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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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Journal ArticleDOI
TL;DR: In this paper, an Ethernet-based motor drive that enables delay analysis for synchronized motor actuation and sensing is proposed to build a scalable and precise industrial transport system. But the authors focus on the delay analysis of the system.
Abstract: Ethernet-based motor drives are hard real-time control systems used to operate servomotors through the industrial Ethernet. Recently, Ethernet-based drives have drawn attention as a solution for industrial transport systems where numerous linear motor drives move magnetic shuttles individually or collectively to accurately position production parts. This paper presents the design and implementation of an Ethernet-based motor drive that enables delay analysis for synchronized motor actuation and sensing to build a scalable and precise industrial transport system. Our software design constructs the drive function using periodic tasks run by a rate-monotonic real-time scheduler and performs worst-case response analysis to determine the end-to-end delay required for the control host to actuate or sense the motor in the drive. Based on the calculated drive delays, clock-based I/O using Ethernet-provided global time realizes synchronized motor operation across multiple drives. In the Ethernet-networked control system, different phases of the host cycle with respect to the drive cycle can result in different actuation and sensing delays. To reduce the delay, we propose a phase-shifted loop method and present a heuristic to find the best phase that minimizes the normalized drive delays. Experimental results obtained using a prototype EtherCAT drive show that the phase-shifted loop significantly reduces the difference between the commanded and feedback currents while properly managing tracking errors. Performance evaluations are performed to investigate the impact of different Ethernet technologies on delays. Elaborated delay models are developed for EtherCAT and Ethernet Powerlink, and a comparative study of delay performance is conducted for various parameters, such as the number of drives, the message size, the network topology, and the bandwidth.

1 citations

Patent
15 Aug 2017
TL;DR: In this paper, a processor in an electronic device maintains coordination between a system clock provided by a clock circuit and an interface clock providing by an external clock circuit by calculating relative drift as a function of time between the interface clock and a second interface clock in the second electronic device.
Abstract: A processor in an electronic device maintains coordination between a system clock provided by a system clock circuit and an interface clock provided by a clock circuit. Then, the processor may calculate, based on differences between transmit times when packets were transmitted by a second electronic device and receive times of the packets, relative drift as a function of time between the interface clock and a second interface clock in the second electronic device. Moreover, the processor adjusts, based on the relative drift, the system clock circuit to eliminate the relative drift. Next, the processor may determine a remaining time offset between the interface clock and the second interface clock. Furthermore, the processor modifies a future time when the electronic device is to perform the playback operation based on the remaining time offset to determine a corrected future time, and may perform the playback operation at the corrected future time.

1 citations

Book ChapterDOI
01 Jan 2021
TL;DR: In this paper, a MATLAB program was written to examine and investigate the communication systems of the OFDM by comparing OFDM and quadratic amplitude modulation (QAM) single carrier.
Abstract: The orthogonal frequency-division multiplexing (OFDM) is a modulation scheme which is spectrally efficient and which decomposes one carrier into several subcarriers. Each subcarrier is orthogonal, allowing the transfer of data on each subcarrier simultaneously. This work performs the strengths of OFDM and quadratic amplitude modulation (QAM) in parallel on a communication channel. A MATLAB program was written to examine and investigate the communication systems of the OFDM by comparing OFDM and QAM single carrier. This technique is significant for the complex simulates systems. The demonstration of two graphical user interfaces (GUI) incorporates the simulation concepts of OFDM.

1 citations

01 Jan 2019
TL;DR: In this article, the authors propose a novel approach to solve the problem of homonymity in homonymization, i.e., homonymonymity-of-homonymity.
Abstract: vii CHAPTER

1 citations


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

  • ...In practice, multiple schemes can be applied to satisfy such clock synchronization requirement to detect the attack [57]....

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Patent
15 Aug 2017
TL;DR: In this paper, an electronic device that reduces relative drift is described, in which an interface circuit in the electronic device may calculate, based on differences between transmit times when packets were transmitted by a second electronic device and receive times of the packets, relative drift as a function of time between a clock in the interface circuit and a second clock in another electronic device.
Abstract: An electronic device that reduces relative drift is described. In particular, an interface circuit in the electronic device may calculate, based on differences between transmit times when packets were transmitted by a second electronic device and receive times of the packets, relative drift as a function of time between a clock in the interface circuit and a second clock in the second electronic device. Then, the interface circuit may adjust, based on the relative drift, a clock circuit that provides the clock to eliminate the relative drift, and may store the adjustments to the clock circuit. Furthermore, when a wireless reset occurs, the interface circuit may adapt the clock circuit based on the stored adjustments to reduce the relative drift while the interface circuit restores frequency lock with the second clock based on additional packets with additional transmit times that are received from the second electronic device.

1 citations

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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