75,750 research outputs found
Expert System for UNIX System Reliability and Availability Enhancement
Highly reliable and available systems are critical to the airline industry. However, most off-the-shelf computer operating systems and hardware do not have built-in fault tolerant mechanisms, the UNIX workstation is one example. In this research effort, we have developed a rule-based Expert System (ES) to monitor, command, and control a UNIX workstation system with hot-standby redundancy. The ES on each workstation acts as an on-line system administrator to diagnose, report, correct, and prevent certain types of hardware and software failures. If a primary station is approaching failure, the ES coordinates the switch-over to a hot-standby secondary workstation. The goal is to discover and solve certain fatal problems early enough to prevent complete system failure from occurring and therefore to enhance system reliability and availability. Test results show that the ES can diagnose all targeted faulty scenarios and take desired actions in a consistent manner regardless of the sequence of the faults. The ES can perform designated system administration tasks about ten times faster than an experienced human operator. Compared with a single workstation system, our hot-standby redundancy system downtime is predicted to be reduced by more than 50 percent by using the ES to command and control the system
Game Theory Meets Network Security: A Tutorial at ACM CCS
The increasingly pervasive connectivity of today's information systems brings
up new challenges to security. Traditional security has accomplished a long way
toward protecting well-defined goals such as confidentiality, integrity,
availability, and authenticity. However, with the growing sophistication of the
attacks and the complexity of the system, the protection using traditional
methods could be cost-prohibitive. A new perspective and a new theoretical
foundation are needed to understand security from a strategic and
decision-making perspective. Game theory provides a natural framework to
capture the adversarial and defensive interactions between an attacker and a
defender. It provides a quantitative assessment of security, prediction of
security outcomes, and a mechanism design tool that can enable
security-by-design and reverse the attacker's advantage. This tutorial provides
an overview of diverse methodologies from game theory that includes games of
incomplete information, dynamic games, mechanism design theory to offer a
modern theoretic underpinning of a science of cybersecurity. The tutorial will
also discuss open problems and research challenges that the CCS community can
address and contribute with an objective to build a multidisciplinary bridge
between cybersecurity, economics, game and decision theory
EPR spectrum via entangled states for an Exchange-Coupled Dimer of Single-Molecule Magnets
Multi-high-frequency electron paramagnetic resonance(EPR) spectrum for a
supermolecular dimer of single-molecule magnets recently reported
[S. Hill, R. S. Edwards, N. Aliaga-Alcalde and G. Christou(HEAC), Science 302,
1015 (2003)] is studied in terms of the perturbation method in which the
high-order corrections to the level splittings of degenerate states are
included. It is shown that the corresponding eigenvectors are composed of
entangled states of two molecules. The EPR-peak positions are calculated in
terms of the eigenstates at various frequencies.
From the best fit of theoretical level splittings with the measured values we
obtain the anisotropy constant and exchange coupling which are in agreement
with the corresponding values of experimental observation. Our study confirms
the prediction of HEAC that the two units within the dimer are coupled
quantum mechanically by the antiferromagnetic exchange interaction and the
supermolecular dimer behaviors in analogy with artificially fabricated quantum
dots.Comment: 16 pages,2 figures, 2 table
Thermoelectric efficiency at maximum power in low-dimensional systems
Low-dimensional electronic systems in thermoelectrics have the potential to
achieve high thermal-to-electric energy conversion efficiency. A key measure of
performance is the efficiency when the device is operated under maximum power
conditions. Here we study the efficiency at maximum power of three
low-dimensional, thermoelectric systems: a zero-dimensional quantum dot (QD)
with a Lorentzian transmission resonance of finite width, a one-dimensional
(1D) ballistic conductor, and a thermionic (TI) power generator formed by a
two-dimensional energy barrier. In all three systems, the efficiency at maximum
power is independent of temperature, and in each case a careful tuning of
relevant energies is required to achieve maximal performance. We find that
quantum dots perform relatively poorly under maximum power conditions, with
relatively low efficiency and small power throughput. Ideal one-dimensional
conductors offer the highest efficiency at maximum power (36% of the Carnot
efficiency). Whether 1D or TI systems achieve the larger maximum power output
depends on temperature and area filling factor. These results are also
discussed in the context of the traditional figure of merit
The nature of symmetric instability and its similarity to convective and inertial instability
It is shown that there exists a local similarity among SI (Symmetric Instability), BI (Buoyancy or Convective Instability), and II (Inertial Instability) even for fully nonlinear viscous motion. The most unstable slope angles for SI and Moist SI motions are analyzed based on parcel energetics. These considerations also suggest qualitatively that CSI (Conditional SI) circulations will be slantwise and lie between the moist most unstable slope and dry least stable slope of the basic state
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Investigate the impacts of assimilating satellite rainfall estimates on rainstorm forecast over southwest United States
Using the MM5-4DVAR system, a monsoon rainstorm case over southern Arizona (5-6 August 2002) was investigated for the influence of assimilating satellite rainfall estimates on precipitation forecasts. A set of numerical experiments was conducted with multiple configurations including using 20-km or 30-km grid distances and none or 3-h or 6-h assimilation time windows. Results show that satellite rainfall assimilation can improve the rainstorm-forecasting pattern and amount to some extent. The minimization procedure of 4DVAR is sensitive to model spatial resolution and the assimilation time window. The 3-h assimilation window with hourly rainfall data works well for the 6-h forecast, and for 12-h or longer forecasts, a 6-h assimilation window will be requested. Copyright 2004 by the American Geophysical Union
Experimental and theoretical research on the electrical conductivity of a liquid desiccant for the liquid desiccant air-conditioning system: LiCl aqueous solution
At present, the energy consumption in buildings occupies a large proportion of total energy use, and air-conditionings cost a large proportion of energy in the buildings. The liquid desiccant air-conditioning system has a good energy saving potential and the electrodialysis (ED) regeneration is a reliable choice for the liquid desiccant regeneration. In order to establish the energy consumption model and the performance coefficient model of liquid desiccant air-conditioning system based on ED regeneration using LiCl, experimental and theoretical research on the electrical conductivity of LiCl aqueous solution with a lot of concentrations and temperatures was conducted in this paper. The results show that when polynomial degrees of the mass concentration and the temperature of the LiCl aqueous solution are both 3, the electrical conductivity model for the LiCl aqueous solution is most suitable as its simplicity and high accuracy. Moreover, when the concentration is 36% and the temperature is 22 °C, the liquid desiccant cooling system has the maximum COP of about 5. Finally, a case study of a small office room was conducted, and the result shows that the liquid desiccant cooling system based on electrodialysis regeneration has a good energy-saving potential
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