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Timing synchronization of low power wireless sensor nodes with largely differing clock frequencies and variable synchronization intervals
... Similar to the power measurement part the system responsible for analyzing the timing behaviour of distributed embedded wireless sensor nodes should have a sufficient high temporal resolution to evaluate jitter of clock synchronization algorithms and data transmission delays in the range of a couple of µs, which is typical for low-cost embedded WSN nodes [5]. ...
... A significant advantage of EPhESOS protocol design is that the nodes are tightly time-synchronized after they are successfully registered in the base station. They can be sent to sleep for 30 s without loosing synchronization [5]. Timing adjustment requires only small corrections in the range of ±1 clock cycle (±30µs). ...
... The measurements show for all nodes a standard deviation less than 30 µs which confirms theoretical and simulation results of [5]. The measurement values around 120 µs and 150 µs are based on slight crystal changes due to temperature variations. ...
... Usually, high accuracy is linked to high complexity which is a contradiction to low power sensor nodes [1]. Nevertheless, energy efficiency and synchronicity is a vital demand for many systems [2], [3] and, therefore, we present an O(N ) complexity algorithm, albeit state-of-the-art algorithms with the same clock frequency estimation accuracy feature at least a complexity of O(N log N ) [4]. The algorithm can be used for frequency estimation in sparse and non-sparse processes. ...
... Here we consider the estimator based on sampling of the observed non-sparse point process (3). According to (6), (7) and (8) which gives an equation for the estimator by using (13) ...
... The algorithm can be implemented iteratively to save most of the computing time in subsequent estimates. There are already existing implementations in the area of energy harvesting wireless sensor nodes [2], [3], which benefit from the high estimation accuracy and less computational complexity. ...
... A significant advantage of our protocol design is that the nodes are tightly time-synchronized after they are successfully registered in the WSN. They can be sent to sleep for 30 s without loosing synchronization [5]. As each message from WNP to a node contains a timestamp, a timing adjustment in the same way as shown in Fig. 4 is possible at any time. ...
... To save energy, one of the core ideas of the data triggered TDMA protocol is that the sending of data to the WNP appears without listening and synchronizing to a beacon before the transmission takes place. This is only possible if the timing synchronization is sufficiently accurate to send the data in the correct timeslot [4], [5]. The nodes listen to the beacon only after the data transmission to receive the information about the successful (or failed) reception of the data. ...
We present the design of a suite of protocols for
wireless sensor networks (WSNs) with respect to a complete life
cycle of a WSN node from warehouse to the end of operation.
While there are numerous publications on various, usually
isolated, aspects of WSNs, the whole life cycle of a node from
registration in an automation system via warehouse, calibration,
mounting, performing measurements to finally unmounting, has
not yet been sufficiently addressed as compound survey. Our
application example is a WSN to be used in automotive test
beds in which a large amount of testing with many different
sensors is performed in controlled environments. While there
is published work on WSNs for performing the measurements
focusing on node hardware and MAC protocol, we now extend
this work by accounting for the whole life cycle of operation of
such a WSN and its nodes. This is mainly achieved by introducing
optimized MAC protocols for wireless communication in all life
cycle phases. Right from beginning of the life cycle the nodes are
synchronized with a base node. Even during long offline periods
nodes stay synchronized. The life cycle is modeled via a set of
states, instantiated in state machines, which control operation in
the base station and the nodes. Besides, considering the whole
life cycle of the sensor nodes, our design minimizes energy
consumption, largely avoids collisions due to suitable multiple
access protocols, and allows tight synchronization even during
long sleep periods. A demonstrator concludes the presentation
and shows functionality and benefits of the concept.
... In addition, there are known custom-made solutions that do not use popular radio interfaces. A protocol based on time-division multiple access (TDMA) for bidirectional sensor-based station communication was presented in [48]. A variance in synchronization error of 7.6 µs was obtained with a clock base of 32.768 kHz. ...
