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Journal ArticleDOI

Clock Synchronization Over IEEE 802.11—A Survey of Methodologies and Protocols

TL;DR: This survey looks into the details of synchronization over IEEE 802.11 with a particular focus on the infrastructure mode which is most relevant for industrial use cases and highlights the different parameters which affect the performance of clock synchronization over WLAN and compares the performances of existing synchronization methods to analyze their shortcomings.
Abstract: Just like Ethernet before, IEEE 802.11 is now transcending the borders of its usage from the office environment toward real-time communication on the factory floor. However, similar to Ethernet, the availability of synchronized clocks to coordinate and control communication and distributed real-time services is not a built-in feature in WLAN. Over the years, this has led to the design and use of a wide variety of customized protocols with varying complexity and precision, both for wired and wireless networks, in accordance with the increasingly demanding requirements from real-time applications. This survey looks into the details of synchronization over IEEE 802.11 with a particular focus on the infrastructure mode which is most relevant for industrial use cases. It highlights the different parameters which affect the performance of clock synchronization over WLAN and compares the performance of existing synchronization methods to analyze their shortcomings. Finally, it identifies new trends and directions for future research as well as features for wireless clock synchronization which will be required by the applications in the near future.
Citations
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Proceedings ArticleDOI
23 Apr 2020
TL;DR: This work considers off-the-shelf platforms and how to detect if the GNSS receiver is attacked or not, by cross-checking the GN SS time and time from other available sources, and proposes a validation approach for absolute and relative time.
Abstract: Global navigation satellite systems (GNSS) provide pervasive accurate positioning and timing services for a large gamut of applications, from Time based One-Time Passwords (TOPT), to power grid and cellular systems. However, there can be security concerns for the applications due to the vulnerability of GNSS. It is important to observe that GNSS receivers are components of platforms, in principle having rich connectivity to different network infrastructures. Of particular interest is the access to a variety of timing sources, as those can be used to validate GNSS-provided location and time. Therefore, we consider off-the-shelf platforms and how to detect if the GNSS receiver is attacked or not, by cross-checking the GNSS time and time from other available sources. First, we survey different technologies to analyze their availability, accuracy and trustworthiness for time synchronization. Then, we propose a validation approach for absolute and relative time. Moreover, we design a framework and experimental setup for the evaluation of the results. Attacks can be detected based on WiFi supplied time when the adversary shifts the GNSS provided time, more than 23.942 μs; with Network Time Protocol (NTP) supplied time when the adversary-induced shift is more than 2.046 ms. Consequently, the proposal significantly limits the capability of an adversary to manipulate the victim GNSS receiver.

9 citations


Additional excerpts

  • ...Protocols such as Reference Broadcast Time Synchronization (RBS), perform well in distributed scenarios [56]....

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Journal ArticleDOI
TL;DR: Preliminary SDMAC implementation on a real testbed and its experimental evaluation showed that integrating this paradigm in the existing protocol stacks constitutes a viable option, whose performance suits a wide range of applications characterized by soft real-time requirements.
Abstract: In distributed control systems where devices are connected through Wi-Fi, direct access to low-level medium access control (MAC) operations may help applications to meet their timing constraints. In particular, the ability to timely control single transmission attempts on air, by means of software programs running at the user space level, eases the implementation of mechanisms aimed at improving communication timeliness and reliability. Relevant examples are deterministic traffic scheduling, seamless channel redundancy, rate adaptation algorithms, and so on. In this paper, a novel architecture is defined, we call software-defined MAC (SDMAC), which in its current embodiment relies on conventional Linux PCs equipped with commercial Wi-Fi adapters. Preliminary SDMAC implementation on a real testbed and its experimental evaluation showed that integrating this paradigm in the existing protocol stacks constitutes a viable option, whose performance suits a wide range of applications characterized by soft real-time requirements.

9 citations


Cites background from "Clock Synchronization Over IEEE 802..."

  • ...If nodes are synchronized, so that they share the same time base [13], [14], it is possible to coordinate their access to the...

