and static nodes, which is a heterogeneous network. Due to the heterogeneity of fog-based VANET, there is a large difference in clock rate between different nodes. It makes clock of nodes soon become inconsistent. The inaccurate clocks would lead to problems such as imprecise duty-cycle scheduling and wrong events sequence, etc. Synchronizing the clocks needs multiple message exchanges among the nodes. However, due to the mobility of the fog-based VANET, the network topology changes dynamically, which may cause synchronization packets to fail to be transmitted to a node that needs to be synchronized. In this paper, a precise time synchronization mechanism for fog-based VANET is proposed. The mechanism uses two-way message exchange method to estimate clock offset through analyzing the mobility of dynamic nodes. Clock skew is corrected using a carrier frequency offset (CFO) based hardware clock frequency estimation and considering the Doppler effect. Theoretical analyses and simulation results show that our time synchronization mechanism achieves highly accurate time synchronization.

Time synchronization is the key supporting technology for wireless sensor networks in many applications such as Internet of things, environmental monitoring and target tracking. On the basis of summarizing and analyzing the time synchronization algorithm of two-way message exchange and the distributed time synchronization algorithm, combining with the research status at home and abroad, the future research direction of time synchronization algorithm is given.

Directional antennas have numerous advantages over traditional omnidirectional antennas, which include more spatial reuse, extended range, less interference, and less energy consumption. Directional antennas introduce deafness and new hidden terminal problems. Deafness may cause more collisions, and hidden terminal problems may result in more retransmissions, poor quality of service, more energy consumption, and less packet delivery ratio. Hence, it is important to design of an effective medium access protocol specifically for directional antennas in order to reap the benefits of directional antennas while managing deafness and hidden terminal problems, otherwise the challenges can adversely affect system performance. In wireless sensor networks, the sensors are battery powered with limited supply of energy. Therefore, energy efficient protocols and solutions are immensely important with the desired goal of extending the network lifetime longer than what is possible through the omnidirectional antennas. In this thesis, I have proposed an energy efficient scheduled directional medium access control protocol (DTRAMA) which is specially designed for the wireless sensor nodes which use directional antennas in wireless sensor networks. It is a traffic adaptive scheduled medium access protocol in which nodes create their transmission and sleep schedules on the basis of their traffic. Scheduled medium access for packet transmission is used to address the deafness and hidden terminal problems caused by the use of directional communication in contention based MAC protocols. Use of directional antenna reduces interference which indirectly improves packet delivery ratio by improving the signal to noise ratio and by reducing the packet error rate. DTRAMA is energy efficient: firstly because the nodes use directional data communication which requires lower node transmit power than the omnidirectional data communication for the same transmission range; and secondly, because the nodes schedule their sleep period to reduce idle listening and overhearing which improves energy efficiency. In DTRAMA, sleep schedule of the node is traffic adaptive which is essential to maximize the sleep period. The node, by using spatial reuse checks, reuses the wireless medium to the maximum extent to reap the spatial reuse benefits of the directional antenna. The nodes reduce their packet latency through spatial reuse. Through simulation and by using different topologies, I have compared the performance of DTRAMA with those of omnidirectional schedule MAC protocol (TRAMA) and contention based directional MAC protocol (DMAC), which clearly shows that DTRAMA outperforms TRAMA and DMAC in packet delivery ratio and outperforms TRAMA in terms of packet latency.

Directional antennas have numerous advantages over traditional omnidirectional antennas, which include more spatial reuse, extended range, less interference, and less energy consumption. Directional antennas introduce deafness and new hidden terminal problems. Deafness may cause more collisions, and hidden terminal problems may result in more retransmissions, poor quality of service, more energy consumption, and less packet delivery ratio. Hence, it is important to design of an effective medium access protocol specifically for directional antennas in order to reap the benefits of directional antennas while managing deafness and hidden terminal problems, otherwise the challenges can adversely affect system performance. In wireless sensor networks, the sensors are battery powered with limited supply of energy. Therefore, energy efficient protocols and solutions are immensely important with the desired goal of extending the network lifetime longer than what is possible through the omnidirectional antennas. In this thesis, I have proposed an energy efficient scheduled directional medium access control protocol (DTRAMA) which is specially designed for the wireless sensor nodes which use directional antennas in wireless sensor networks. It is a traffic adaptive scheduled medium access protocol in which nodes create their transmission and sleep schedules on the basis of their traffic. Scheduled medium access for packet transmission is used to address the deafness and hidden terminal problems caused by the use of directional communication in contention based MAC protocols. Use of directional antenna reduces interference which indirectly improves packet delivery ratio by improving the signal to noise ratio and by reducing the packet error rate. DTRAMA is energy efficient: firstly because the nodes use directional data communication which requires lower node transmit power than the omnidirectional data communication for the same transmission range; and secondly, because the nodes schedule their sleep period to reduce idle listening and overhearing which improves energy efficiency. In DTRAMA, sleep schedule of the node is traffic adaptive which is essential to maximize the sleep period. The node, by using spatial reuse checks, reuses the wireless medium to the maximum extent to reap the spatial reuse benefits of the directional antenna. The nodes reduce their packet latency through spatial reuse. Through simulation and by using different topologies, I have compared the performance of DTRAMA with those of omnidirectional schedule MAC protocol (TRAMA) and contention based directional MAC protocol (DMAC), which clearly shows that DTRAMA outperforms TRAMA and DMAC in packet delivery ratio and outperforms TRAMA in terms of packet latency.

We analyze various critical transmitting/sensing ranges for connectivity and coverage in three-dimensional sensor networks. As in other large-scale complex systems, many global parameters of sensor networks undergo phase transitions. For a given property of the network, there is a critical threshold, corresponding to the minimum amount of the communication effort or power expenditure by individual nodes, above (respectively, below) which the property exists with high (respectively, a low) probability. For sensor networks, properties of interest include simple and multiple degrees of connectivity/coverage. First, we investigate the network topology according to the region of deployment, the number of deployed sensors, and their transmitting/sensing ranges. More specifically, we consider the following problems: assume that n nodes, each capable of sensing events within a radius of r, are randomly and uniformly distributed in a 3-dimensional region R of volume V, how large must the sensing range R/sub SENSE/ be to ensure a given degree of coverage of the region to monitor? For a given transmission range R/sub TRANS/, what is the minimum (respectively, maximum) degree of the network? What is then the typical hop diameter of the underlying network? Next, we show how these results affect algorithmic aspects of the network by designing specific distributed protocols for sensor networks.