Showing papers by "University of Piraeus published in 2009"

A survey on jamming attacks and countermeasures in WSNs

Abstract: Jamming represents the most serious security threat in the field of wireless sensor networks (WSNs), as it can easily put out of order even WSNs that utilize strong highlayer security mechanisms, simply because it is often ignored in the initial WSN design. The objective of this article is to provide a general overview of the critical issue of jamming in WSNs and cover all the relevant work, providing the interested researcher pointers for open research issues in this field. We provide a brief overview of the communication protocols typically used in WSN deployments and highlight the characteristics of contemporary WSNs, that make them susceptible to jamming attacks, along with the various types of jamming which can be exercised against WSNs. Common jamming techniques and an overview of various types of jammers are reviewed and typical countermeasures against jamming are also analyzed. The key ideas of existing security mechanisms against jamming attacks in WSNs are presented and open research issues, with respect to the defense against jamming attacks are highlighted.

Additive fabrication technologies applied to medicine and health care: a review

Abstract: Additive fabrication (AF) and rapid prototyping (RP) technologies are mostly associated with applications in the product development and the design process as well as with small batch manufacturing. Due to their relatively high speed and flexibility, however, they have also been employed in various non-manufacturing applications. A field that attracts increasingly more attention by the scientific community is related to the application of AF technologies in medicine and health care. The associated research is focused both on the development of specifically modified or new methods and systems based on AF principles, as well as on the applications of existing systems assisting health care services. In this paper, representative case studies and research efforts from the field of AF medical applications are presented and discussed in detail. The case studies included cover applications like the fabrication of custom implants and scaffolds for rehabilitation, models for pre-operating surgical planning, anatomical models for the mechanical testing and investigation of human bones or of new medical techniques, drug delivery devices fabrication, as well as the development of new AF techniques specifically designed for medical applications.

Club Convergence in Carbon Dioxide Emissions

Abstract: We examine convergence in carbon dioxide emissions among 128 countries for the period 1960–2003 by means of a new methodology introduced by Phillips and Sul (Econometrica 75(6):1771–1855, 2007a). Contrary to previous studies, our approach allows us to examine for evidence of club convergence, i.e. identify groups of countries that converge to different equilibria. Our results suggest convergence in per capita CO2 emissions among all the countries under scrutiny in the early years of our sample. However, there seem to be two separate convergence clubs in the recent era that converge to different steady states. Interestingly, we also find evidence of transitioning between the two convergence clubs suggesting either a slow convergence between the two clubs or a tendency for some countries to move from one convergence club to the other.

Do Country or Firm Factors Explain Capital Structure? Evidence from SMEs in France and Greece

Abstract: We investigate the capital structure determinants of small and medium sized enterprises (SMEs) using a sample of Greek and French firms. We address the following questions: Are the capital structure determinants of SMEs in the two countries driven by similar factors? Are potential differences driven by country-specific or firm-specific factors? Are the size and structure of their financial markets important factors to explain any cross-country differences on SME capital structure? To answer these questions we apply panel data methods to the sample of firms for the period 1998 to 2002. We assess the extent to which the debt to assets ratio of firms depends upon their asset structure, size, profitability and growth rate. The results show that the SMEs in both countries exhibit similarities in their capital structure choices. Asset structure and profitability have a negative relationship with leverage, whereas firm size is positively related to their debt to assets ratio. Growth is statistically significant only for France and is positively related to debt. We attribute these similarities to their institutional characteristics and in particular the commonality of their civil law systems. We find differences in the intensity of the capital structure relationship between the two countries. We provide evidence that these differences are due to firm-specific rather than country factors.

