Search

The inter-satellite link (ISL) has been received increasing attention, as it is a potential way to achieve autonomous clock synchronization for envisioned space-based satellite networks with minimal ground segment capability. Existing satellite clock synchronization solutions either rely on prior information or modelling the relative motion by an approximated polynomial. In this paper, we propose a deep learning approach based on long short-term memory (LSTM) to decouple the clock parameters from pseudo-range measurements. The process of clock parameter estimation solely relies on the observed pseudo-range measurements, and the prior information of position and velocity are not required and the nonlinear relative motion process is modelled by training on historical data. The simulation results show that the proposed method outperforms the benchmark solutions in terms of accuracy. [ABSTRACT FROM AUTHOR]

Clock synchronization is one of the most fundamental and crucial network communication strategies. With the expansion of the Industrial Internet in numerous industrial applications, a new requirement for the precision, security, complexity, and other features of the clock synchronization mechanism has emerged in various industrial situations. This paper presents a study of standardized clock synchronization protocols and techniques for various types of networks, and a discussion of how these protocols and techniques might be classified. Following that is a description of how certain clock synchronization protocols and technologies, such as PROFINET, Time-Sensitive Networking (TSN), and other well-known industrial networking protocols, can be applied in a number of industrial situations. This study also investigates the possible future development of clock synchronization techniques and technologies. [ABSTRACT FROM AUTHOR]

Clock synchronization provides fundamental timing reference for various services in the Internet of Things. Unlike centralized synchronization algorithm, which relies heavily on the synchronization routing and brings about error accumulation consequently, the distributed synchronization algorithm achieves clock synchronization through time message exchange and calculation among adjacent nodes, hence the error accumulation can be greatly eliminated. However, the convergence speed of existing distributed synchronization algorithms is generally slow, and network topology information is seldom utilized to improve the convergence speed, which may cause additional energy consumption, especially in resource-constrained scenarios. To improve the convergence speed of distributed clock synchronization, a consensus tracking-based clock synchronization (CTCS) algorithm is proposed in this paper. By analyzing the synchronization process in the state space framework, the convergence acceleration term is designed to optimize the eigenvalue distribution of synchronization error matrix, hence the convergence speed can be greatly improved. To be suitable for clock synchronization in the dynamic network, a reference clock input term is designed in the clock update formula, which eliminates the oscillation of synchronized clocks triggered by synchronization of newly joined nodes. To prove the convergence of CTCS, an in-depth analysis of the algorithm is conducted. Both simulation and experimental results validate the effectiveness of CTCS in comparison with other solutions. [ABSTRACT FROM AUTHOR]

The Mössbauer rotor effect recently gained a renewed interest due to the discovery and explanation of an additional effect of clock synchronization which has been missed for about 50 years, i.e. starting from a famous book of Pauli, till some recent experimental analyses. The theoretical explanation of such an additional effect is due to some recent papers in both the general relativistic and the special relativistic frameworks. In the first case (general relativistic framework) the key point of the approach is the Einstein’s equivalence principle (EEP), which, in the words of the same Einstein, enables “the point of view to interpret the rotating system K’ as at rest, and the centrifugal field as a gravitational field”. In this paper, we analyse both the history of the Mössbauer rotor effect and its interpretation from the point of view of Einstein’s general theory of relativity (GTR) by adding some new insight. In particular, it will be shown that, if on one hand the “traditional” effect of redshift has a strong analogy with the gravitational redshift, on the other hand the additional effect of clock synchronization has an intriguing analogy with the cosmological redshift. Finally, we show that a recent claim in the literature that the second effect of clock synchronization does not exist is not correct. [ABSTRACT FROM AUTHOR]

This paper studies the secure and accurate clock synchronization problem for sensor networks with time-varying delays and malicious attacks. A novel clock synchronization scheme based on the attack detection mechanism, attack compensation, and maximum consensus protocol is proposed. The proposed scheme starts with the detection of the malicious attacks and the clock data under attacks is eliminated. On the basis, software clock parameters are updated so that all the nodes in the network can have the same software skew and offset, so the clock synchronization can be achieved. Furthermore, it is theoretically proved that the proposed scheme can achieve the attack detection correctly, and further can guarantee a secure and accurate clock synchronization. In addition, extensive simulations are also conducted to validate the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

