1,721,179 research outputs found
Evaluation of customer’s losses and value-at-risk under cloud outages
No full textCloud services may suffer from outages, which interrupt customers' activities and result in economic losses. A linear model is proposed here to translate outages into economic losses, accounting for both the frequency and the duration of outages. Two tools, the exceedance loss probability and the Value-at-Risk, are employed to assess the maximum economic exposure of a customer. The use of both tools is demonstrated by applying them to a dataset concerning five major cloud providers. Those tools can be employed to drive the choice of a cloud provider towards the least risky one
Concentration indices and Zipf's law
No full textThe relationships between Zipf s law (a relevant model for the firms' size and the financial investment amounts) and the major concentration indices are derived. The Hirschman-Herfindahl index is the most sensitive index in contexts where Zipf s law applies. (C) 2002 Elsevier Science B.V. All rights reserved
Connectivity of Waxman topology models
No full textWaxman graphs are a popular class of random graphs used for modelling the Internet topology, especially for the intra-domain part. When used for network modelling purposes their connectedness properties are particularly relevant, both for the characteristics of the realized graph and for the generation time. In this paper, the probability of obtaining a connected Waxman graph is derived analytically for the general case of nodes disposed over a rectangular area (as opposed to the special case of an underlying square area already investigated in the literature) and is related to the values of the Waxman parameters used in the generation of the graph. (c) 2005 Elsevier B.V. All rights reserved
The impact of the temperature dependent thermal conductivity of insulating materials on the effective building envelope performance
No full textConstant thermal conductivity values of insulating materials are typically used in building design and assessment. However, the thermal conductivity depends on many factors, such as on the temperature and moisture content. Linear temperature-dependent laws have been occasionally proposed for inorganic fibrous materials such as fiberglass or rock wool that exhibit a decreased thermal conductivity (better performance) at low temperatures. However, the petrochemical-foamed insulating materials such as the polyisocyanurate, have less regular temperature dependent behaviors with a poorer performance at both extremely cold and warm temperatures. This means that the use of constant thermal conductivity values results in actual building envelope performance different from the design predictions, with increasing building energy consumptions, greater risks of condensation issues, and decreased occupants’ comfort. This paper aims to quantify the impact of the temperature dependency of the thermal conductivity in exterior walls and flat roofs. Experimental results over a large temperature range (from −20 °C to +60 °C) for different insulating materials were used in hygrothermal simulations in both continental and humid temperate climates. Common insulating materials were taken into accounts, such as fiberglass, rock wool, polyisocyanurate, and extruded polystyrene. The increase in the energy fluxes from common building envelopes once the effective thermal conductivity was considered resulted below 10% for walls and as high as 70% for roofs, especially in the cold weather. Finally, a hybrid insulation system constituted by two layers of different materials, i.e. a polyisocyanurate and a rock wool is investigated
Detection of anomalous bids in procurement auctions
Full text linkProcurement auctions may be affected by abnormally low bids, whose acceptance may have negative consequences on the auctioneer. A method, based on the average submitted bid, is considered to detect such anomalous bids and aid the auctioneer in the possible rejection decision. Analytical expressions or simulation results are provided for the detection probability and for the false alarm probability. The performances heavily depend on the number of tenderers and on the dispersion of bid values. Both performance indices improve as the number of tenderers grows and generally degrade as the dispersion grows. The presence of multiple anomalous bids leads to a significant worsening of the performance, while courtesy bids raise both the false alarm probability and the detection probability. The use of the average-bid criterion, though officially endorsed in national legislations, is therefore recommended as a strongly precautionary criterion, i.e. when the need to avoid anomalous bids is considered much more relevant than the costs associated to deeper investigation of anomalous bids or to the erroneous rejection of regular bids. (c) 2008 Elsevier B.V. All rights reserved
Accuracy of third-party cloud availability estimation through ICMP
No full textThird-party measurements of cloud availability are needed as a checkpoint of cloud providers' quality statements, and ICMP has been proposed to carry on such measurements. Simple ICMP-based measurement schemes, however, provide a poor discrimination between false outages and true cloud outages, unless the cloud outage probability is quite higher than the packet loss rate incurred by probing packets due to network failures. The use of a closely spaced sequence of probing instances, proposed to get rid of false outages, is ineffective for the purpose, unless the cloud itself is free of unavailability glitches
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