An ASIC supporting external clock synchronization for distributed real-time systems

Synchronization of the clocks is one of the essential thing for many applications in distributed systems. Clock synchronization is very important because they improve the performance and reliability of distributed systems. The main purpose of clock synchronization algorithms is to provide the common time to essential parts of the distributed systems. In this paper the problem considered is synchronization of clock with bounded clock drift and proposing a two level synchronization algorithm which synchronizes the processors local clocks by combining both internal and external clock synchronization.

A probabilistic algorithm for external clock synchronization is proposed. The algorithm works under the assumption that it is possible to identify a virtual ring among the servers involved, and does not require an upper bound to message transmission delays. The approach is probabilistic, since clock synchronization is not guaranteed, but the probability of success can be made arbitrarily close to one. The precision is comparable with that achievable with other probabilistic protocols, with the advantage that all the nodes can synchronize their clocks within the same attempt. A Pascal-like description of the protocol is given, and a formal proof of its correctness is outlined. >

年图灵奖获得 者 Lamport 教授就指出时钟同步是分布式系统的关键依赖性条件 [4] .随着 21 世纪科学技术的高速发 展, 人们对网络化系统中分散时钟的有效同步提出了更高要求, 这正是因为网络化系统日益渗透到不 同行业

In practical applications, communication devices usually use communication message packets to synchronize time through electromagnetic wave transmission. However, due to complex external electromagnetic interference, communication equipment often encounter problems such as time synchronization errors, poor synchronization accuracy, and long synchronization periods through the above synchronization methods. Based on the wide application of GPS in daily life and production, this paper proposes a GPS-based external synchronization technology for communication equipment, which synchronizes the time within the entire network by synchronizing the communication equipment with GPS. The external clock synchronization hardware architecture is introduced, and the external synchronization process is explained in detail. Combined with the characteristics of clock drift, the Kalman filtering algorithm is introduced to track the clock drift. The external synchronization technology based on GPS timing has the advantages of high synchronization accuracy and fast synchronization speed.

Fault-tolerant clock synchronization is the foundation of synchronous architectures such as the Time-Triggered Architecture (TTA) for dependable cyber-physical systems. Clocks are typically local counters that are increased with a given rate according to real time, and clock synchronization algorithms ensure that any two clocks in the system read about the same value at about the same point in real time. This is achieved by a clock synchronization algorithm that changes the current values of the clocks, the clocks' rate, or both. This paper presents a diagnosis algorithm and a clock-rate correction algorithm as layered services on top of the TTEthernet clock synchronization algorithm, which itself is a clock-state correction algorithm. We analyze the algorithms' properties and explore and understand their behavior using a bounded model checker for infinite data types. We use our formal framework for both simulation and formal proof. To the best knowledge of the authors this has been the first time that formal methods, should they be theorem provers or model checkers, have been applied to the problem of rate-correction for fault-tolerant clock synchronization. Furthermore, the formal development process itself demonstrates how easily existing models can be utilized in the development of new algorithms and their formal verification.

Secure and self-stabilizing clock synchronization in sensor networks

This paper introduces a novel lightweight clock synchronization protocol (LCSP) and BUB clock synchronization algorithm based on real-time micro-kernel. Unlike other clock synchronization approaches which rely on redundant exchanged messages, this approach takes advantages of the traffic smoothing technique at each end node to suppress or block other traffics during the clock synchronization duration. The BUB algorithm efficiently decreases the number of synchronization messages exchanged during each synchronization round and can easily extend to large distributed real-time systems. Performance evaluation results prove that the LCSP protocol and the BUB algorithm can provide perfect synchronization precision and are appropriate for most distributed hard real-time and embedded control systems, when integrated with the traffic smoothing technique.

Time-delay interferometry (TDI) is a data processing technique for LISA designed to suppress the otherwise overwhelming laser noise by several orders of magnitude. It is widely believed that TDI can only be applied once all phase or frequency measurements from each spacecraft have been synchronized to a common time frame. We demonstrate analytically, using as an example the commonly used Michelson combination $X$, that TDI can be computed using the raw, unsynchronized data, thereby avoiding the need for an initial synchronization processing step and significantly simplifying the initial noise reduction pipeline. Furthermore, the raw data are free of any potential artifacts introduced by clock synchronization and reference frame transformation algorithms, which allow one to operate directly on the megahertz beatnotes. As a consequence, in-band clock noise is directly suppressed as part of TDI, in contrast to the approach previously proposed in the literature (in which large trends in the beatnotes are removed before the main laser-noise reduction step, and clock noise is suppressed in an extra processing step). We validate our algorithm with full-scale numerical simulations that use lisa instrument and pytdi and show that we reach the same performance levels as the previously proposed methods, ultimately limited by the clock sideband stability.