Functional architecture

Abstract: In this article, we propose and describe a functional architecture (FA) for the efficient radio and spectrum resource management of the anticipated future compound communication systems. This work comprises an advancement of the FA that has been initially proposed within end-to-end reconfigurability - phase 2 (E2R II) project and has been enhanced to incorporate cognitive and self-x capabilities to address the newly born B3G challenges. Furthermore, the proposed FA is currently being elaborated within the Working Group 3 (WG3) of the Reconfigurable Radio Systems Technical Committee (RRS TC). This committee was created by European Standards Telecommunication Institute (ETSI) Board with the aim to study the feasibility of standardization activities related to reconfigurable radio systems (including software defined and cognitive radios). It should be also mentioned that a relevant functional architecture for optimized radio resource usage in heterogeneous wireless networks is currently under standardization within IEEE.

The relationship between internal and external service quality

Abstract: Purpose – The purpose of this paper is to provide experimental evidence supporting the view that internal service quality has a direct effect on external service quality.Design/methodology/approach – The study focuses on the restaurant industry in Greece. Waiters are considered as internal customers and kitchen personnel as internal suppliers. Inferential analysis included factor analysis on individual waiter and customer data as well as canonical correlation analysis on a restaurant level.Findings – Factor analysis of external service quality revealed six factors including product, organizational image, safety and choice, empathy, reliability as well as responsiveness. Internal service quality factors, additional to those found in external service quality research, included professionalism and internet. Canonical correlation revealed that the internal service quality dimensions of safety, reliability and internet exert a direct positive influence on the external service quality dimensions of organization...

Azathioprine is superior to budesonide in achieving and maintaining mucosal healing and histologic remission in steroid-dependent Crohn's disease.

Abstract: Background: The effects of azathioprine (AZA) and budesonide (BUD) on mucosal healing and histologic remission of Crohn's disease (CD) are insufficiently studied. In this prospective study we evaluated the comparative effects of AZA and BUD on endoscopic and histologic activity in patients with steroid-dependent Crohn's ileocolitis or proximal colitis who had achieved clinical remission on conventional steroids. Methods: Patients were randomized to AZA (2.0–2.5 mg/kg a day) or BUD (6–9 mg a day) for 1 year. The study protocol included clinical examination, laboratory tests, calculation of the Crohn's Disease Activity Index (CDAI), completion of the Inflammatory Bowel Disease Questionnaire (IBDQ), at baseline and then every 2 months for 1 year. Ileocolonoscopy with regional biopsies was performed at baseline and then at the end of the study to assess mucosal healing and the histologic activity of CD. Results: Thirty-eight patients were randomized to AZA and 39 to BUD. At the end of the study 32 and 25 patients in the AZA and BUD groups, respectively, were in clinical remission (P = 0.07). The Crohn's Disease Endoscopic Index of Severity (CDEIS) score fell significantly only in the AZA group (P < 0.0001). Complete or near complete healing was achieved in 83% of AZA-treated patients compared with only 24% of BUD-treated patients (P < 0.0001). Histologic activity as assessed by an average histology score (AHS) fell significantly only in the AZA group (P < 0.001 versus baseline) and was significantly lower than in the BUD group at the end of the study (P < 0.001). Eight patients in the AZA group were withdrawn for adverse events (n = 6) or relapse of disease compared with 14 patients in the BUD group who were withdrawn for relapse of disease. Conclusions: In patients with steroid-dependent inflammatory Crohn's ileocolitis or proximal colitis who achieve clinical remission with conventional steroids, a 1-year treatment with AZA was superior to BUD in achieving and maintaining mucosal healing and histologic remission. (Inflamm Bowel Dis 2008)

Energy efficiency in wireless sensor networks using sleep mode TDMA scheduling

Abstract: Power saving is a very critical issue in energy-constrained wireless sensor networks. Many schemes can be found in the literature, which have significant contributions in energy conservation. However, these schemes do not concentrate on reducing the end-to-end packet delay while at the same time retaining the energy-saving capability. Since a long delay can be harmful for either large or small wireless sensor networks, this paper proposes a TDMA-based scheduling scheme that balances energy-saving and end-to-end delay. This balance is achieved by an appropriate scheduling of the wakeup intervals, to allow data packets to be delayed by only one sleep interval for the end-to-end transmission from the sensors to the gateway. The proposed scheme achieves the reduction of the end-to-end delay caused by the sleep mode operation while at the same time it maximizes the energy savings.

