Oblivious Gradient Clock Synchronization

Abstract. We study the gradient clock synchronization (GCS) problem, in which the worst-case clock skew between neighboring nodes has to be minimized. In particular, we consider oblivious clock synchronization algorithms which base their decision on how to adapt the clock solely on the most accurate timing information received from each neighbor. For several intuitive clock synchronization algorithms, which attempt to minimize the skew at all times, we show that the clock skew between neighboring nodes can be significantly larger than the proven lower bound of Ω( log D log log D), where D denotes the diameter of the network. All of these natural algorithms belong to the class of oblivious clock synchronization algorithms. Additionally, we present an oblivious algorithm with a worstcase skew of O(d + √ D) between any two nodes at distance d

Moisture dependent thermal properties of cassia

Thermal and mechanical properties of cassia are primarily required in the study of its different thermal and mechanical operations. The thermal properties of cassia bark viz., specific heat, glass transition temperature (Tg), bulk thermal conductivity, and bulk thermal diffusivity were determined for a moisture range of 8–14% dry basis (d.b). Compression test was performed by Universal Testing Machine to examine the breaking characteristics of cassia along its three orthogonal directions at 11.10% d.b. moisture level. Results revealed that the specific heat increased from 1,553.12 to 3,232.90 J kg−1 °C−1 with an increase in moisture at 30°C. Whereas Tg decreased linearly from −66.67 to −86.09°C and the bulk thermal conductivity increased linearly from 0.10 to 0.14 W m−1 °C−1 with a rise in moisture content of the sample. Moreover, the bulk thermal diffusivity of cassia bark decreased from 2.75 × 10−7 to 1.62 × 10−7 m2 s−1 with an increase in moisture level. The average rupture forces for cassia bark through the minor axis, major and intermediate axis were 0.130, 0.123 and 0.109 kN, respectively, and corresponding average deformations at rupture were 1.36, 1.30 and 0.91 mm, respectively. In conclusion, the thermal properties of cassia were significantly affected by the variation in the moisture level. Also, the mechanical properties such as rupture force and rupture deformation demonstrated a prominent difference in their values along the three axes of the sample

Clock Synchronization for Wireless Networks

Abstract. Time synchronization is a fundamental service in many wireless applications. While the synchronization problem is well-studied in traditional wired networks, physical constraints of the wireless medium impose a unique set of challenges. We present a novel time synchronization algorithm which is highly energy efficient and failure/recoverytolerant. Our algorithm allows nodes to synchronize to sources of real time such as GPS when such signals are available, but continues to synchronize nodes to each other, even in the absence of GPS. In addition, the algorithm satisfies a relaxed gradient property, in which the degree of synchronization between nodes varies as a linear function of their distance. Thus, nearby nodes are highly synchronized, which is desirable in many wireless applications.

Unifying Byzantine consensus algorithms with Weak Interactive Consistency

The paper considers the consensus problem in a partially synchronous system with Byzantine processes. In this context, the literature distinguishes (1) authenticated Byzantine faults, where messages can be signed by the sending process (with the assumption that the signature cannot be forged by any other process), and (2) Byzantine faults, where there is no mechanism for signatures (but the receiver of a message knows the identity of the sender). The paper proposes an abstraction called weak interactive consistency (WIC) that unifies consensus algorithms with and without signed messages. WIC can be implemented with and without signatures. The power of WIC is illustrated on two seminal Byzantine consensus algorithms: the Castro-Liskov PBFT algorithm (no signatures) and the Martin-Alvisi FaB Paxos algorithms (signatures). WIC allows a very concise expression of these two algorithms. Moreover, using a implementation of WIC without signatures allows us to derive a signature-free variant o

Le surhomme à bout de souffle: le Belmondo des années 1975-1985

Abstract. Reference-Broadcast Synchronization (RBS) is a technique that allows a set of receivers in a broadcast network to accurately estimate each others ’ clock values. RBS provides a relative time-frame for conversion between the local clocks of different nodes, and can be used to synchronize nodes to an external time-source such as GPS. However, RBS by itself does not output a logical clock at every node, and so it does not solve internal clock synchronization. In this work we study the theoretical properties of RBS in the worstcase model, in which the performance of a clock synchronization algorithm is measured by the worst-case skew it can incur. We suggest a method by which RBS can be incorporated in standard internal clock synchronization algorithms. This is achieved by separating the task of estimating the clock values of other nodes in the network from the task of using these estimates to output a logical clock value. The separation is modelled using a virtual estimate graph, overlaidon top of the real network graph, which represents the information various nodes can obtain about each other. RBS estimates are represented in the estimate graph as edges between nodes at distance 2 from each other in the original network graph. A clock synchronization algorithm then operates on the estimate graph as though it were the original network. To illustrate the merits of this approach, we modify a recent optimal gradient clock synchronization algorithm to work in this setting. The modified algorithm transparently takes advantage of RBS estimates. Its quality of synchronization depends on the diameter of the estimate graph, which is typically much smaller than the diameter of the original network graph. Keywords: Gradient Clock Synchronization, Wireless Networks.