This paper presents an energy-efficient and high-accuracy sampling synchronization approach for real-time synchronous data acquisition in wireless sensor networks (saWSNs). A proprietary protocol based on time-division multiple access (TDMA) and deep energy-efficient coding in sensor firmware is proposed. A real saWSN model based on 2.4 GHz nRF52832 system-on-chip (SoC) sensors was designed and experimentally tested. The obtained results confirmed significant improvements in data synchronization accuracy (even by several times) and power consumption (even by a hundred times) compared to other recently reported studies. The results demonstrated a sampling synchronization accuracy of 0.8 μs and ultra-low power consumption of 15 μW per 1 kb/s throughput for data. The protocol was well designed, stable, and importantly, lightweight. The complexity and computational performance of the proposed scheme were small. The CPU load for the proposed solution was <2% for a sampling event handler below 200 Hz. Furthermore, the transmission reliability was high with a packet error rate (PER) not exceeding 0.18% for TXPWR ≥ −4 dBm and 0.03% for TXPWR ≥ 3 dBm. The efficiency of the proposed protocol was compared with other solutions presented in the manuscript. While the number of new proposals is large, the technical advantage of our solution is significant.
... As conventional BLE only supports 20 simultaneous connections, the number of devices in the WSN would be limited. This limitation could be circumvented by using custom or proprietary wireless protocols based on the BLE physical stack, e.g., [26], where hundreds of wireless sensor nodes could connect to a WSN based on BLE. With ROS as the used middleware, a problem can occur when there are too many active ROS topics and insufficient hardware to support it. ...
In robotics, there are many different sensors and actuators mounted onto a robot which may also, in the case of modular robotics, be interchanged during operation. During development of new sensors or actuators, prototypes may also be mounted onto a robot to test functionality, where the new prototypes often have to be integrated manually into the robot environment. Proper, fast and secure identification of new sensor or actuator modules for the robot thus becomes important. In this work, a workflow to add new sensors or actuators to an existing robot environment while establishing trust in an automated manner using electronic datasheets has been developed. The new sensors or actuators are identified via near field communication (NFC) to the system and exchange security information via the same channel. By using electronic datasheets stored on the sensor or actuator, the device can be easily identified and trust can be established by using additional security information contained in the datasheet. In addition, the NFC hardware can simultaneously be used for wireless charging (WLC), thus allowing for wireless sensor and actuator modules. The developed workflow has been tested with prototype tactile sensors mounted onto a robotic gripper.
... The time granularity of the wake-up mechanism is defined by the sleep clock. To overcome the resulting problems, concepts for synchronization and determining the wake-up interval are adapted according to [27], [28], which enables synchronicity of ±1 clock period. As for WirelessHART or ISA100.11a, ...
Minimized and nearly deterministic end-to-end latency facilitates real-time data acquisition and actuator control. In addition, defined latency is an integral part of quality oriented service in order to get closer to the reliability of wired networks and at the same time take advantage of wireless networking. This paper introduces a QoS routing protocol capable of balancing power consumption between wireless sensor and actuator nodes while minimizing end-to-end latency. We introduce a TDMA scheme in the routed wireless network to enable defined latency and in addition it improves the energy efficiency by avoiding collisions which eliminates time and energy consuming retries. Our novel routing method allows latency and round-trip times to be calculated in advance. We implemented a demonstrator and show experimental results of a wireless sensor network with our proposed routing scheme.
... Many applications rely on period or frequency estimation such as carrier frequency recovery in communication systems, vital sign monitoring or synchronization in wireless sensor networks (WSNs) [1,2,3,4]. Within networks, beacon signals are sent out periodically from a master and received by many communication partners. The time stamping with the arrival time enables the estimation of the beacon period and the synchronization the local clock. ...
... Therefore, the system time granularity is defined by the standby clock and synchronization is done by periodic beacons sent each SF. Synchronization and frequency estimation is an integral part of the system and supported by several concepts presented in [22], [23]. Similar to WirelessHART or ISA100.11.a, ...
... Many applications rely on period or frequency estimation, such as carrier frequency recovery in communication systems, vital sign monitoring, or synchronization in wireless sensor networks (WSNs) [1,2,3,4]. Within networks beacon signals are sent out periodically from a master and received by many communication partners. The time stamping of the arrival time allows to estimate the beacon period and to synchronize the local clock. ...