With the rapid development, Wireless Sensor Networks (WSNs) technology has been widely applied in many fields, such as military intelligence detection, medical condition detection, urban facility construction and smart home, etc. Clock synchronization is one of the key technologies in WSNs, precise node time synchronization is critical for many networked control applications in WSNs. Once the clock of sensor nodes loses synchronization, the information collected by nodes will lose timeliness and value for users. Therefore, the key technology of clock synchronization plays an important role in WSNs. The main purpose of clock synchronization is to control the interval time that between sending and receiving timestamps of wireless sensor nodes within acceptable clock synchronization accuracy. However, because there are many kinds of link delays in the process of information transmission, the transmission time is prolonged and the time precision is reduced. Therefore, it is necessary to study the exact causes and effects of these errors. In ZigBee wireless sensor network, through theoretical model and data experimental analysis, we got the result that the random delay distributions of the uplink and downlink obey normal distribution when the sensor nodes exchange some information. [ABSTRACT FROM AUTHOR]

This paper proposes a novel three-dimensional guidance method based on clock synchronization algorithms, creatively solving the problems of randomized change, non-connection and even communication outage caused by packet loss and delay in the co-guidance of loitering munitions. The proposed method is characterized by fast convergence of the aspect angle rate in the longitudinal plane, and the attack time constrains in the lateral plane. Normal accelerations on the two sub-planes are thereby calculated, so as to control the munitions' co-attack. Simulation experiments show that the proposed method can ensure reliable and stable asymptotic agreement of attack time expectation for the cooperative multiple munitions under the switching and noisy networks. [ABSTRACT FROM AUTHOR]

In underwater navigation, the global positioning system is unavailable; hence, other solutions must be pursued in the development of navigation systems. In this paper, filters for the position, linear velocity, and acceleration of underwater vehicles are derived by combining a long baseline acoustic positioning system with an inertial navigation system. A dynamic model is devised via state augmentation, including the bias of the pseudo-ranges, which accounts for the effect of the unknown offset between the emitters' and receivers' clocks. With this technique, the resulting dynamics are linear, in spite of the original nonlinear nature of the problem. The proposed solution, which includes the explicit dynamic estimation of the bias of the pseudo-ranges, is a linear Kalman filter. The observability of the system is assessed, which allows to establish globally exponentially stable error dynamics. The performance of the solution is evaluated with realistic simulation results, considering sensor noise and discrete-time measurements. Finally, the comparison with the extended Kalman filter (EKF), the unscented Kalman filter (UKF), and the Bayesian Cramér–Rao bound is presented, including Monte Carlo simulations. This comparison shows the goodness of the proposed solution, which converges for all initial conditions and exhibits performance comparable to the EKF and the UKF, whereas the EKF and UKF are shown to lack global convergence. [ABSTRACT FROM AUTHOR]

Synchronization of clock within a wireless sensor network epitomizes crucial problems in the efficient and reliable operation of the sensors. This paper discusses a novel probability theory based clock offset estimator for various clock synchronization schemes of wireless sensor networks is proposed. The motivation is to utilize local clock timing for achieving the global clock synchronization. It presents a probabilistic model to estimate the most expected value of clock offset for sensor nodes. This model uses a statistical tools based on dispersion and central tendency. The proposed model was compared with the existing clock offset estimating models. It was observed that the proposed model gives better results with 1.008% accuracy, 0.065% precision and 99.8% efficiency. [ABSTRACT FROM AUTHOR]