Multi-port vs. Hub-and-Spoke port calls by containerships

Abstract: This paper addresses the design of container liner shipping networks taking into consideration container management issues including empty container repositioning. We examine two typical service networks with different ship sizes: multi-port calling by conventional ship size and hub-and-spoke by mega-ship. The entire solution process is performed in two phases: the service network design and container distribution. A wide variety of numerical experiments are conducted for the Asia–Europe and Asia–North America trade lanes. In most scenarios the multi-port calling is superior in terms of total cost, while the hub-and-spoke is more advantageous in the European trade for a costly shipping company.

Handoff prioritization and decision schemes in wireless cellular networks: a survey

Abstract: Handoff is a key element in wireless cellular networks in order to provide quality of service (QoS) to the users and to support users' mobility. Handoff failure will result in the forced termination of an ongoing call. From the user's point of view, the service of a handoff request is more important, as the forced termination of an ongoing call is more annoying than the blocking of new calls. Therefore, in order to support QoS to the users and to provide ubiquitous coverage, the handoff procedure ought to be further investigated. This paper provides a comprehensive survey of the basic elements, the different types and phases of the handoff procedure. Moreover, particular interest has been given in the horizontal handoff execution phase by discussing and classifying the most recent handoff prioritization schemes into categories based on the concepts that these schemes adopt, e.g. channel reservation, handoff queueing, channel transferred, subrating, genetic and hybrid schemes and in the vertical handoff decision phase by presenting different decision algorithms.

Biofuels: A Survey on Pros and Cons

Abstract: A survey of research findings focusing on issues related to the pros and cons of using biofuels to substitute conventional fuels, namely, fossil fuels is presented in this paper. This controversial topic has attracted great political, economic, and social attention because it is touching interests of world significance during recent years. The paper presents the main biofuel types that are used today or can be used in the future, their properties and characteristics, and their production technologies and focuses on the evaluation and main economic, environmental, and social impacts of biofuels, measuring the pros and cons of their use in energy production. The future of biofuels is also discussed. The paper is concluded with a discussion, some conclusions, and suggestions for further research.

Containership routing with time deadlines and simultaneous deliveries and pick-ups

Abstract: In this paper we seek to determine optimal routes for a containership fleet performing pick-ups and deliveries between a hub and several spoke ports. A capacitated vehicle routing problem with pick-ups, deliveries and time deadlines is formulated and solved using a hybrid genetic algorithm for establishing routes for a dedicated containership fleet. Results on the performance of the algorithm and the feasibility of the approach show that a relatively small fleet of containerships could provide efficient services within deadlines. Moreover, through sensitivity analysis we discuss performance robustness and consistency of the developed algorithm under a variety of problem settings and parameters values.

The Interaction between Mortgage Financing and Housing Prices in Greece

Abstract: Although the close empirical relationship between the evolution of mortgage lending and housing prices is well established in the literature, the direction of causation is less clear from a theoretical standpoint. We apply multivariate cointegration techniques in order to address this issue empirically for the Greek economy. Our results, based on a cointegration relationship that we identify as a mortgage loan demand equation, indicate that housing prices do not adjust to disequilibria in the market for housing loans. This suggests that in the long run the causation does not run from mortgage lending to housing prices. In the short run we find evidence of a contemporaneous bi-directional dependence.

