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.
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TL;DR: In this article , the clock synchronization issues in IoT installations where applications require a unified perception of time are discussed. And a review of time synchronization techniques is provided to aid IoT practitioners in selecting the most relevant aspects for deployment.
Abstract: The Internet of Things (IoT) aims to revolutionize users' daily lives by enabling seamless data exchange among various ubiquitous devices over the Internet. However, achieving this broad objective can pose significant challenges for applications that require synchronized timing, such as ensuring the temporal sequencing of data or synchronized performance of activities. IoT networks typically comprise entities with varying resources, making it challenging to implement existing time synchronization methods like N.T.P. (Network Time Protocol) on resource-constrained devices. On the other hand, solutions that work for constrained systems may not be scalable across diverse IoT deployments. This survey paper delves into the clock synchronization issues in IoT installations where applications require a unified perception of time. The article describes clock synchronization solutions for various models and their associated performance and discusses several research challenges related to clock synchronization. Additionally, a review of time synchronization techniques is provided to aid IoT practitioners in selecting the most relevant aspects for deployment.
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TL;DR: 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.
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 paper, 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.
TL;DR: In this paper , an all-digital frequency-tunable clock source is used as on-chip syntonistor to physically compensate the clock frequency errors of nodes in a network.
Abstract: The emergence of Internet of Everything has made time awareness a feature of high priority. One of the enabling factors is time synchronization. High-quality synchronization demands frequency source of high stability, which is pricey and bulky. When low-end source is used, synchronization quality would be severely impaired. This situation can be improved by the technique of time synchronization. In mainstream approaches, however, the frequency of clock pulse train is not tuned due to the lack of appropriate tool. As requirement for synchronization advances into microsecond or even nanosecond regime, frequency tuning in hardware is expected to be beneficial. Moreover, wireless networks do not have frequency transfer mechanism. This exacerbates the problem of frequency deviation. Those issues make syntonization (synchronization of frequency) become necessary. In this work, an all-digital frequency-tunable clock source is used as on-chip syntonistor to physically compensate the clock frequency errors of nodes in a network. Time discrepancies are hence alleviated. A wireless network has been built to validate this scheme. Time synchronization quality has reached to sub-µs range. The key contribution is the suggestion of adopting on-chip syntonistor for improving the quality of network synchronization. This work serves as a demonstration of this new scheme.
21 Nov 2022
TL;DR: Theorem of Inaccessibility as mentioned in this paper is focused on the domain of time, where its application allows all the uncertainty related to the movement of energy through the space domain, which is associated with electromagnetic waves, to get confined and transparent.
Abstract: Every disregarded aspect of the network operation becomes a real threat for safety-critical environments. As a measure to better assess the dependability and timeliness properties of networking communications, this paper advances the state-of-the-art on dependable real-time networking communications by providing a powerful yet simple formalisation, which supports the characterisation, analysis, and understanding of temporal deviations observed on communication networks, as consequence of network errors. As part of such formalisation, the Theorem of Inaccessibility is focused on the domain of time, where its application allows all the uncertainty related to the movement of energy through the space domain, which is associated with electromagnetic waves, to get confined and transparent. Our approach can help researchers, professionals, and practitioners to recognise the existence of network inaccessibility, in order to understand how it affects wired/wireless networks and critical services that rely on them for exchanging data on a dependable, timely, and therefore predictable way.
TL;DR: The authors proposed dynamically reconfigurable Local time controller for implementation of time synchronization mechanisms for the SpaceFibre network, and evaluated their characteristics.
Abstract: Most onboard embedded systems have real-time requirements. The SpaceFibre standard is developed for onboard local networks. However, the current version of the SpaceFibre standard does not specify any time synchronization mechanisms. The authors consider the mechanisms of time synchronization that are used in the data transmission standards, which are currently used for networks with real-time requirements. In the paper, the authors proposed possible time synchronization mechanisms for the SpaceFibre network, evaluate their characteristics. The authors proposed dynamically reconfigurable Local time controller for implementation of these mechanisms with ASIC.
References
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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
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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1,368 citations
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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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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