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System Availability (system + availability)

Distribution by Scientific Domains

Mathematics and Statistics	57%

Selected Abstracts

Comparison of steady system availability with imperfect repair

APPLIED STOCHASTIC MODELS IN BUSINESS AND INDUSTRY, Issue 1 2004
Ji Hwan Cha
Abstract Availability is a very important measure of performance for a repairable system which can be in one of two states: operational and non-operational. In this paper we compare steady system availabilities of two different systems which are subject to imperfect repair policy. Comparisons of the measure are made based on failure rate orderings, usual stochastic orderings, repair policy orderings and classifications of life distributions. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Concepts for computer center power management

CONCURRENCY AND COMPUTATION: PRACTICE & EXPERIENCE, Issue 2 2010
A. DiRienzo
Abstract Electrical power usage contributes significantly to the operational costs of large computer systems. At the Hypersonic Missile Technology Research and Operations Center (HMT-ROC) our system usage patterns provide a significant opportunity to reduce operating costs since there are a small number of dedicated users. The relatively predictable nature of our usage patterns allows for the scheduling of computational resource availability. We take advantage of this predictability to shut down systems during periods of low usage to reduce power consumption. With interconnected computer cluster systems, reducing the number of online nodes is more than a simple matter of throwing the power switch on a portion of the cluster. The paper discusses these issues and an approach for power reduction strategies for a computational system with a heterogeneous system mix that includes a large (1560-node) Apple Xserve PowerPC supercluster. In practice, the average load on computer systems may be much less than the peak load although the infrastructure supporting the operation of large computer systems in a computer or data center must still be designed with the peak loads in mind. Given that a significant portion of the time, systems loads can be less than full peak, an opportunity exists for cost savings if idle systems can be dynamically throttled back, slept, or shut off entirely. The paper describes two separate strategies that meet the requirements for both power conservation and system availability at HMT-ROC. The first approach, for legacy systems, is not much more than a brute force approach to power management which we call Time-Driven System Management (TDSM). The second approach, which we call Dynamic-Loading System Management (DLSM), is applicable to more current systems with ,Wake-on-LAN' capability and takes a more granular approach to the management of system resources. The paper details the rule sets that we have developed and implemented in the two approaches to system power management and discusses some results with these approaches. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Approximate urban area call drop-out probability estimation in mobile multi-satellite systems

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING, Issue 2 2003
F. Perez-Fontan
Land mobile satellite (LMS) systems require constellations of multiple satellites to compensate for the large blockage probabilities suffered in urban areas. An approximate methodology based on the street masking function (MKF) or mask concept is presented for the evaluation of system availability. MKFs are graphical representations of the angles (azimuths and elevations) for which a satellite is visible from the terminal's location in the street. MKFs can also be used in the evaluation of second order statistics such as durations of fades or connections. In this paper, the applicability of the mask concept is demonstrated by performing a study of the probability of successful call completion in a LEO-based satellite telephony service. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Dynamic fade restoration in Ka-band satellite systems

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING, Issue 4 2002
A. Paraboni
Atmospheric precipitation can seriously affect the propagation of centimeter and millimeter electromagnetic waves. As a consequence, in some applications, it is necessary to make use of a fade countermeasure technique in order to satisfy the system availability and quality requirements. This study analyses the performance of a satellite-based system in geo-stationary orbit operating at 20 GHz, dynamically assigning the antenna directivity pattern to counteract tropospheric attenuation. The on-board power is spatially distributed over the covered region to minimize, at any time, the number of users undergoing outage because of the tropospheric attenuation. Both the aspects of broadcasting and telecommunication services are addressed. The reflector antenna of the system is supposed to be illuminated by a cluster of feeds driven by a set of excitation coefficients, continuously modified and optimized according to the meteorological information derived by processing METEOSAT satellite data and ECMWF (European Centre for Medium-Range Weather Forecasts) data. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Optimal burn-in procedure for periodically inspected systems

NAVAL RESEARCH LOGISTICS: AN INTERNATIONAL JOURNAL, Issue 7 2007
Ji Hwan Cha
Abstract Burn-in is a widely used method to improve the quality of products or systems after they have been produced. In this paper, we study burn-in procedure for a system that is maintained under periodic inspection and perfect repair policy. Assuming that the underlying lifetime distribution of a system has an initially decreasing and/or eventually increasing failure rate function, we derive upper and lower bounds for the optimal burn-in time, which maximizes the system availability. Furthermore, adopting an age replacement policy, we derive upper and lower bounds for the optimal age parameter of the replacement policy for each fixed burn-in time and a uniform upper bound for the optimal burn-in time given the age replacement policy. These results can be used to reduce the numerical work for determining both optimal burn-in time and optimal replacement policy. © 2007 Wiley Periodicals, Inc. Naval Research Logistics, 2007 [source]

Optimal corrective maintenance contract planning for aging multi-state system

APPLIED STOCHASTIC MODELS IN BUSINESS AND INDUSTRY, Issue 5 2009
Yi Ding
Abstract This paper considers an aging multi-state system, where the system failure rate varies with time. After any failure, maintenance is performed by an external repair team. Repair rate and cost of each repair are determined by a corresponding corrective maintenance contract with a repair team. The service market can provide different kinds of maintenance contracts to the system owner, which also can be changed after each specified time period. The owner of the system would like to determine a series of repair contracts during the system life cycle in order to minimize the total expected cost while satisfying the system availability. Operating cost, repair cost and penalty cost for system failures should be taken into account. The paper proposes a method for determining such optimal series of maintenance contracts. The method is based on the piecewise constant approximation for an increasing failure rate function in order to assess lower and upper bounds of the total expected cost and system availability by using Markov models. The genetic algorithm is used as the optimization technique. Numerical example is presented to illustrate the approach. Copyright © 2009 John Wiley & Sons, Ltd. [source]