Abstract—Global synchronization is important for many sensor network applications that require precise mapping of collected sensor data with the time of the events, for example, in tracking and surveillance. It also plays an important role in energy conservation in MAC layer protocols. This paper describes four methods to achieve global synchronization in a sensor network: a node-based approach, a hierarchical cluster-based method, a diffusion-based method, and a fault-tolerant diffusion-based method. The diffusion-based protocol is fully localized. We present two implementations of the diffusion-based protocol for synchronous and asynchronous systems and prove its convergence. Finally, we show that, by imposing some constraints on the sensor network, global clock synchronization can be achieved in the presence of malicious nodes that exhibit Byzantine failures. Index Terms—Sensor networks, fault tolerance. æ

A highly accurate active probing measurement infrastructure is described based on GPS synchronised DAG cards as timestamping monitors, and Real-Time Linux as a probe stream sender. A comparison of less accurate systems including senders based on FreeBSD and Linux is given and diverse sources of timing errors are described and discussed, as is NTP synchronization. Preliminary results using the infrastructure are presented, and its application to a new approach for bottleneck bandwidth estimation outlined.

Advances in microelectronics, array processing, and wireless networking, have motivated the analysis and design of low-cost integrated sensing, computating, and communicating nodes capable of performing various demanding collaborative space-time processing tasks. In this paper, we consider the problem of coherent acoustic sensor array processing and localization on distributed wireless sensor networks. We first introduce some basic concepts of beamforming and localization for wideband acoustic sources. A review of various known localization algorithms based on time-delay followed by LS estimations as well as maximum likelihood method is given. Issues related to practical implementation of coherent array processing including the need for fine-grain time synchronization are discussed. Then we describe the implementation of a Linux-based wireless networked acoustic sensor array testbed, utilizing commercially available iPAQs with built in microphones, codecs, and microprocessors, plus wireless Ethernet cards, to perform acoustic source localization. Various field-measured results using two localization algorithms show the effectiveness of the proposed testbed. An extensive list of references related to this work is also included.

Abstract — A new distributed algorithm for cooperative estimation of a slowly time-varying signal using a wireless sensor network is presented. The estimate in each node is based on a so called consensus algorithm, which weights measurements and estimates of neighboring nodes. The algorithm is therefore scalable with the number of network nodes. It requires only limited information exchange between nodes and computations in each node. The weights are locally optimized based on a minimum variance criterion. Numerical results show that the proposed algorithm exhibits good performance compared to other distributed algorithms proposed in the literature. I.

this paper are chosen from actual measurements made on the UNIX kernel running the Ficus operating system, 16,17 which is a modified version of SunOS 4.1.1, on a Sparcstation IPC. Since kitrace is designed for kernel development, its use presupposes familiarity with the code being measured. For those readers unfamiliar with the aspects of the UNIX kernel used in our examples, a brief summary is in order

This paper traces the origins and evolution of the Network Time Protocol (NTP) over two decades of continuous operation. The technology has been continuously improved from hundreds of milliseconds in the rowdy Internet of the early 1980s to tens of nanoseconds in the Internet of the new century. It includes a blend of history lessons, technology milestones and series of experiments that shape, define and record the early history of the Internet and NTP. This narrative is decidedly personal, since the job description for an Internet timekeeper is highly individualized and invites very few applicants. There is no attempt here to present a comprehensive tutorial, only a almanac of personal observations, eclectic minutiae and fireside chat. Many souls have contributed to the technology, some of which are individually acknowledged in this paper, the rest too numerous left to write their own memoirs.

Sensornets must allocate limited computation and energy resources efficiently to maximise utility and lifetime. This task is complicated by the need to coordinate activity between nodes as sensornets are necessarily real-time collaborative systems. In this paper we present and evaluate lightweight adaptive protocols based on pulse-coupled oscillators to synchronise tasks within a unicellular sensornet. A near-optimal schedule is constructed and dynamically maintained under non-ideal network conditions. 1

In distributed applications, a group of multiple objects are cooperated. On receipt of request messages, the objects send back the responses. Kinds of group communication protocols have been discussed so far, which support the reliable and ordered delivery of messages at the network level. Only messages to be ordered at the application level, not necessarily all messages, are required to be causally delivered in the required order. The state of the object depends on in what order the requests are computed and the responses and requests are transmitted. In this paper, we would like to define the significant precedence order of messages based on the conflicting relation among the requests. We would like to discuss a protocol which supports the significantly ordered delivery of request and response messages. 1 Introduction Distributed applications like teleconferences [7] are composed of multiple application objects. Objects support operations for manipulating the states of the objects....

Off-line clock synchronisation algorithms, in which synchronisation is performed by adjusting a collection of recorded timestamps, is suitable for use with many monitors for distributed systems. Off-line synchronisation can often achieve very good synchronisation without the need for extra messages. The work described here builds on earlier work in this area [4] by introducing new synchronisation algorithms, developing ways of evaluating algorithms, and performing an extensive set of experiments based on five different algorithms and a considerable amount of data collected by a monitor for Amoeba. The best algorithms achieve excellent synchronisation, and are used in the Amoeba monitor.

This memorandum explores issues in performing experiments of this type and summarizes various techniques found useful in practice.