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Journal ArticleDOI
TL;DR: The simulation results demonstrate the impact of the PTP-aided URLLC in maintaining network reliability, latency, and jitter in full coordination with the TTE-network.
Abstract: With the rapid growth of time-critical applications in smart grid, robotics, autonomous vehicles, and industrial automation, demand for high reliability, low latency and strictly bounded jitter is sharply increasing. High-precision time synchronization communications, such as Time Triggered Ethernet (TTE), have been successfully developed for wired networks. However, the high cost of deploying additional equipment and extra wiring limits the scalability of these networks. Therefore, in this article, a hybrid wired/wireless high-precision time synchronization network based on a combination of high-speed TTE and 5G Ultra-Reliable and Low-Latency Communications (URLLC) is proposed. The main motivation is to comply with the low latency, low jitter, and high reliability requirements of time critical applications, such as smart grid synchrophasor communications. Therefore, in the proposed hybrid network architecture, a high-speed TTE is considered as the main bus (i.e., backbone network), whereas a Precision Time Protocol (PTP) aided 5G-URLLC-based wireless access is used as a sub-network. The main challenge is to achieve interoperability between the PTP aided URLLC and the TTE, while ensuring high precision timing and synchronization. The simulation results demonstrate the impact of the PTP-aided URLLC in maintaining network reliability, latency, and jitter in full coordination with the TTE-network.

8 citations


Cites methods from "Clock Synchronization Over IEEE 802..."

  • ...PTP, which was originally proposed for wired networks, has been successfully used for time critical wireless applications, such as URLLC [11, 12]....

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Journal ArticleDOI
TL;DR: A synchronization scheme based on periodic extinctions of the GPS receiver, which demonstrates that GPS power switching is an efficient solution to reduce energy costs while maintaining a high synchronization accuracy.
Abstract: Synchronization is a challenging problem for wireless nodes, especially for applications demanding good synchronization accuracy over wide areas. In that case, the GPS is a valuable solution as the nodes can independently synchronize to UTC. However, the energy consumption of a GPS receiver (over 100 mW when switched on) is not sustainable on a wireless node. Therefore, in this work, we developed a synchronization scheme based on periodic extinctions of the GPS receiver. The goal is to study the GPS power switching effect on the synchronization accuracy. To do so, a node with dedicated timestamping hardware was designed. Two clock models were compared to predict the node time when the GPS is off and the impact of a Kalman filter, to remove the GPS noise, was evaluated. From experimental data, we show that the choice of the clock model depends on the accuracy needed and that the Kalman filter improves the estimation of the clock frequency for both models. In our design, the GPS can be off from 60% up to 95% of the time for mean synchronization errors of 20 ns to 420 ns, respectively. This work demonstrates that GPS power switching is an efficient solution to reduce energy costs while maintaining a high synchronization accuracy.

8 citations


Cites background from "Clock Synchronization Over IEEE 802..."

  • ...Typically, the frequency stability of these oscillators is in the ppb range for sampling intervals of one second [18]....

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Journal ArticleDOI
TL;DR: A novel “criss-cross-pattern” method of constructing synchronous-symmetric channel-hopping sequences is introduced, which carries the best balance of desirable properties and performance metrics than previous works.
Abstract: In this letter, a novel “criss-cross-pattern” method of constructing synchronous-symmetric channel-hopping sequences is introduced. This simplified and improved construction works with any number of licensed channels and, at the same time, carries desirable properties including enlarged cardinality, full degree-of-rendezvous, optimal maximum-time-to-rendezvous, and even channel use. It also achieves full channel utilization per time slot so that multiple pairs of users can rendezvous using all licensed channels in all time slots without channel collisions. The studies of computational complexity and performance metrics, system throughput, and rendezvous-success rate show that the criss-cross construction carries the best balance of desirable properties and performance metrics than previous works.

8 citations


Cites background from "Clock Synchronization Over IEEE 802..."

  • ...When synchronization is achievable, say through a GPS clock or timing signals from nearby users [9], [10], synchronous CH sequences [1]–[5] can achieve better properties [e....

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References
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Journal ArticleDOI
09 Dec 2002
TL;DR: Reference Broadcast Synchronization (RBS) as discussed by the authors is a scheme in which nodes send reference beacons to their neighbors using physical-layer broadcasts, and receivers use their arrival time as a point of reference for comparing their clocks.
Abstract: Recent advances in miniaturization and low-cost, low-power design have led to active research in large-scale networks of small, wireless, low-power sensors and actuators. Time synchronization is critical in sensor networks for diverse purposes including sensor data fusion, coordinated actuation, and power-efficient duty cycling. Though the clock accuracy and precision requirements are often stricter than in traditional distributed systems, strict energy constraints limit the resources available to meet these goals.We present Reference-Broadcast Synchronization, a scheme in which nodes send reference beacons to their neighbors using physical-layer broadcasts. A reference broadcast does not contain an explicit timestamp; instead, receivers use its arrival time as a point of reference for comparing their clocks. In this paper, we use measurements from two wireless implementations to show that removing the sender's nondeterminism from the critical path in this way produces high-precision clock agreement (1.85 ± 1.28μsec, using off-the-shelf 802.11 wireless Ethernet), while using minimal energy. We also describe a novel algorithm that uses this same broadcast property to federate clocks across broadcast domains with a slow decay in precision (3.68 ± 2.57μsec after 4 hops). RBS can be used without external references, forming a precise relative timescale, or can maintain microsecond-level synchronization to an external timescale such as UTC. We show a significant improvement over the Network Time Protocol (NTP) under similar conditions.