Architecture and enablers for optimized radio resource usage in heterogeneous wireless access networks: The IEEE 1900.4 Working Group

Abstract: Over the past decade or so, the wireless industry has undergone many significant changes. Radio systems have moved toward forming heterogeneous wireless networks: collaborations of multiple radio access networks, which in some cases operate different radio access technologies, such as second- and third-generation cellular RATs, IEEE 802.x wireless standards, and so on. On the other hand, multimode reconfigurable user devices with the ability to choose among various supported RATs have become a reality, and devices and networks with dynamic spectrum access capabilities, allowing real-time sharing of spectrum resource usage among different systems, are expected to be a part of the future radio eco-space. As a result of these changes, there is a need to develop a standard that addresses the requirements and leverages the opportunities posed by such a versatile radio environment. To this end, IEEE 1900.4 aims to standardize the overall system architecture and information exchange between the network and mobile devices, which will allow these elements to optimally choose from available radio resources. In other words, the standard facilitates the distributed dynamic optimization of the usage of spectrum offered by the heterogeneous wireless network, relying on a collaborative information exchange between networks and mobile devices, thereby acting as a common means to improve overall composite capacity and quality of service for the served networks. This article provides a snapshot of IEEE P1900.4 in its current form, covering the scope and purpose of the standard, reference use cases for which the standard is applicable, its system and functional architectures, and finally, the information model for its main interfaces.

A 60 GHz Radio-Over-Fiber Network Architecture for Seamless Communication With High Mobility

Abstract: We demonstrate a 60 GHz broadband picocellular Radio-over-Fiber network architecture that enables seamless connectivity for highly mobile end-users. Its seamless communication capabilities arise by the supported handover scheme that relies on a novel Moving Extended Cell (MEC) concept. MEC exploits user-centric virtual groups of adjacent cells that transmit the same data content to the user and utilizes a switch mechanism for restructuring the virtual multi-cell area according to the user's mobility pattern, so that a virtual antenna group moves together with the mobile user. We present the theoretical formulation for MEC and show that it can provide zero packet loss and call dropping probability values in high-rate wireless services for a broad range of mobile speeds up to 40 m/sec, independently of the fiber link distances. We also demonstrate the physical layer network architecture and switch mechanism both for a RoF network with a single 60 GHz radio frequency (RF) over each wavelength, as well as for a RoF configuration supporting simultaneous multi-RF channel transmission over each optical wavelength. The performance of the multi-RF-over-lambda network implementation is evaluated via simulations showing successful 100 Mb/s radio signal transmission over fiber links longer than 30 km. To this end, MEC can enable seamless connectivity and bandwidth guarantees in 60 GHz picocellular RoF networks being also capable of serving multiple users over the same wavelength in a RF frequency-division-multiplexed (FDM) approach.

Performance and Merton-type default risk of listed banks in the EU: A panel VAR approach

Abstract: This paper provides empirical evidence that sheds new light into the dynamic interactions between risk and efficiency, a highly debated issue. First, we estimate three alternative measures of bank performance, by employing a directional distance function framework, along with a cost frontier and a profit function. As a second step, we calculate a Merton-type bank default risk. Then, we employ a panel VAR analysis, which allows the examination of the underlying relationships between efficiency and risk without applying any a priori restrictions. Most evidence shows that the effect of a one standard deviation shock of the distance to default on inefficiency is negative and substantial. There is some evidence of a reverse causation. As part of a sensitivity analysis, we extent our study to investigate the relationship between efficiency and default risk for banks with different types of ownership structures and across financial systems with different levels of development.

Power Conservation through Energy Efficient Routing in Wireless Sensor Networks

Abstract: The power awareness issue is the primary concern within the domain of Wireless Sensor Networks (WSNs). Most power dissipation ocurrs during communication, thus routing protocols in WSNs mainly aim at power conservation. Moreover, a routing protocol should be scalable, so that its effectiveness does not degrade as the network size increases. In response to these issues, this work describes the development of an efficient routing protocol, named SHPER (Scaling Hierarchical Power Efficient Routing).

