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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01 Apr 2018
TL;DR: A family of synchronous-asymmetric channel-hopping sequences for cognitive radio (CR) networks is constructed and their maximum-time-to-rendezvous (MTTR) lower bound is derived and they have the unique property of enlarging cardinality without scarifying the MTTR.
Abstract: In this paper, a family of synchronous-asymmetric channel-hopping (CH) sequences for cognitive radio (CR) networks is constructed and their maximum-time-to-rendezvous (MTTR) lower bound is derived. The new sequences achieve full degree-of-rendezvous and optimal MTTR. They also have the unique property of enlarging cardinality without scarifying the MTTR. Their rendezvous-success (RS) rate and MTTR are compared with prior works. This family provides a flexible choice of CH sequences that have adjustable cardinality and RS rate for the CR networks with variable QoS.
5 citations
Cites background from "Clock Synchronization Over IEEE 802..."
...(Synchronous CH sequences can be designed with low and even zero channel-collision probability and thus are good for baseline studies in the CR networks and useful in wireless networks requiring clock synchronization [13]....
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TL;DR: A novel framework, data repairing of time synchronization problems (DR-TSP), which can detect the time-unsynchronized ERI readers and correct timestamp-deviated ERI data, and can significantly improve data quality.
Abstract: Timestamps are often problematic in Internet-of-Things (IoT) systems due to time synchronization problems of distributed radio-frequency identification (RFID) readers or sensors. This issue may seriously affect the data quality in some fields, such as transportation. A typical IoT application in transportation is electronic registration identification of the motor vehicle (ERI), an emerging traffic data acquisition technology based on RFID. ERI data play a vital role in intelligent transportation. However, the data quality is often affected seriously by the inaccurate timestamps, which arise from the time-unsynchronized distributed ERI readers. To solve this issue, we propose a novel framework, data repairing of time synchronization problems (DR-TSP), which can detect the time-unsynchronized ERI readers and correct timestamp-deviated ERI data. Precisely, DR-TSP consists of three components. Problem reader discovery component employs a statistics-based method to detect the time-unsynchronized ERI readers and discovers the clock leaps of the problematic ERI reader through a smoothing-based method. Travel-time estimation component constructs a spatial correlative travel-time estimation model based on the neural network to infer timestamp deviation. The influence of clock deviation is considered in the model training. Data correction component utilizes the above results to correct the timestamp-deviated data. Experiments over large-scale ERI data collected from a big China city, Chongqing, show that our method can significantly improve data quality.
5 citations
TL;DR: An asymptotic performance analysis verifies the maximum achievable diversity of the dCDD even in the overpopulated case within the considered channel environment, and link-level simulations are conducted to verify themaximum achievable diversity gain.
Abstract: This paper investigates the diversity gain of a distributed cyclic delay diversity (dCDD) scheme for cyclic-prefixed single carrier systems in non-identical fading channels. Non-identical small-scale fading is assumed in the environment, in which non-identical line-of-sight and non-line-of-sight fading coexist. A condition for dCDD resulting in intersymbol interference free reception at the receiver, is extended to this new channel environment. For an overpopulated system setup, a generalized performance analysis, is not available from existing works, is conducted after developing closed-form expressions for the distribution of the signal-to-noise ratio (SNR) realized at the receiver. Since the order statistics are involved in the statistical properties of the SNR, the corresponding spacing statistics are utilized to derive feasible closed-form expressions. The finalized closed-form expressions are shown to provide very reliable outage probability and spectral efficiency of dCDD for underpopulated and overpopulated systems. An asymptotic performance analysis verifies the maximum achievable diversity of the dCDD even in the overpopulated case within the considered channel environment. Link-level simulations are conducted and these verify the maximum achievable diversity gain.
5 citations
Cites methods from "Clock Synchronization Over IEEE 802..."
...11-based precision time protocol [40], [41], the RRUs...
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TL;DR: In this paper , the authors review the existing wireless clock-synchronization approaches and their attainable performances, and discuss their feasibility to enable wireless Time-Sensitive Networking (TSN).
Abstract: Industrial control systems in the era of Industry 4.0 present significant challenges from a communication perspective, that is, low latency, ultrahigh reliability, and accurate synchronization. Time-sensitive networking (TSN) has emerged as the main solver of these challenges. As a research trend, TSN and wireless technologies are expected to converge in the wireless TSN paradigm. This convergence starts with the adoption of accurate clock synchronization over wireless systems. In this article, we review the existing wireless clock-synchronization approaches and their attainable performances, and we discuss their feasibility to enable wireless TSN. We conclude that the existing clock-synchronization techniques are enough to enable wireless TSN, although significant implementation efforts are required to incorporate accurate clock synchronization over wireless systems.
5 citations
20 Jul 2020
TL;DR: This work compares the synchronization achieved between RPi using the commonly available clock sources, synchronization procedures and software, and concludes that Chrony achieves the best result, and GPS is not always the best solution.
Abstract: Real-time experimentation in multiple areas relies on time synchronization across devices. Protocols are used to sync time between nodes, but the accuracy of synchronization achieved by those different protocols is sometimes overlooked and not taken into account. This is a major issue, especially in less reliable hardware commonly used in research like the Raspberry Pi (RPi).We compare the synchronization achieved between RPi using the commonly available clock sources (Global Positioning Systems (GPSs) and time-servers), synchronization procedures and software. Across all sources the software Chrony achieves the best result, and GPS is not always the best solution.
4 citations
Cites background or result from "Clock Synchronization Over IEEE 802..."
...Later, in [15], the work is extended to compare with more techniques, non-802....
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...Some studied solutions in [14], [15] are not easy applicable in real-world scenarios, due the lack of easy to find Wi-Fi interfaces that support timestamping at hardware level or in software at driver level....
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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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