The quality of the links between the different space geodetic techniques (VLBI, SLR, GNSS and DORIS) is still one of the major limiting factors for the realization of a unique global terrestrial reference frame that is accurate enough to allow the monitoring of the Earth system, i.e., of processes like sea level change, postglacial rebound and silent earthquakes. According to the specifications of the global geodetic observing system of the International Association of Geodesy, such a reference frame should be accurate to 1 mm over decades, with rates of change stable at the level of 0.1 mm/year. The deficiencies arise from inaccurate or incomplete local ties at many fundamental sites as well as from systematic instrumental biases in the individual space geodetic techniques. Frequently repeated surveys, the continuous monitoring of antenna heights and the geometrical mount stability (Lösler et al. in J Geod 90:467-486, 2016. 10.1007/s00190-016-0887-8) have not provided evidence for insufficient antenna stability. Therefore, we have investigated variations in the respective system delays caused by electronic circuits, which is not adequately captured by the calibration process, either because of subtle differences in the circuitry between geodetic measurement and calibration, high temporal variability or because of lacking resolving bandwidth. The measured system delay variations in the electric chain of both VLBI- and SLR systems reach the order of 100 ps, which is equivalent to 3 cm of path length. Most of this variability is usually removed by the calibrations but by far not all. This paper focuses on the development of new technologies and procedures for co-located geodetic instrumentation in order to identify and remove systematic measurement biases within and between the individual measurement techniques. A closed-loop optical time and frequency distribution system and a common inter-technique reference target provide the possibility to remove variable system delays. The main motivation for the newly established central reference target, locked to the station clock, is the combination of all space geodetic instruments at a single reference point at the observatory. On top of that it provides the unique capability to perform a closure measurement based on the observation of time. [ABSTRACT FROM AUTHOR]

Due to constraints of cost and size for mobile devices, a digital self-synchronous OFDM receiver is preferable in mobile applications since it only requires economical crystal oscillators plus free-running analog front-end circuits. However, there are more stringent requirements for clock timing recovery tasks in such a self-synchronous receiver. This paper presents an efficient and robust clock timing recovery scheme for the digital self-synchronous OFDM receiver. It is based on the canonical Gardner structure, but new techniques are devised for performance improvement. A joint iterative estimation algorithm is used for error detection, which can not only improve the detection precision, but also maintain the loop's dynamic performance. A direct control method based on a simple dynamic delay technique instead of the numerical-controlled oscillator mechanism is adopted to smoothly adjust the timing recovery by adaptively performing asynchronous resampling on the discrete sequence of the received signal. Theoretical analysis and numerical simulation results are also presented to verify the effectiveness of the proposed clock timing recovery scheme and conduct performance analysis and evaluation. [ABSTRACT FROM AUTHOR]

Channel based clock synchronization in packet switched networks (PSNs) is considered for, but not limited to, the time and safety/security critical application of power system protection. The synchronization accuracy requirement of power system protection devices used for line current differential protection is $$10\,\upmu $$ s, which could be achieved in time division multiplexing networks that were traditionally used in that domain. In PSN, highly accurate synchronization can be achieved with the standard synchronization method IEEE 1588-2008 Precision Time Protocol (PTP) when devices in the communication network are equipped with so called boundary clocks (BCs) or transparent clocks (TCs). However, when BCs or TCs are not available, the required accuracy can hardly be achieved. In this work, a modification of the PTP is proposed that replaces the clock parameter estimation and the computation of the clock control signal. Thereby, the statistics of measured packet delays are considered to select optimum estimation schemes. It is shown that the here proposed method outperforms the Linux PTP in terms of timing accuracy by a factor of 2 in enterprise local area networks and by a factor of 10 in Carrier Ethernet wide area networks. [ABSTRACT FROM AUTHOR]

Copyright of e & i Elektrotechnik und Informationstechnik is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Clock synchronization is one of the most crucial and fundamental issues in distributed networks. Inaccurate factors in synchronizing clocks between nodes in the network can occur at any point in the network layers. Most uncertainties caused at the upper layers can be eliminated by hardware-assisted time stamping. However, eliminating the uncertainty of a physical layer is difficult. This paper proposes a multi-phase correlator-based clock synchronization method to mitigate the time representation error at the physical layer and improve synchronization accuracy by introducing a time representation error, which is one of physical uncertainties. Further, to apply the proposed method to a realistic environment, we implement and evaluate the proposed multi-phase correlator. Our experimental results show that the accuracy of the proposed method is better than that of a conventional approach in terms of minimizing the time representation error. [ABSTRACT FROM AUTHOR]