Genetic-algorithm-based multi-objective optimization of the build orientation in stereolithography

Abstract: Build orientation is an important fabrication parameter in layer manufacturing (LM) since it affects the part fabrication accuracy, cost, and time. Despite its importance, orientation selection relies quite heavily on the experience and skill of the operator of the LM system, which does not guarantee optimality of the decision. In the present work, a decision support system that automates the orientation selection task is proposed. The proposed system utilizes genetic algorithms and multi-criteria optimization techniques for the definition of (near) optimum build orientation for parts fabricated with stereolithography. Build time, surface roughness, and post-processing time are considered as the main optimization criteria.

A Pattern Similarity Scheme for Medical Image Retrieval

Abstract: In this paper, we propose a novel scheme for efficient content-based medical image retrieval, formalized according to the PAtterns for Next generation DAtabase systems (PANDA) framework for pattern representation and management. The proposed scheme involves block-based low-level feature extraction from images followed by the clustering of the feature space to form higher-level, semantically meaningful patterns. The clustering of the feature space is realized by an expectation-maximization algorithm that uses an iterative approach to automatically determine the number of clusters. Then, the 2-component property of PANDA is exploited: the similarity between two clusters is estimated as a function of the similarity of both their structures and the measure components. Experiments were performed on a large set of reference radiographic images, using different kinds of features to encode the low-level image content. Through this experimentation, it is shown that the proposed scheme can be efficiently and effectively applied for medical image retrieval from large databases, providing unsupervised semantic interpretation of the results, which can be further extended by knowledge representation methodologies.

Clustering Trajectories of Moving Objects in an Uncertain World

Abstract: Mining Trajectory Databases (TD) has recently gained great interest due to the popularity of tracking devices. On the other hand, the inherent presence of uncertainty in TD (e.g., due to GPS errors) has not been taken yet into account during the mining process. In this paper, we study the effect of uncertainty in TD clustering and introduce a three-step approach to deal with it. First, we propose an intuitionistic point vector representation of trajectories that encompasses the underlying uncertainty and introduce an effective distance metric to cope with uncertainty. Second, we devise CenTra, a novel algorithm which tackles the problem of discovering the Centroid Trajectory of a group of movements. Third, we propose a variant of the Fuzzy C-Means (FCM) clustering algorithm, which embodies CenTra at its update procedure. The experimental evaluation over real world TD demonstrates the efficiency and effectiveness of our approach.

A randomized phase III study comparing three anthracycline-free taxane-based regimens, as first line chemotherapy, in metastatic breast cancer

Abstract: Background Effective anthracycline-free combinations need to be evaluated in metastatic breast cancer (MBC), due to the increased number of patients treated with anthracycline-based adjuvant chemotherapy. Patients and methods Patients with MBC were randomized to paclitaxel and carboplatin (PCb) every 3 weeks or docetaxel and gemcitabine (GDoc) every 3 weeks or weekly paclitaxel (Pw). Trastuzumab was given to patients with HER-2 over-expressing tumors. The primary endpoint of the study was survival. Quality of life (QoL) and cost were assessed. Results Totally, 416 eligible patients entered the study. Median survival times were 29.9 months for PCb, 26.9 for GDoc and 41.0 for Pw (P = 0.037). According to multivariate analysis, adjuvant chemotherapy, >1 metastatic sites, lack of maintenance hormonal therapy, and worse performance status (PS) were significant adverse prognostic factors for survival, while Pw when compared to GDoc improved survival (P = 0.03), as well as when compared to PCb in the subgroup of patients with PS = 1 (P = 0.01, treatment by PS interaction P = 0.03). No significant differences in terms of time to progression were found. Severe myelotoxicity and mucositis were more frequent with GDoc, while severe neuropathy with PCb and Pw. QoL changes did not differ significantly between treatment groups, while cost analysis favored Pw. Conclusions Pw appears to be the most preferable choice among the 3 anthracycline-free taxanes-based regimens tested in the present study.