... There are many ways to synchronize sensor nodes. One of them is the use of an RTC, like the one proposed by [18], which impacts in the sensor node energy. Another alternative, is to find the difference between time from coordinator and sensor node, which is proposed to be used. ...
This research presents a WSN computer system model applied to precision irrigation, specifically for conditions in which sensor nodes may be located far from the coordinator. The proposed model is based on a tailored hardware and software aiming to extend both sensor nodes and WSN lifetime. The coordinator is aware of its sensor nodes' available energy and such feature enables estimating the remaining lifetime of each sensor node. The so-called Sensor nOdes' system for data acqUisition of Long range (SOUL) is the conceptual model for a sensor node hardware and software system design, concerning energy management and messages grouping, improving both sensor nodes and network lifetime. The sensor nodes were employed to monitor soil moisture and thus the SOUL computer system model was validated in field conditions. Sensor nodes lifetime was increased by 5.7 times when compared to a conventional use.
Existing packet-based synchronization approaches take extensive energy for industrial wireless sensor networks (IWSNs) due to substantial timing message exchanges. Combined with acknowledgement (ACK), this paper proposes an energy-efficient synchronization protocol without exchanging timestamps. Also, a mapping rule of ACK response time is presented for the joint estimation of skew and fixed delay without requiring a following response in each cycle, so that estimation can be implemented in parallel to data transmission. Then the timestamp-free synchronization technique is extended to the receiver-only synchronization (ROS) scenario to further reduce communication overhead. Assuming Gaussian variable delays, the maximum likelihood estimators and corresponding lower bounds for slave node and silent node are developed, as well as a low-complexity estimator for silent node. In addition, considering the dynamics of clock, a timestamp-free clock skew tracking algorithm using Kalman filtering is developed. Simulation results validate the efficiency of the proposed estimators.
With the emerging Internet of Things (IoT) technology becoming reality, a number of applications are being proposed. Several of these applications are highly dependent on wireless sensor networks (WSN) to acquire data from the surrounding environment. In order to be really useful for most of applications, the acquired data must be coherent in terms of the time in which they are acquired, which implies that the entire sensor network presents a certain level of time synchronization. Moreover, to efficiently exchange and forward data, many communication protocols used in WSN rely also on time synchronization among the sensor nodes. Observing the importance in complying with this need for time synchronization, this work focuses on the second synchronization problem, proposing, implementing and testing a time synchronization service for low-power WSN using low frequency real-time clocks in each node. To implement this service, three algorithms based on different strategies are proposed: one based on an auto-correction approach, the second based on a prediction mechanism, while the third uses an analytical correction mechanism. Their goal is the same, i.e., to make the clocks of the sensor nodes converge as quickly as possible and then to keep them most similar as possible. This goal comes along with the requirement to keep low energy consumption. Differently from other works in the literature, the proposal here is independent of any specific protocol, i.e., it may be adapted to be used in different protocols. Moreover, it explores the minimum number of synchronization messages by means of a smart clock update strategy, allowing the trade-off between the desired level of synchronization and the associated energy consumption. Experimental results, which includes data acquired from simulations and testbed deployments, provide evidence of the success in meeting this goal, as well as providing means to compare these three approaches considering the best synchronization results and their costs in terms of energy consumption.
We analyse the use of a ΔΣ-modulator in the nodes of a wireless sensor network, which is a new method to achieve long term synchronization. We consider star topology WSNs (Wireless Sensor Networks) with a central base station and address timing synchronization using low frequency realtime clocks. The WSN uses a beacon driven TDMA-protocol for bidirectional node/base communication. Between the beacons, which are sent by the base station, lie the superframe time intervals to handle data transmission from node to base. The ΔΣ-modulator is used to generate — at average — the accurate superframe duration for any rational number of clock ticks, by generating a sequence of superframes with different time durations, but each consisting of integer multiples of clock ticks. We discuss the synchronization accuracy based on the internal arithmetic of the ΔΣ-modulator and show by theory a relation between synchronization accuracy and word length of the internal arithmetic. Additionally the fractional part of a crystal clock module is responsible for variations in the synchronization quality. We present an equation that allows us to interpret measurements showing periodic variations of synchronization quality.