We consider a stochastic model of clock synchronization in a wireless network of $$N$$ sensors interacting with one dedicated accurate time server. For large $$N$$ we find an estimate of the final time sychronization error for global and relative synchronization. The main results concern the behavior of the network on different timescales $$t_{N}\rightarrow \infty $$ , $$N\rightarrow \infty $$ . We discuss the existence of phase transitions and find the exact timescales for which an effective clock synchronization of the system takes place. [ABSTRACT FROM AUTHOR]

Modern manufacturing industry requires computer numerical control (CNC) systems to be more open, extendable, and networked. Real-time Ethernet is introduced and its use in CNC is discussed in this paper. Compared to traditional fieldbus, a real-time Ethernet which modifies the Ethernet data link layer to achieve fast delivery time and high synchronization accuracy is discussed. The method of synchronizing distributing clocks to reduce cumulative jitter of transmission for a synchronization frame is proposed. Some special considerations of this real-time Ethernet are discussed, e.g., incorporation with Windows NT and making feedback returned faster. A CNC system based on such a real-time Ethernet and Windows NT has been developed and presented. The system involves the service-oriented Windows Communication Foundation as an up-layer interface to the Internet, and a real-time Ethernet for connecting to hardware components of a machine tool. The non-real-time aspect of Windows NT is accounted for; for example, a driver rewriting the interrupt descriptor table is introduced and packets to be sent are buffered. Design and implementation of this system, such as main classes and services in the NC kernel, main modules (e.g., data link layer) in salve nodes, are also discussed. At last, experimental results are presented to illustrate the Windows NT's performance and the real-time accuracy of the synchronization method. [ABSTRACT FROM AUTHOR]

Efficient and accurate clock synchronization is a challenge for wireless sensor networks (WSNs). Unlike previous works on clock synchronization in WSNs that consider communication delay as the main cause of clock inaccuracy, we propose a new adaptive synchronization scheme, AdaSynch, which considers the principium of the clock. We aim to overcome the challenges posed by WSNs' resource constraints such as limited energy and bandwidth, low precision oscillators and random factors. By implementing some experiments on TelosB platform, we find that the clock system switches between multiple clock models. Motivated by experiment results, we establish a general clock model which describes the clock offset in terms of the oscillators. We then design two kinds of basic Kalman filter models to achieve clock synchronization. In order to execute Kalman filtering, we propose a recursion method based on the Expectation-Maximization (EM) algorithm to access the parameters of the Kalman filter model adaptively. To describe alternation in the clock model, we propose a Multimodel Kalman filter, and put forward an adaptive method based on hypothesis testing to describe these complexities in the clock model. We demonstrate the performance gains of our scheme through experiments using different Kalman filter models based on experiment data. [ABSTRACT FROM AUTHOR]

Classic distributed computing abstractions do not match well the reality of digital logic gates, which are the elementary building blocks of Systems-on-Chip (SoCs) and other Very Large Scale Integrated (VLSI) circuits: Massively concurrent, continuous computations undermine the concept of sequential processes executing sequences of atomic zero-time computing steps, and very limited computational resources at gate-level make even simple operations prohibitively costly. In this paper, we introduce a modeling and analysis framework based on continuous computations and zero-bit message channels, and employ this framework for the correctness & performance analysis of a distributed fault-tolerant clocking approach for Systems-on-Chip (SoCs). Starting out from a 'classic' distributed Byzantine fault-tolerant tick generation algorithm, we show how to adapt it for direct implementation in clockless digital logic, and rigorously prove its correctness and derive analytic expressions for worst case performance metrics like synchronization precision and clock frequency. Rather than on absolute delay values, both the algorithm's correctness and the achievable synchronization precision depend solely on the ratio of certain path delays. Since these ratios can be mapped directly to placement & routing constraints, there is typically no need for changing the algorithm when migrating to a faster implementation technology and/or when using a slightly different layout in an SoC. [ABSTRACT FROM AUTHOR]

