Ubiquitous computing environments are typically based upon ad hoc networks of mobile computing devices. These devices may be equipped with sensor hardware to sense the physical environment and may be attached to real world artifacts to form so{called smart things. The data sensed by various smart things can then be combined to derive knowledge about the environment, which in turn enables the smart things to "react" intelligently to their environment. For this so{called sensor fusion, temporal relationships (X happened before Y) and real{time issues (X and Y happened within a certain time interval) play an important role. Thus physical time and clock synchronization are crucial in such environments. However, due to the characteristics of sparse ad hoc networks, classical clock synchronization algorithms are not applicable in this setting. We present a time synchronization scheme that is appropriate for sparse ad hoc networks.

This paper describes the motivation, design, and performance of Porcupine, a scalable mail server. The goal of Porcupine is to provide a highly available and scalable electronic mail service using a large cluster of commodity PCs. We designed Porcupine to be easy to manage by emphasizing dynamic load balancing, automatic configuration, and graceful degradation in the presence of failures. Key to the system’s manageability, availability, and performance is that sessions, data, and underlying services are distributed homogeneously and dynamically across nodes in a cluster.

We discuss the problem of detecting errors in measurements of the total delay experienced by packets transmitted through a wide-area network. We assume that we have measurements of the transmission times of a group of packets sent from an originating host, A, and a corresponding set of measurements of their arrival times at their destination host, B, recorded by two separate clocks. We also assume that we have a similar series of measurements of packets sent from B to A (as might occur when recording a TCP connection), but we do not assume that the clock at A is synchronized with the clock at B, nor that they run at the same frequency. We develop robust algorithms for detecting abrupt adjustments to either clock, and for estimating the relative skew between the clocks. By analyzing a large set of measurements of Internet TCP connections, we find that both clock adjustments and relative skew are sufficiently common that failing to detect them can lead to potentially large errors when an...

research relevant to the design and application of high performance scientific computers. We test our ideas by designing, building, and using real systems. The systems we build are research prototypes; they are not intended to become products. There are two other research laboratories located in Palo Alto, the Network Systems

This paper describes the design, implementation, and evaluation of a Federated Array of Bricks (FAB), a distributed disk array that provides the reliability of traditional enterprise arrays with lower cost and better scalability. FAB is built from a collection of bricks, small storage appliances containing commodity disks, CPU, NVRAM, and network interface cards. FAB deploys a new majority-votingbased algorithm to replicate or erasure-code logical blocks across bricks and a reconfiguration algorithm to move data in the background when bricks are added or decommissioned. We argue that voting is practical and necessary for reliable, high-throughput storage systems such as FAB. We have implemented a FAB prototype on a 22-node Linux cluster. This prototype sustains 85MB/second of throughput for a database workload, and 270MB/second for a bulk-read workload. In addition, it can outperform traditional masterslave replication through performance decoupling and can handle brick failures and recoveries smoothly without disturbing client requests.

The current Internet provides a single class best effort service. From an application's point of view, this service amounts in practice to providing channels with time-varying characteristics such as delay and loss distributions. One way to support real time applications such as interactive audio given this service is to use control mechanisms that adapt the audio coding and decoding processes based on the characteristics of the channels, the goal be gin to maximize the quality of the audio delivered to the destinations. In this paper, we describe and analyze a set of such control mechanisms. They include a jitter control mechanism and a combined error and rate control mechanism. These mechanisms have been implemented and evaluated over the Internet and the MBone. Experiments indicate that they make it possible to establish and maintain reasonable quality audioconferences even across fairly congested connections.

. In this paper, we develop and analyze a simple interval-based algorithm suitable for fault-tolerant external clock synchronization. Unlike usual internal synchronization approaches, our convergence function-based algorithm provides approximately synchronized clocks maintaining both precision and accuracy w.r.t. external time. This is accomplished by means of a time representation relying on intervals that capture external time, providing accuracy information encoded in interval lengths. The algorithm, which is generic w.r.t. the convergence function and relies on either instantaneous correction or continuous amortization for clock adjustment, is analyzed by utilizing a novel, interval-based framework for establishing worst-case precision and accuracy bounds subject to a fairly detailed system model. Apart from individual clock rate and transmission delay bounds, our system model incorporates non-standard features like clock granularity and broadcast latencies as well. Relying on a su...

This paper gives an introduction to the issues involved in asynchronous video transfers. Brief overviews of video coding, rate control, multiplexing, as well as delay, error and loss control are given. Section 1. Background A collection of the world's telecommunication operators and their equipment suppliers are currently standardizing the asynchronous transfer mode (ATM) within the International Telecommunication Union (ITU) as a basis for broadband integrated services digital networks. In parallel with the ITU, the ATM Forum is furthering the development process by including computer communication manufacturers, as well as end--users in the standardization process. Although the representations give different perspectives on the proposed network solutions, the overall goal of building networks capable of transferring multimedia efficiently is shared between the ITU and the ATM Forum. In addition to the work on ATM, new developments for internet protocols lean towards supporting full--fledged multimedia communications. The simple internet protocol plus (RFC 1710) has been selected by the Internet Engineering Task Force as a basis for the next generation internet protocol (version 6, often denoted IPng; see RFC 1752). It features connection--oriented services, denoted flows, and signaling by means of the resource reservation protocol (RSVP) [103]. An implied assumption in the development of both ATM and IPng is that network users will get access to copious amounts of transmission capacity at affordable prices, even in the local loop. This capacity is needed to lower the latency of bulk data transfers and to enable audio and video communications. The provision of the latter has received considerable attention among researchers and is the topic of this paper. Asynchronous ...

Ordering and time are two different aspects of consistency of shared objects in a distributed system. One avoids conflicts between operations, the other addresses how quickly the effects of an operation are perceived by the rest of the system. Consistency models such as sequential consistency and causal consistency do not consider the particular time at which an operation is executed to establish a valid order among all the operations of a computation. Timed consistency models require that if a write operation is executed at time t, it must be visible to all nodes by time t+D. Timed consistency generalizes several existing consistency criteria and it is well suited for interactive and collaborative applications, where the action of one user must be seen by others in a timely fashion.

INTRODUCTION Sensor networks are used to monitor real-world phenomena. For such monitoring applications, physical time often plays a crucial role. We will discuss these applications of time in Section 1.1.1. Providing synchronized physical time is a complex task due to various challenging characteristics of sensor networks. In Section 1.1.2, we present these challenges and discuss why synchronization algorithms for traditional distributed systems often do not meet these challenges. ii (a) (b) (c) Fig. 1.1 Applications of physical time: (a) interaction of an external observer with the sensor network, (b) interaction among sensor nodes, and (c) interaction of the sensor network with the real world. 1.1.1 The need for synchronized time Physical time plays a crucial role for many sensor-network applications. While many traditional applications of time also apply to sensor networks, we will focus here on areas specific to sensor networks. Figure 1.1 illustrates a rough classificatio