In industrial applications of wireless sensor networks (WSNs), synchronized sampling of data on each sensor node is often required. Thus, the wireless communication protocol needs to support accurate timing synchronization. If due to a high sampling rate also high data throughput is required, WSNs based on the IEEE 802.15.4 physical layer often do not provide sufficient data rate. Wireless communications based on the well-established IEEE 802.11 wireless local area network (WLAN) standard provides high data throughput but not an accurate timing synchronization unless the protocol stack is severely changed. We propose two low-complexity consensus-based synchronization algorithms for the hybrid WSN introduced, which are executable at limited embedded computing capacity, e.g., on an 8-b microcontroller. A time division multiple access-based synchronization packet broadcasting with three-step-controlled or proportional-integral (PI)-controlled clock adjustment enables 1-kHz sensor sampling rate with a sampling jitter <15 mu s for the three-step-controlled synchronization algorithm and for the PI-controlled algorithm.
In this paper, we present Guard Beacon, which is a beacon strategy to reduce the energy consumption of time synchronization in wireless sensor networks (WSNs). In a low duty-cycled sensor network, a node may miss incoming synchronization beacons because of wrong active/sleep mode due to clock drift. Therefore, it is critical to guarantee that beacons arrive at the receiver at the right time. The proposed method, i.e., Guard Beacon, can reduce the overall power consumption of synchronization while guaranteeing a very high probability that the synchronization packet is received, by sending multiple beacons within a synchronization round. By investigating on the energy tradeoff between sending and receiving beacons, we find an optimal iterative solution as well as a suboptimal analytical solution as to how many beacons should be sent and when to send them. The strategy is implemented in a real-world testbed for experiment validation. The results show that the proposed Guard Beacon may save more than 40% synchronization power consumption compared with the existed Single Beacon strategy and is also more energy efficient than RTSP in multihop networks.
Wireless sensor networks (WSN) are emerging as a valuable tool in many fields of science and industry. Time synchronization is an important issue for WSN's due to the collaborative and distributed nature of the tasks they perform. This paper describes a new technique for clock synchronization in WSNs called consensus clock synchronization that provides internal synchronization to a virtual consensus clock. It is sensitive to the limited resources available to sensor nodes and is robust to many of the challenges faced in dynamic ad-hoc networks. Simulations are presented to analyse the effectiveness of the synchronization protocol in a mesh network.
Clock synchronization is a critical component in the operation of wireless sensor networks (WSNs), as it provides a common time frame to different nodes. It supports functions such as fusing voice and video data from different sensor nodes, time-based channel sharing, and coordinated sleep wake-up node scheduling mechanisms. Early studies on clock synchronization for WSNs mainly focused on protocol design. However, the clock synchronization problem is inherently related to parameter estimation, and, recently, studies on clock synchronization began to emerge by adopting a statistical signal processing framework. In this article, a survey on the latest advances in the field of clock synchronization of WSNs is provided by following a signal processing viewpoint. This article illustrates that many of the proposed clock synchronization protocols can be interpreted and their performance assessed using common statistical signal processing methods. It is also shown that advanced signal processing techniques enable the derivation of optimal clock synchronization algorithms under challenging scenarios.
The paper introduces an improved low power time synchronization algorithm operating Kalman filtering and frequency deviation compensation for WirelessHART sensor networks and simulates in the single-hop and whole-network situation. The communication between nodes with WirelessHART communication is performed by using timeslot unit of 10ms precision. It is crucial to ensure that there is no time deviation during the communication so that the network can operate normally in such a short timeslot. The simulation results show that the proposed time synchronization algorithm is efficient and effective for our target WirelessHART network system. In contrast with the currently used algorithm, the time deviation after 20 times of synchronization is reduced approximately 33% by using the improved synchronization algorithm in the single-hop test, and the error variance is significantly decreased in the whole network time synchronization as the hop number increases.