We use the U ppaal model checker for timed automata to verify the Timing-Sync time-synchronization protocol for sensor networks (TPSN), the clock-synchronization algorithm of Lenzen, Locher and Wattenhofer (LLW) for general distributed systems, and the clock-thread technique of the software monitoring with controllable overhead algorithm (SMCO). Clock-synchronization algorithms such as TPSN, LLW, and SMCO must be able to perform arithmetic on clock values to calculate clock drift and network propagation delays. They must also be able to read the value of a local clock and assign it to another local clock. Such operations are not directly supported by the theory of timed automata. To overcome this formal-modeling obstacle, we augment the U ppaal specification language with the integer clock-derived type. Integer clocks, which are essentially integer variables that are periodically incremented by a global pulse generator, greatly facilitate the encoding of the operations required to synchronize clocks as in the TPSN, LLW, and SMCO protocols. With these integer-clock-based models in hand, we use U ppaal to verify a number of key correctness properties, including network-wide time synchronization, bounded clock skew, bounded overhead skew, and absence of deadlock. We also use the U ppaal Tracer tool to illustrate how integer clocks can be used to capture clock drift and resynchronization during protocol execution. [ABSTRACT FROM AUTHOR]

In this paper we generalize the Continuous Adversarial Queuing Theory (CAQT) model (Blesa et al. in MFCS, Lecture Notes in Computer Science, vol. 3618, pp. 144-155, ) by considering the possibility that the router clocks in the network are not synchronized. We name the new model Non Synchronized CAQT (NSCAQT). Clearly, this new extension to the model only affects those scheduling policies that use some form of timing. In a first approach we consider the case in which although not synchronized, all clocks run at the same speed, maintaining constant differences. In this case we show that all universally stable policies in CAQT that use the injection time and the remaining path to schedule packets remain universally stable. These policies include, for instance, Shortest in System (SIS) and Longest in System (LIS). Then, we study the case in which clock differences can vary over time, but the maximum difference is bounded. In this model we show the universal stability of two families of policies related to SIS and LIS respectively (the priority of a packet in these policies depends on the arrival time and a function of the path traversed). The bounds we obtain in this case depend on the maximum difference between clocks. This is a necessary requirement, since we also show that LIS is not universally stable in systems without bounded clock difference. We then present a new policy that we call Longest in Queues (LIQ), which gives priority to the packet that has been waiting the longest in edge queues. This policy is universally stable and, if clocks maintain constant differences, the bounds we prove do not depend on them. To finish, we provide with simulation results that compare the behavior of some of these policies in a network with stochastic injection of packets. [ABSTRACT FROM AUTHOR]

In a broad sense, any parametric family of quantum states can be viewed as a quantum clock. The time, which is the parameter, is encoded in the corresponding quantum states. The quality of such a clock depends on how precisely we can distinguish the states or, equivalently, estimate the parameter. In view of the quantum Cramér-Rao inequalities, the quality of quantum clocks can be characterized by the quantum Fisher information. We address the issue of quantum clock synchronization in terms of quantum Fisher information and demonstrate its fundamental difference from the classical paradigm. The key point is the superadditivity of Fisher information, which always holds in the classical case but can be violated in quantum mechanics. The violation can occur for both pure and mixed states. Nevertheless, we establish the superadditivity of quantum Fisher information for any classical-quantum state. We also demonstrate an alternative form of superadditivity and propose a weak form of superadditivity. The violation of superadditivity can be exploited to enhance quantum clock synchronization. [ABSTRACT FROM AUTHOR]

Edwards transformations relating inertial frames with arbitrary clock synchronization are reviewed and put in more general setting. Their group-theoretical context is described. [ABSTRACT FROM AUTHOR]

The locality hypothesis is generally considered necessary for the study of the kinematics of non-inertial systems in special relativity. In this paper we discuss this hypothesis, showing the necessity of an improvement, in order to get a more clear understanding of the various concepts involved, like coordinate velocity and standard velocity of light. Concrete examples are shown, where these concepts are discussed. [ABSTRACT FROM AUTHOR]