2,537 citations

Proceedings ArticleDOI
03 Nov 2004
TL;DR: The FTSP achieves its robustness by utilizing periodic flooding of synchronization messages, and implicit dynamic topology update and comprehensive error compensation including clock skew estimation, which is markedly better than that of the existing RBS and TPSN algorithms.
Abstract: Wireless sensor network applications, similarly to other distributed systems, often require a scalable time synchronization service enabling data consistency and coordination. This paper describes the Flooding Time Synchronization Protocol (FTSP), especially tailored for applications requiring stringent precision on resource limited wireless platforms. The proposed time synchronization protocol uses low communication bandwidth and it is robust against node and link failures. The FTSP achieves its robustness by utilizing periodic flooding of synchronization messages, and implicit dynamic topology update. The unique high precision performance is reached by utilizing MAC-layer time-stamping and comprehensive error compensation including clock skew estimation. The sources of delays and uncertainties in message transmission are analyzed in detail and techniques are presented to mitigate their effects. The FTSP was implemented on the Berkeley Mica2 platform and evaluated in a 60-node, multi-hop setup. The average per-hop synchronization error was in the one microsecond range, which is markedly better than that of the existing RBS and TPSN algorithms.

2,267 citations


"Clock Synchronization Over IEEE 802..." refers background in this paper

  • ...For P2P synchronization, all clients can communicate directly with each other and there is no device acting as the reference....

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Journal ArticleDOI
TL;DR: A tutorial review of some time-domain methods of characterizing the performance of precision clocks and oscillators is presented, and both the systematic and random deviations are considered.
Abstract: A tutorial review of some time-domain methods of characterizing the performance of precision clocks and oscillators is presented. Characterizing both the systematic and random deviations is considered. The Allan variance and the modified Allan variance are defined, and methods of utilizing them are presented along with ranges and areas of applicability. The standa,rd deviation is contrasted and shoun not to be. in general. a good measure for precision clocks and oscillators. Once a proper characterization model has been developed, then optimum estimation and prediction techniques can be employed. Some important cases are illustrated. As precision clocks and oscillators become increasingly important in society. communication of their characteristics and specifications among the vendors, manufacturers. design engineers. managers, and metrologists of this equipment becomes increasingI> important.

784 citations


"Clock Synchronization Over IEEE 802..." refers background in this paper

  • ...In contrast, generating timestamps by software means creates indeterministic delays due to scheduling, caches, concurrency....

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Book
28 Oct 2017
TL;DR: In this article, the spectral density S y (f) of the function y(t) where the spectrum is considered to be one-sided on a per hertz basis is defined.
Abstract: Consider a signal generator whose instantaneous output voltage V(t) may be written as V(t) = [V 0 + ??(t)] sin [2??v 0 t + s(t)] where V 0 and v 0 are the nominal amplitude and frequency, respectively, of the output. Provided that ??(t) and ??(t) = (d??/(dt) are sufficiently small for all time t, one may define the fractional instantaneous frequency deviation from nominal by the relation y(t) - ??(t)/2??v o A proposed definition for the measure of frequency stability is the spectral density S y (f) of the function y(t) where the spectrum is considered to be one sided on a per hertz basis. An alternative definition for the measure of stability is the infinite time average of the sample variance of two adjacent averages of y(t); that is, if y k = 1/t ??? tk+r = y(t k ) y(t) dt where ?? is the averaging period, t k+1 = t k + T, k = 0, 1, 2 ..., t 0 is arbitrary, and T is the time interval between the beginnings of two successive measurements of average frequency; then the second measure of stability is ?? y 2(??) ??? (y k+1 - y k )2/2 where denotes infinite time average and where T = ??. In practice, data records are of finite length and the infinite time averages implied in the definitions are normally not available; thus estimates for the two measures must be used. Estimates of S y (f) would be obtained from suitable averages either in the time domain or the frequency domain.

725 citations


"Clock Synchronization Over IEEE 802..." refers background or methods in this paper

  • ...According to [21], different types of industrial applications must be supported by industrial communication networks, such as control, or monitoring and diagnostics....

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  • ...Based on this taxonomy, different methods to synchronize clock in IEEE 802.11 for the infrastructure mode are presented in this section....

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