SpaceWire fiber-optic Bus is a development of the SpaceWire ECSS-E-50-12C standard. Its transmission rate can reach 2 Gbps. However, it suffers from the problems of imprecise clock synchronization and link blocking in networks. Therefore, we analysis the problems and propose following methods. Firstly, we design clock synchronization algorithms to guarantee the precision of clock synchronization. Furthermore, we design static time distribution scheduling table which takes time-triggered traffic for real-time messages. Through simulation verification, the time precision can be within 58 ns while using clock synchronization algorithm. The use of scheduling table can effectively avoid network conflicts and reduce the delay of data transmission. The improved scheduling method can meet the real-time requirements of SpaceWire fiber-optic Bus systems, and will effectively promote the space application of fiber-optic technology.

Execution of a task in a synchronized manner in a distributed setting, one needs the common idea of time. To accomplish this task, clock synchronization is conceptualized. Clock drifting is an intrinsic property of a clock, which necessitates the synchronization and resynchronization. A network consists of nodes, which may behave appropriately or behave badly. The mischievous node may pose difficulty in clock synchronization. We require a method to overcome the effect of mischievous nodes. Synchronization can be achieved in two ways namely accuracy and precision. In this paper, we present an accurate weighted average clock synchronization algorithm to execute a coordinated activity in a fault-tolerant manner. We use trusted clock value from GPS/GLONASS/IRNSS as a reference clock to achieve better accuracy. We also use behavior of the node to compute normalized weight in a localized manner, which lies between unit intervals. This weight assignment enables us to contain the effect of misbehavior up to some extent. This algorithm offers better accuracy and precision while tolerating mischievous nodes. The upper bound of tolerance boundary is one-third of the network size.

Execution of a task in a synchronized manner in a distributed setting, one needs the common idea of time. To accomplish this task, clock synchronization is conceptualized. Clock drifting is an intrinsic property of a clock, which necessitates the synchronization and resynchronization. A network consists of nodes, which may behave appropriately or behave badly. The mischievous node may pose difficulty in clock synchronization. We require a method to overcome the effect of mischievous nodes. Synchronization can be achieved in two ways namely accuracy and precision. In this paper, we present an accurate weighted average clock synchronization algorithm to execute a coordinated activity in a fault-tolerant manner. We use trusted clock value from GPS/GLONASS/IRNSS as a reference clock to achieve better accuracy. We also use behavior of the node to compute normalized weight in a localized manner, which lies between unit intervals. This weight assignment enables us to contain the effect of misbehavior up to some extent. This algorithm offers better accuracy and precision while tolerating mischievous nodes. The upper bound of tolerance boundary is one-third of the network size.

Sensor localization in wireless sensor networks is an important component of many applications. Previous work has demonstrated how localization can be achieved using various methods. In this paper we focus on achieving fine-grained localization that does not require external infrastructure, specialized hardware support, or excessive sensor resources. We use a real sensor network and provide measurements on the actual system. We adopt a localization approach that relies on acoustic sounds and clock synchronization. The contribution of our work is achieving consistent sound pulse detection at each sensor and precise range estimation using a high-precision clock synchronization implementation. We first describe our technique and then we evaluate our approach using a real setup. Our results show that our approach achieves an average clock synchronization accuracy of 5μs. We verify this accuracy using an external global clock via an interrupt mechanism. Our sound detection technique is able to consistently identify sound pulses up to 10m distances in indoor environments. Combining the two techniques, we find that our localization method results in accurate range estimation with an average error of 11cm in distances up to 7m and in consistent range estimation up to 10m in various indoor environments.