We introduce the distributed gradientclock synchronization problem. As in traditional distributed clock synchronization, we consider a network of nodes equipped with hardware clocks with bounded drift. Nodes compute logical clock values based on their hardware clocks and message exchanges, and the goal is to synchronize the nodes' logical clocks as closely as possible, while satisfying certain validity conditions. The new feature of gradient clock synchronization GCS for short) is to require that the skew between any two nodesy' logical clocks be bounded by a nondecreasing function of the uncertainty in message delay (call this the distance) between the two nodes, and other network parameters. That is, we require nearby nodes to be closely synchronized, and allow faraway nodes to be more loosely synchronized. We contrast GCS with traditional clock synchronization, and discuss several practical motivations for GCS, mostly arising in sensor and ad-hoc networks. Our main result is that the worst case clock skew between two nodes at distance d or less from each other is Ω( d + $$\frac{{\rm log} D}{{\rm log} {\rm log} D})$$ , where D is the diameter of the network. This means that clock synchronization is not a localproperty, in the sense that the clock skew between two nodes depends not only on the distance between the nodes, but also on the size of the network. Our lower bound implies, for example, that the TDMA protocol with a fixed slot granularity will fail as the network grows, even if the maximum degree of each node stays constant. [ABSTRACT FROM AUTHOR]

The experimental evidence for electromagnetic signals propagating with superluminal group velocity is recalled. Transformations of space and time depending on a synchronization parameter, e1, indicate the existence of a privileged inertial system. The Lorentz transformations are obtained for a particular e1≠0. No standard experiment on relativity depends on e1, but if accelerations are considered only e1=0 remains possible. The causal paradox generated by superluminal signals (SLS) in the theory of relativity does not exist in the theory with e1=0. The irrelevance of SLS for the Einstein, Podolsky and Rosen paradox is pointed out. [ABSTRACT FROM AUTHOR]

Our recently proposed inertial transformations of the space and time variables based on absolute simultaneity imply the existence of a single isotropic inertial reference system (‘privileged system’). We show, however, that a resynchronization of clocks in all inertial systems is possible leading to a different, arbitrarily chosen, isotropic ‘privileged’ system. Such a resynchronization does not modify any one of the empirical consequences of the theory, which is thus compatible with a formulation of the relativity principle weaker than adopted in Einstein's theory of special relativity. [ABSTRACT FROM AUTHOR]

Atomic clocks distributed around the world communicate with one another by means of radio signals. The synchronization signals sent by a transmitting station always reach the receiving station ‘on time,’ at any hour of the day and in any season, despite the motion of the Earth. For some authors this means that these signals propagate isotropically (with one way velocity c), even with respect to the Earth's surface. In fact this may not be so; we show that the proper working of the network says nothing about the one-way velocity, as it is consistent with another theory, empirically (almost) equivalent to special relativity, in which the one-way speed of light has a directional dependence in moving frames. [ABSTRACT FROM AUTHOR]

This paper gives two complete and elementary proofs that if the speed of light over closed paths has a universal value c, then it is possible to synchronize clocks in such a way that the one-way speed of light is c. The first proof is an elementary version of a recent proof. The second provides high precision experimental evidence that it is possible to synchronize clocks in such a way that the one-way speed of light has a universal value. We also discuss an old incomplete proof by Weyl which is important from an historical perspective. [ABSTRACT FROM AUTHOR]

An alternator is a network of concurrent processes, which satisfies the following conditions. (1) If one process executes the critical step, no neighbor of the process executes the critical step in the same computing step. (2) Along any infinite computing steps, each process executes the critical step infinitely often. (3) An alternator is self-stabilizing to the above conditions. An alternator is said to be 1-fair if condition (2) is changed as: A process can execute the critical step twice only if all other processes execute the critical step at least once. In this paper, we proposed an alternator for rings of odd size. The design has the snap property in the sense that it satisfies condition (1) even when transient faults occur. The alternator allows each process execute the critical step once every three steps when it stabilizes. The design is optimal 1-fair in the sense that no other 1-fair design can have better performance. Based on the above design, we fine-tune the alternator to achieve maximal performance. That is, our final alternator is a maximal alternator: a process is allowed to execute the critical step when both its two neighbors do not execute the critical step. [ABSTRACT FROM AUTHOR]

