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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TL;DR: A secure far proximity identification approach that determines whether or not a remote device is far away from the unforgeable "fingerprint" of the proximity is proposed.
Abstract: As wireless mobile devices are more and more pervasive and adopted in critical applications, it is becoming increasingly important to measure the physical proximity of these devices in a secure way. Although various techniques have been developed to identify whether a device is close, the problem of identifying the far proximity (i.e., a target is at least a certain distance away) has been neglected by the research community. Meanwhile, verifying the far proximity is desirable and critical to enhance the security of emerging wireless applications. In this article, we propose a secure far proximity identification approach that determines whether or not a remote device is far away. The key idea of the proposed approach is to estimate the far proximity from the unforgeable “fingerprint” of the proximity. We have validated and evaluated the effectiveness of the proposed far proximity identification method through experiments on real measured channel data. The experiment results show that the proposed approach can detect the far proximity with a successful rate of 0.85 for the non-Line-of-sight (NLoS) scenario, and the successful rate can be further increased to 0.99 for the Line-of-sight (LoS) scenario.
2 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 [21]....
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01 May 2019
TL;DR: Studies of performance metrics, such as rendezvous-success (RS) rate, show that the new CH sequences provide better balance and flexible choices between cardinality, DoR, MTTR, and RS rate than previous works.
Abstract: A family of synchronous-symmetric channel-hopping (CH) sequences for cognitive radio networks is constructed. The CH sequences satisfies the design criteria of channel overlap, even channel use, and full channel utilization. They also carry desirable properties of enlarged cardinality, optimal maximum-time-to-rendezvous (MTTR), full degree-of-rendezvous (DoR), and minimum channel competition. They achieve the unique full-channel-utilization-per-time-slot property and can support the largest number of users without creating channel collisions. Studies of performance metrics, such as rendezvous-success (RS) rate, show that the new CH sequences provide better balance and flexible choices between cardinality, DoR, MTTR, and RS rate than previous works.
2 citations
Cites background from "Clock Synchronization Over IEEE 802..."
...In some applications where synchronization can be achieved by, for example, global positioning system or clock synchronization algorithms [4], [14], [15], synchronous CH sequences can support better parameters, properties, and performance metrics than asynchronous ones....
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Patent•
15 Aug 2017
TL;DR: In this paper, an audio/video (A/V) hub that coordinates playback of audio content is described, in which the A/V hub may calculate current time offsets between clocks in electronic devices and a clock in the hub based on differences between transmit times of frames from the electronic devices, and receive times when the frames were received.
Abstract: An audio/video (A/V) hub that coordinates playback of audio content is described. In particular, the A/V hub may calculate current time offsets between clocks in electronic devices and a clock in the A/V hub based on differences between transmit times of frames from the electronic devices and receive times when the frames were received. For example, the current time offsets may be calculated using wireless ranging by ignoring distances between the A/V hub and the electronic devices. Then, the A/V hub may transmit, to the electronic devices, one or more frames that include audio content and playback timing information, which may specify playback times when the electronic devices are to playback the audio content based on the current time offsets. Furthermore, the playback times of the electronic devices may have a temporal relationship so that the playback of the audio content by the electronic devices is coordinated.
2 citations
TL;DR: In this paper, the authors consider the problem of joint position and clock tracking of a mobile wireless node by a set of reference nodes, and apply extended and unscented Kalman filters to estimate the position and the velocity of the mobile, and the skew and offset of its clock.
2 citations
01 Jun 2021
TL;DR: In this article, a multiple cluster-based transmission diversity scheme is proposed for asynchronous joint transmissions in private networks, in which the use of multiple clusters or small cells is preferable to increase transmission speeds, reduce latency, and bring transmissions closer to the users.
Abstract: In this paper, a multiple cluster-based transmission diversity scheme is proposed for asynchronous joint transmissions (JT) in private networks, in which the use of multiple clusters or small cells is preferable to increase transmission speeds, reduce latency, and bring transmissions closer to the users. To increase the spectral efficiency and coverage, and to achieve flexible spatial degrees of freedom, a distributed remote radio unit system (dRRUS) is installed in each of the clusters. When the dRRUS is disposed in the private environments, it will be associated with multipath-rich and asynchronous delay propagation. Taking into account of this unique environment of private networks, asynchronous multiple signal reception is considered in the development of operation at the remote radio units to make an intersymbol interference free distributed cyclic delay diversity (dCDD) scheme for JT to achieve a full transmit diversity gain without full channel state information. A spectral efficiency of the proposed dCDD-based JT is analyzed by deriving the closed- form expression, and then compared with link-level simulations for non-identically distributed frequency selective fading over the entire private network.
2 citations
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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