Clock synchronization in virtual rings

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.

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

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.

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.

Secure and self-stabilizing clock synchronization in sensor networks

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.

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.

Existing set-top boxes experiences very large jitter in terrestrial environment due to multi path reflections. Existing algorithms for achieving clock recovery for DVB-T relies on very complex calculations which are not suitable for real time data processing. In digital set-top box most of the CPU bandwidth is consumed in audio & video data decoding so clock synchronization algorithms get very less CPU time to execute. This paper presents Low cost weighing filter approach to solve the color loss problem in digital set-top box. This algorithm has been tested in practical satellite and terrestrial environment. Its performance has been compared with linear regression based algorithm and shows distinct advantages.

Towards a real-time distributed computing model

Designing clock synchronization algorithms for distributed real-time systems based on packet networks requires to decide on the amount and functionality of proper hardware support. Targeting a 1 /spl mu/s precision and similar accuracy measures in case of external UTC supply from GPS-receivers, this paper describes pertinent features of a peripheral device called UTCSU manufactured as an ASIC. The most salient one is the introduction of an adder-based clock, that allows a fine grained rate adjustment, continuous amortization, and maintenance of local accuracy intervals.

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.

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.

Clock synchronization is an important service in wireless sensor networks (WSN) since it can be used for data integration, for the implementation of TDMA based approaches or for synchronous data acquisition. One main issue in WSN concerns the power consumption since the batteries have a limited capacity an often they cannot be replaced. In this paper we present a dynamic continuous clock synchronization algorithm which provides a virtual common clock among a master and a group of slaves. The main feature of the approach proposed is the ability to allow nodes to sleep for long times and exchange only rare synchronization packets however providing a good quality common clock. The synchronization algorithm has been implemented inside embedded devices as a middleware over the MAC, without any modification to the original communication protocol.

This brief announcement presents a fault-tolerant self-stabilizing distributed clock synchronization protocol for an arbitrary, non-partitioned digraph. Synchronization algorithms are essential for managing the use of resources and controlling communication in a distributed system. Synchronization of a distributed system is the process of achieving and maintaining a bounded skew among independent local time clocks. A distributed system is said to be self-stabilizing if, from an arbitrary state, it is guaranteed to reach a legitimate state in a finite amount of time and remain in a legitimate state. For clock synchronization, a legitimate state is a state where all parts in the system are in synchrony. The self-stabilizing distributed-system clock synchronization problem is, therefore, to develop an algorithm (i.e., a protocol) to achieve and maintain synchrony of local clocks in a distributed system after experiencing system-wide disruptions in the presence of network element imperfections. The convergence and closure properties address achieving and maintaining networksynchrony, respectively.