The range of applications for wireless sensor networks (WSNs) is continuously growing and is expanding into new branches and markets rapidly. Largely different requirements regarding network structure, powering possibility, latency, data rates, and reliability, lead to the need of suitable systems and protocols. An approved way to implement an energy aware wireless network is the Time Division Multiple Access (TDMA) strategy. In this work we propose two different TDMA schedules for highly reliable sensor data collection with sample rates of 10 Hz and analyze and compare them regarding reliability and energy consumption. A standard coin cell can power a sensor node for more than 2 month with a sensor data loss smaller than 10-3 percent at 3% PER in the radio channel.
for efficient data transmission in WSNs with a broad range of sample- and transmission rates. Our proposal is based on beacon-triggered time-inte"a1s, supports wireless sensor nodes with a wide range of data rates and enables self organized joining and leaving of nodes. We present measurement results of a demonstration system in star topology using evaluation boards of the CC2510, a low-power System-on-Chip solution with integrated micro-controller and RF -transceiver, with two different sleep mode strategies to minimize power consumption. Index Terms-Wireless sensor network (WSN), real-time sys tem, time division multiple access (TDMA), low-power system.
Clock synchronization is an important service in wireless sensor networks (WSN) since it can be used for data integration, for the implementation of TDMA based approaches or for synchronous data acquisition. One main issue in WSN concerns the power consumption since the batteries have a limited capacity an often they cannot be replaced. In this paper we present a dynamic continuous clock synchronization algorithm which provides a virtual common clock among a master and a group of slaves. The main feature of the approach proposed is the ability to allow nodes to sleep for long times and exchange only rare synchronization packets however providing a good quality common clock. The synchronization algorithm has been implemented inside embedded devices as a middleware over the MAC, without any modification to the original communication protocol.
Clock synchronization is an extremely important requirement of wireless sensor networks(WSNs). There are many application scenarios such as weather monitoring and forecasting etc. where external clock synchronization may be required because WSN itself may consists of components which are not connected to each other. A usual approach for external clock synchronization in WSNs is to synchronize the clock of a reference node with an external source such as UTC, and the remaining nodes synchronize with the reference node using an internal clock synchronization protocol. In order to provide highly accurate time, both the offset and the drift rate of each clock with respect to reference node are estimated from time to time, and these are used for getting correct time from local clock reading. A problem with this approach is that it is difficult to estimate the offset of a clock with respect to the reference node when drift rate of clocks varies over a period of time. In this paper, we first propose a novel internal clock synchronization protocol based on weighted averaging technique, which synchronizes all the clocks of a WSN to a reference node periodically. We call this protocol weighted average based internal clock synchronization(WICS) protocol. Based on this protocol, we then propose our weighted average based external clock synchronization(WECS) protocol. We have analyzed the proposed protocols for maximum synchronization error and shown that it is always upper bounded. Extensive simulation studies of the proposed protocols have been carried out using Castalia simulator. Simulation results validate our theoretical claim that the maximum synchronization error is always upper bounded and also show that the proposed protocols perform better in comparison to other protocols in terms of synchronization accuracy. A prototype implementation of the proposed internal clock synchronization protocol using a few TelosB motes also validates our claim.
This paper describes a new consensus-based protocol, referred to as Average TimeSync (ATS), for synchronizing the clocks of a wireless sensor network. This algorithm is based on a cascade of two consensus algorithms, whose main task is to average local information. The proposed algorithm has the advantage of being totally distributed, asynchronous, robust to packet drop and sensor node failure, and it is adaptive to time-varying clock drifts and changes of the communication topology. In particular, a rigorous proof of convergence to global synchronization is provided in the absence of process and measurement noise and of communication delay. Moreover, its effectiveness is shown through a number of experiments performed on a real wireless sensor network.
Dis-tributed synchronization in wireless networks
- O Simeone
- U Spagnolini
- Y Bar-Ness
- S H Strogatz