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.

/pdf/the-flooding-time-synchronization-protocol-2j5x8h86k4.pdf

The flooding time synchronization protocol

The network time protocol (NTP), which is designed to distribute time information in a large, diverse system, is described. It uses a symmetric architecture in which a distributed subnet of time servers operating in a self-organizing, hierarchical configuration synchronizes local clocks within the subnet and to national time standards via wire, radio, or calibrated atomic clock. The servers can also redistribute time information within a network via local routing algorithms and time daemons. The NTP synchronization system, which has been in regular operation in the Internet for the last several years, is described, along with performance data which show that timekeeping accuracy throughout most portions of the Internet can be ordinarily maintained to within a few milliseconds, even in cases of failure or disruption of clocks, time servers, or networks. >

/pdf/internet-time-synchronization-the-network-time-protocol-3b6kqc4t85.pdf

Internet time synchronization: the network time protocol

Originating from basic research conducted in the 1970's and 1980's, the parallel and distributed simulation field has matured over the last few decades. Today, operational systems have been fielded for applications such as military training, analysis of communication networks, and air traffic control systems, to mention a few. The article gives an overview of technologies to distribute the execution of simulation programs over multiple computer systems. Particular emphasis is placed on synchronization (also called time management) algorithms as well as data distribution techniques.

/pdf/parallel-and-distributed-simulation-systems-2ovm9qn1cd.pdf

Parallel and Distributed Simulation Systems

This document describes the Network Time Protocol (NTP), specifies its
formal structure and summarizes information useful for its
implementation. [STANDARDS-TRACK]

/pdf/network-time-protocol-version-3-specification-implementation-320diiyv7x.pdf

Network Time Protocol (Version 3) Specification, Implementation and Analysis

The time-triggered architecture (TTA) provides a computing infrastructure for the design and implementation of dependable distributed embedded systems. A large real-time application is decomposed into nearly autonomous clusters and nodes, and a fault-tolerant global time base of known precision is generated at every node. In the TTA, this global time is used to precisely specify the interfaces among the nodes, to simplify the communication and agreement protocols, to perform prompt error detection, and to guarantee the timeliness of real-time applications. The TTA supports a two-phased design methodology, architecture design, and component design. During the architecture design phase, the interactions among the distributed components and the interfaces of the components are fully specified in the value domain and in the temporal domain. In the succeeding component implementation phase, the components are built, taking these interface specifications as constraints. This two-phased design methodology is a prerequisite for the composability of applications implemented in the TTA and for the reuse of prevalidated components within the TTA. This paper presents the architecture model of the TTA, explains the design rationale, discusses the time-triggered communication protocols TTP/C and TTP/A, and illustrates how transparent fault tolerance can be implemented in the TTA.

/pdf/the-time-triggered-architecture-34o2qlqe64.pdf

The time-triggered architecture

The generation of a fault-tolerant global time base with known accuracy of synchronization is one of the important operating system functions in a distributed real-time system. Depending on the types and number of tolerated faults, this paper presents upper bounds on the achievable synchronization accuracy for external and internal synchronization in a distributed real-time system. The concept of continuous versus instantaneous synchronization is introduced in order to generate a uniform common time base for local, global, and external time measurements. In the last section, the functions of a VLSI clock synchronization unit, which improves the synchronization accuracy and reduces the CPU load, are described. With this unit, the CPU overhead and the network traffic for clock synchronization in state-of-the-art distributed real-time systems can be reduced to less than 1 percent.

Wilhelm Ochsenreiter

Papers

Clock Synchronization in Distributed Real-Time Systems

Interval Measurements in Distributed Real Time Systems.