Vulnerability of Urban Water Distribution Networks under Intermittent Water Supply Operations

The work presented herein investigates the effects of intermittent water supply (IWS) on the condition and breakage rate of urban water distribution piping networks (WDN), by studying the change in the rate of occurrence of failures before, during and after IWS periods, using statistical and survival analyses. The analyses, based on a seven-year dataset (2003–2010) from a major urban center of about 300,000 residents, take into account information related with breakage incidents and with operating system parameters, as well as external factors and vulnerability assessments of the network’s key components. The results show an increase in the number of waterloss incidents during and immediately after the periods during which IWS practices were implemented, and they reinforce the belief that IWS practices negatively affect the vulnerability of WDNs.     

Using computer supported collaborative learning strategies for helping students acquire self-regulated problem-solving skills in mathematics

The main objective of this paper is to investigate the effectiveness of a proposed computer-based instructional method in Primary Education for self-regulated problem solving. The proposed instructional method is based on Sternberg’s model of problem solving within an authentic context. It consists of three main phases: observation, collaboration and semi-structured guidance. The ultimate learning objective is to augment the autonomous problem-solving skills of primary school children. In our study the Synergo tool was used, which is a synchronous computer supported collaborative learning tool, as well as the Moodle learning management system. The context which frames the method is authentic, very close to a students’ realistic learning situation. The findings of this study advocate that students can increase their problem-solving skills in a relatively short period of time. At the same time, they can improve their approach to the solution of a given mathematical problem, performing significant signs of autonomy.     

Malware-Propagative Mobile Ad Hoc Networks: Asymptotic Behavior Analysis

In this paper, the spreading of malicious software over ad hoc networks, where legitimate nodes are prone to propagate the infections they receive from either an attacker or their already infected neighbors, is analyzed. Considering the Susceptible-Infected-Susceptible （SIS） node infection paradigm we propose a probabilistic model, on the basis of the theory of closed queuing networks, that aims at describing the aggregated behavior of the system when attacked by malicious nodes. Because of its nature, the model is also able to deal more effectively with the stochastic behavior of attackers and the inherent probabilistic nature of the wireless environment. The proposed model is able to describe accurately the asymptotic behavior of malware-propagative large scale ad hoc networking environments. Using the Norton equivalent of the closed queuing network, we obtain analytical results for its steady state behavior, which in turn is used for identifying the critical parameters affecting the operation of the network. Finally, through modeling and simulation, some additional numerical results are obtained with respect to the behavior of the system when multiple attackers are present, and regarding the time-dependent evolution and impact of an attack.     

An efficient and fair cooperative approach for resource management in wireless networks

In cellular networks, the implementation of various resource management processes, such as bandwidth reservation and location updates, has been based on the one‐to‐one resource management information exchange paradigm, between the mobile nodes and the base stations. In this paper, we design and demonstrate the use of a distributed cooperative scheme that can be applied in the future wireless networks to improve the energy consumption for the routine management processes of mobile terminals, by adopting the peer‐to‐peer communication concept of wireless ad hoc networks. In our approach, the network is subdivided into one‐hop ad hoc clusters where the members of each cluster cooperate to perform the required management functions, and conventional individual direct report transmissions of the mobile terminals to the base stations are replaced by two‐hop transmissions. The performance evaluation and the corresponding numerical results presented in this paper confirm that our proposed scheme reduces significantly the overall system energy consumption when compared with the conventional one‐to‐one direct information management exchange approach. Furthermore the issue of fairness in dynamically selecting the various cluster heads in successive operational cycles of the proposed scheme is analyzed, and an enhanced algorithm is proposed and evaluated, which improves significantly the cluster head selection fairness, in order to balance the energy consumption among the various mobile terminals. Copyright © 2005 John Wiley & Sons, Ltd.