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: R-Sync is presented, a robust time synchronization scheme for IIoT that makes all the nodes get synchronized and gets the better performance in terms of accuracy and energy consumption, compared with three existing time synchronization algorithms TPSN, GPA, STETS.
Abstract: Energy-efficient and robust-time synchronization is crucial for industrial Internet of things (IIoT). Some energy-efficient time synchronization schemes that achieve high accuracy have been proposed recently. However, some unsynchronized nodes namely isolated nodes exist in the schemes. To deal with the problem, this paper presents R-Sync, a robust time synchronization scheme for IIoT. We use a pulling timer to pull isolated nodes into synchronized networks whose initial value is set according to level of spanning tree. Then, another timer is set up to select backbone node and its initial value is related to the distance to parent node. Moreover, we do experiments based on simulation tool NS-2 and testbed based on wireless hardware nodes. The experimental results show that our approach makes all the nodes get synchronized and gets the better performance in terms of accuracy and energy consumption, compared with three existing time synchronization algorithms TPSN, GPA, STETS.
124 citations
Cites background from "Clock Synchronization Over IEEE 802..."
...Therefore, many existing time synchronization protocols have been proposed for sensor networks [13]....
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05 Mar 2019
TL;DR: An innovative approach for high-performance industrial wireless networks [wireless high performance (WirelessHP)] is presented, based on a substantial redesign of the lower layers of the industrial wireless protocol stack, with the aim of supporting the requirements of critical industrial control applications.
Abstract: Wireless networks are ever more deployed in the industrial control scenario, thanks to the numerous benefits they can bring, especially in terms of costs and flexibility. However, some critical fields of application, such as motion control, power systems automation, or power electronics control, to mention some, have extremely tight requirements in terms of timeliness, reliability, and determinism, which nowadays can only be satisfied by wired communication networks. Indeed, the available industrial wireless solutions are far from offering adequate performance levels, especially in the timing budget, due to the native limitations of their physical (PHY) layers. In this paper, an innovative approach for high-performance industrial wireless networks [wireless high performance (WirelessHP)] is presented, based on a substantial redesign of the lower layers of the industrial wireless protocol stack, with the aim of supporting the requirements of critical industrial control applications. The required levels of timeliness, reliability, and determinism are first derived through a comprehensive survey that looks at real-world application scenarios as well as at the performance of wired networks for industrial control, such as real-time Ethernet networks. The design of a new solution, which is able to satisfy these targets, is then discussed in detail, introducing a low-latency PHY layer that aims at reducing the transmission time of short packets to $1~\mu \text{s}$ , or even less. The feasibility of the proposed solution is presented through an experimental demonstrator based on software-defined radios, while its performance bounds are computed through theoretical analyses. Finally, future activities in the context of WirelessHP are widely discussed, providing an overview of the directions that will have to be addressed, particularly in the design of the upper layers.
87 citations
Cites background from "Clock Synchronization Over IEEE 802..."
...This is a reasonable value, since the TDMA MAC should be able to provide submicrosecond synchronization accuracy to support ultracritical industrial applications and this looks feasible to some extent [63]....
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...The problem of maintaining synchronization in wireless networks has been tackled by many researchers [27], [63], [64], and achieving microsecondlevel synchronization accuracy in TDMA networks looks possible....
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...4, and it has been studied widely in the scientific literature [27], [63], [64]....
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21 Mar 2019
TL;DR: An overview of the potential applications, requirements, and unique research challenges to extend time-synchronized networking capabilities over wireless and a classification of wireless applications and a reference architecture for enabling the integration of wired and wireless TSN capabilities in future industrial automation systems are provided.
Abstract: Many industrial automation systems rely on time-synchronized (and timely) communication among sensing, computing, and actuating devices. Advances in Ethernet enabled by time-sensitive networking (TSN) standards, being developed by the IEEE 802.1 TSN Task Group, are significantly improving time synchronization as well as worst case latencies. Next-generation industrial systems are expected to leverage advances in distributed time coordinated computing and wireless communications to enable greater levels of automation, efficiency, and flexibility. Significant progress has been made in extending accurate time synchronization over the air (e.g., 802.1AS profile for IEEE 802.11/Wi-Fi). Given the inherently unreliable, varying capacity and latency prone characteristics associated with wireless communications, proving the feasibility of worst case latency performance over the wireless medium is a major research challenge. More specifically, understanding what levels of capacity, reliability, and latency could be guaranteed over wireless links with high reliability are important research questions to guide the development of new radios, protocols, and time coordinated applications. This paper provides an overview of the potential applications, requirements, and unique research challenges to extend TSN capabilities over wireless. The paper also describes advances in wireless technologies (e.g., next-generation 802.11 and 5G standards) toward achieving reliable and accurate time distribution and timeliness capabilities. It also provides a classification of wireless applications and a reference architecture for enabling the integration of wired and wireless TSN capabilities in future industrial automation systems.
79 citations
TL;DR: The purpose of this research is to show a systematic review of the most recent studies about the architecture, security, latency, and energy consumption that FC presents at industrial level and thus provide an overview of the current characteristics and challenges of this new technology.
71 citations
Posted Content•
TL;DR: The key features of IEEE 802.11be are introduced and the benefits and requirements of the most representative Internet of Things low-latency use cases for WiFi 7 are reviewed: multimedia, healthcare, industrial, and transport.
Abstract: Short time after the official launch of WiFi 6, IEEE 802.11 working groups are already designing its successor in the wireless local area network (WLAN) ecosystem: WiFi 7. With the IEEE 802.11be amendment as one of its main constituent parts, future WiFi 7 aims to include time-sensitive networking (TSN) capabilities to support low latency and ultra reliability in license-exempt spectrum bands. This article first introduces the key features of IEEE 802.11be, which are then used as the basis to discuss how TSN functionalities could be implemented in WiFi 7. Finally, benefits and requirements of the most representative low-latency use cases for WiFi 7 are reviewed.
49 citations
Cites background from "Clock Synchronization Over IEEE 802..."
...10 ns) and some other minor changes that do not hamper backwards compatibility [38]....
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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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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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