This paper presents an innovative method to synchronize physical clocks for a computational grid, in particular for a computational grid linked through the asynchronous Intranet or Internet environments. The method discussed is an asynchronous self-adaptive clock synchronization mechanism. Two strategies for clock synchronisationare introduced. (1), Use continuous time intervals to calculate the precision of clocks, which can reduce the effect of network delay efficiently. (2) Every node synchronizes its clock, with its leader actively. In addition, a node self-adaptive model is presented, and the relationship between the clock precision and synchronization time is induced, hence a node can predict when it should begin the synchronization process. Detailed simulation and extension of this issue are provided at the end of the paper. The presented model is both practical and feasible. [ABSTRACT FROM AUTHOR]

Transformations of space and time, depending on a synchronisation parameter e1, indicate the existence of a privileged inertial system S0. The Lorentz transformations are obtained for a particular e1≠0. No classical experiment on relativity is expected to depend on the choice of e1. We show, consistently with expectations, that the result of the Fizeau experiment is explained equally well by theories adopting different values of e1. In previous papers we showed that if accelerated reference frames are considered only e1=0 remains possible. [ABSTRACT FROM AUTHOR]

Starting from the experimental fact that light propagates over a closed path at speed c ( L/ c law), we show to what extent the isotropy of the speed of light can be considered a matter of convention. We prove the consistence of anisotropic and inhomogeneous conventions, limiting the allowed possibilities. All conventions lead to the same physical theory even if its formulation can change in form. The mathematics involved is that of gauge theories and the choice of a simultaneity convention is interpreted as a choice of the gauge. Moreover, we prove that a Euclidean space where the L/ c law holds, gives rise to a spacetime with Minkowskian causal structure, and we exploit the consequences for the causal approach to the conventionality of simultaneity. [ABSTRACT FROM AUTHOR]

In a recent paper, F. Selleri argued that the result of the Sagnac experiment is incompatible with the assumption that the one-way speed of light is isotropic in every Lorentz frame. We present an intuitive argument which allows one to argue that a rational description of the Sagnac effect can be reached on the basis of special relativity and of an invariant one-way speed of light as a pure desynchronization effect. [ABSTRACT FROM AUTHOR]

We show how synchronized clocks can be realized in a distributed system as a byproduct of a common communication paradigm where processors periodically perform broadcasts. Our approach decouples the precision concern of clock synchronization-limiting how much correct clocks can differ from each other-from the accuracy concern-limiting the rate at which any correct clock may drift from real time. Given a system that guarantees only precision, we develop a protocol whereby high accuracy can be achieved on demand. In this manner, the 'lazy' protocol we obtain incurs the cost of high accuracy only when needed while keeping the basic synchronization procedure extremely simple and cheap. [ABSTRACT FROM AUTHOR]

We investigate the power of a broadcast mechanism in a distributed network. We do so by considering the problem of synchronizing clocks in an errorfree network, under the assumption that there is no upper bound on message transmission time, but that broadcast messages are guaranteed to be received within an interval of size ε, for some fixed constant ε. This is intended to be an idealization of what happens in multiple access networks, such as the Ethernet. We then consider tradeoffs between the type and number of broadcasts, and the tightness of synchronization. Our results include (1) matching upper and lower bounds of (1+1/ K)ε on the precision of clock synchronization attainable for n≧3 process using K ( n−1)-casts, 3≦ K≦ n, (2) matching upper and lower bounds of (1+1/ n)ε on the precision of clock synchronization attainable for n≧3 processes using an arbitrary number of ( n−1)-casts, and (3) matching upper and lower bounds of (1+ n−2/ n)ε on the precision attainable using 2-casting. [ABSTRACT FROM AUTHOR]

A probabilistic method is proposed for reading remote clocks in distributed systems subject to unbounded random communication delays. The method can achieve clock synchronization precisions superior to those attainable by previously published clock synchronization algorithms. Its use is illustrated by presenting a time service which maintains externally (and hence, internally) synchronized clocks in the presence of process, communication and clock failures. [ABSTRACT FROM AUTHOR]

In this paper we present a new probabilistic clock synchronization algorithm, its prototype implementation and experimental results. The algorithm follows the client-server programming paradigm and is designed to work in a departmental environment with few servers and a number of clients connected through an arbitrary network topology. At the core of the algorithm is a remote clock reading method that mitigates the negative effects of message delay uncertainty. The implementation proves the effectiveness of this approach and corroborates the theoretical speculations. [ABSTRACT FROM AUTHOR]