Steward: Scaling Byzantine Fault-Tolerant Replication to Wide Area Networks

This paper presents the first hierarchical Byzantine fault-tolerant replication architecture suitable to systems that span multiple wide-area sites. The architecture confines the effects of any malicious replica to its local site, reduces message complexity of wide-area communication, and allows read-only queries to be performed locally within a site for the price of additional standard hardware. We present proofs that our algorithm provides safety and liveness properties. A prototype implementation is evaluated over several network topologies and is compared with a flat Byzantine fault-tolerant approach. The experimental results show considerable improvement over flat Byzantine replication algorithms, bringing the performance of Byzantine replication closer to existing benign fault-tolerant replication techniques over wide area networks.     

How to calculate symmetries of Petri nets

Symmetric net structure yields symmetric net behaviour. Thus, knowing the symmetries of a net, redundant calculations can be skipped. We present a framework for the calculation of symmetries for several net classes including place/transition nets, timed nets, stochastic nets, self–modifying nets, nets with inhibitor arcs, and many others. Our approach allows the specification of different symmetry groups. Additionally it provides facilities either to calculate symmetries on demand while running the actual analysis algorithm, or to calculate them in advance. For the latter case we define and calculate a ground set of symmetries. Such a set has polynomial size and is sufficient for an efficient implementation of the for all symmetries loop and the partition of net elements into equivalence classes. These two constructions are the usual way to integrate symmetries into an analysis algorithm. Received 7 July 1997 / 10 August 1999     

Advanced networking services for distributed multimedia streaming applications

Increased network speeds coupled with new services delivered via the Internet have increased the demand for intelligence and flexibility in network systems. This paper argues that both can be provided by new hardware platforms comprised of heterogeneous multi-core systems with specialized communication support. We present and evaluate an experimental network service platform that uses an emergent class of devices—network processors—as its communication support, coupled via a dedicated interconnect to a host processor acting as a computational core. A software infrastructure spanning both enables the dynamic creation of application-specific services on the network processor, mediated by middleware and controlled by kernel-level communication support. Experimental evaluations use a Pentium IV-based computational core coupled with an IXP 2400 network processor. The sample application services run on both include an image manipulation application and application-level multicasting.

Learning automata-based solutions to the nonlinear fractional knapsack problem with applications to optimal resource allocation.

This paper considers the nonlinear fractional knapsack problem and demonstrates how its solution can be effectively applied to two resource allocation problems dealing with the World Wide Web. The novel solution involves a "team" of deterministic learning automata (LA). The first real-life problem relates to resource allocation in web monitoring so as to "optimize" information discovery when the polling capacity is constrained. The disadvantages of the currently reported solutions are explained in this paper. The second problem concerns allocating limited sampling resources in a "real-time" manner with the purpose of estimating multiple binomial proportions. This is the scenario encountered when the user has to evaluate multiple web sites by accessing a limited number of web pages, and the proportions of interest are the fraction of each web site that is successfully validated by an HTML validator. Using the general LA paradigm to tackle both of the real-life problems, the proposed scheme improves a current solution in an online manner through a series of informed guesses that move toward the optimal solution. At the heart of the scheme, a team of deterministic LA performs a controlled random walk on a discretized solution space. Comprehensive experimental results demonstrate that the discretization resolution determines the precision of the scheme, and that for a given precision, the current solution (to both problems) is consistently improved until a nearly optimal solution is found--even for switching environments. Thus, the scheme, while being novel to the entire field of LA, also efficiently handles a class of resource allocation problems previously not addressed in the literature.     

Three‐dimensional numerical modelling of the flushing process of the Kali Gandaki hydropower reservoir

Sediments filling reservoirs is a common problem in the world today, with an estimated 1% of the capacity of hydropower reservoirs being lost annually through sedimentation. One of the most used techniques for reducing this problem is reservoir flushing. During a flood, the water level is drawn down, causing increased velocities, therefore facilitating erosion and sediment transport. During the flushing, water from the reservoir will be lost, resulting in significant economic implications for the reservoir owner. The success of reservoir flushing depends on several parameters, including water discharge, sediment properties and reservoir geometry. This study describes the use of Computational Fluid Dynamics (CFD) as a modern method to predict the reservoir flushing process. A three‐dimensional numerical model (SSIIM 2), with an adaptive, non‐orthogonal and unstructured grid has been used. Through the application of special modified algorithms (e.g., wetting/drying, free water surface), numerical modelling of sediment movement can be an alternative for planning and optimizing the flushing process for complex reservoir geometries. The numerical model was tested against data from a physical model study of the Kali Gandaki hydropower reservoir in Nepal. The total quantity of flushed out sediments, and the bed deformation in six cross‐sections, were compared, highlighting a good correspondence between the results. These include the cross‐sectional shape of a 90‐degree bend, for which secondary currents influenced the results. The study indicates that numerical models might become a useful tool for reservoir flushing predictions.     

Penetration of fluorescent homologous serum proteins into the wall of the aorta in rats with acute angiotensin hypertension

A few hours of acute angiotensin hypertension in rats increased the permeability of the thoracic part of the aorta for plasma components. The permeability, which was demonstrated by means of homologous circulating fluorescent serum proteins, seemed to be diffuse rather than focal. The fluorescent proteins penetrated into the whole thickness of the aortic wall, but were to a great extent deposited in the subendothelial space and the most luminal part of the tunica media. The deposition took place in the greater part of the circumference of the aortic wall. These results are in agreement with the insudative theory of atherogenesis, and the initial stage seems to be a "damming back" of some of the serum or plasma proteins for deposition in the subendothelial space and the most luminal part of the tunica media.     

Appropriate Material Properties for Advective–Diffusive Solute Flux in Membrane Soil

A generalized advective–diffusive equation for describing electrolytic solute transport in fine-grained soil (clay) membranes is presented. Clarification is made between the current theory and the widely cited coupled-flow theory by Yeung and Mitchell. Results of recently published experimental data are evaluated to demonstrate that the effective diffusion coefficient associated with Fick’s first law is not an appropriate independent material property for solute transport considerations in membrane soil. Three independent and experimentally attainable material properties are identified, specifically: osmotic efficiency, membrane diffusion coefficient, and hydraulic conductivity. Simple physical experiments by which these properties may be measured are described.     

Experimental setup for camera-based measurements of electrically and optically stimulated luminescence of silicon solar cells and wafers

We report in detail on the luminescence imaging setup developed within the last years in our laboratory. In this setup, the luminescence emission of silicon solar cells or silicon wafers is analyzed quantitatively. Charge carriers are excited electrically (electroluminescence) using a power supply for carrier injection or optically (photoluminescence) using a laser as illumination source. The luminescence emission arising from the radiative recombination of the stimulated charge carriers is measured spatially resolved using a camera. We give details of the various components including cameras, optical filters for electro- and photo-luminescence, the semiconductor laser and the four-quadrant power supply. We compare a silicon charged-coupled device (CCD) camera with a back-illuminated silicon CCD camera comprising an electron multiplier gain and a complementary metal oxide semiconductor indium gallium arsenide camera. For the detection of the luminescence emission of silicon we analyze the dominant noise sources along with the signal-to-noise ratio of all three cameras at different operation conditions.