Global Navigation Satellite Systems (GNSS) rely on one-way signal travel time measurements from satellites to receivers. To ensure accurate ranging, GNSS must estimate and compensate for clock offsets on at least one end of the transmitter-receiver system. Consequently, clock offsets are highly correlated with GNSS-derived parameters such as station coordinates, tropospheric delay estimates, satellite radial orbit parameters, and apparent geocenter coordinates. Improper handling of clock offsets can lead to their absorption into these parameters, degrading geodetic product accuracy. Traditionally, clocks have been handled within individual techniques without being specially addressed. However, some research efforts and experimental trials have explored using VLBI to synchronize clocks for GNSS and refining clock models to reduce correlations with other estimated parameters in GNSS, thereby improving overall precision.With recent advancements, high-precision optical clocks and fiber-optic time transfer technologies provide new possibilities for enhancing clock synchronization in GNSS. Optical fiber offers a stable, interference-free medium for time and frequency transfer, eliminating atmospheric effects such as ionospheric and tropospheric delays that impact GNSS-based synchronization. Moreover, fiber-optic time transfer is inherently two-way, removing the need for external clock offset estimation. Thus, fiber-optic clock synchronization represents the most precise timing technique currently available. Establishing a common clock by connecting multiple GNSS receivers via optical fiber could significantly reduce the number of estimated clock parameters in GNSS solutions, leading to improved geodetic measurement precision and a more accurate geodetic reference frame.Despite its potential, the approach of implementing a common clock with optical fiber for GNSS receivers remains largely untested. Although optical fiber links have been successfully used in clock synchronization, their integration into GNSS receiver clocking is still in an experimental stage. Notable existing efforts include: (1) the Geodetic Observatory Wettzell, where multiple receivers are connected to a single clock via optical fiber, representing a local baseline setup, and (2) the GFZ experiment, where a receiver is linked to the ultra-stable clock of PTB, representing a regional baseline approach.This study characterizes existing fiber-optic connections among GNSS receivers to identify practical challenges and evaluate their impact on GNSS parameter estimation. We process data from receivers sharing a common clock using Bernese GNSS Software. Initially, PPP will be performed to compare GNSS-derived clock parameters across receivers sharing a common clock, verifying the effectiveness of fiber-optic clock synchronization. Following this validation, we will leverage GNSS-derived clock parameters to evaluate inter-receiver clock differences measured via optical fiber. Subsequently, we will implement a strategy in which only one receiver’s clock parameters are estimated, using fiber-measured clock differences as a priori constraints for other receivers in the PPP solution. This approach reduces the number of estimated clock parameters and serves as a preliminary test of the feasibility of a GNSS common clock framework.Future developments will focus on refining our analysis framework to enable simultaneous estimation of GNSS receivers operating under a common clock. This research contributes to the broader goal of integrating ultra-stable clocks into global GNSS networks, ultimately enhancing the stability of geodetic reference frames and improving GNSS-derived geodetic products.

Clock synchronization is an important component of wireless sensor networks both for co-ordination of node communications and for time stamping sensor data. The properties of the previously presented clock sampling mutual network synchronization algorithm (CS-MNS) are explored. The desirable properties of this algorithm include simplicity, low communication and processing overhead, and fully decentralized operation. Past simulation and analysis has shown clock convergence under specific conditions in single hop networks. We show that in the absence of offset errors the network clocks converge. In the presence of offset error we present conditions on the degree on clock asynchrony under which the network clock rates show convergent behavior. The analysis is applicable as long as the network topology is connected and thus is of interest in both single-hop and multi-hop environments. We show how the network designer can use these conditions to add a bias term to the CS-MNS algorithm and thus improve the start-up dynamics of the algorithm. Additionally, we discuss the algorithm from a security standpoint. Finally, we propose a method for adding external reference synchronization that is compatible with our security discussion.

The birth of computer networks and distributed systems has led to the appearance of the clock synchronization problem. This issue has gained increasing importance with the emergence of new resource constrained networks such as wireless sensor networks. In this paper, we propose a new distributed clock synchronization algorithm, referred to as Weighted Consensus Clock Synchronization (WCCS), whose objective is to achieve a consensus clock among network nodes. In this distributed approach and in contrast to centralized schemes, each node periodically exchanges the local clock reading with its immediate neighbor nodes. Then, each node employs these time informations to calculate its relative offset and skew with respect to its neighbor nodes using a weighted average consensus based technique. The effectiveness of WCCS is proved through both simulations and an experimental study on TelosB mote using TinyOS.