1,720,960 research outputs found
Measuring the Connectivity of a Cognitive Radio Ad-Hoc Network
No full textIn Cognitive Radio Ad-Hoc Networks (CRAHN) the behavior of the primary users influences the secondary network connectivity and the relevant performance. The methodologies used to evaluate network connectivity have to be worked up to take into account this aspect. In this letter we propose the use of Laplacian matrix and its second smallest eigenvalue to measure the network algebraic connectivity of a CRAHN. We re-elaborate the Laplacian matrix in order to have in its second smallest eigenvalue a function of the primary users behavior expressed as an activity factor. In this way we are able to monitor the algebraic connectivity of CRAHNs. This metric can be a useful instrument for network planning, data routing and network maintenance. Performance results show how this methodology can be efficiently applied in this kind of networks
Performance analysis of IEEE 802.15.4 wireless sensor networks: An insight into the topology formation process
No full textTopology formation is an important issue in a wireless sensor network. Performance parameters such as energy consumption, network lifetime, data delivery delay. sensor field coverage depend on the network topology. In this paper, we analyze the process of formation of a wireless sensor network according to the IEEE 802.15.4/ZigBee standards. We focus on both single-sink scenarios and multi-sink ones where: (i) we characterize the topology in terms of network depth and nodes distribution at different network levels; (ii) we analyze some network performance as a function of the number of sinks; (iii) we investigate the effects of some topology constraints on network performance; (iv) we study the effects of nodes mobility on the network formation. The whole study is performed by taking into account the specific features and recommendations of the IEEE 802.15.4/ZigBee standards: thus, our results can be used to configure IEEE 802.15.4/ZigBee procedures and set the related parameters, as a function of the desired application requirements. (C) 2009 Elsevier B.V. All rights reserved
Constraining the network topology in IEEE 802.15.4
No full textThe IEEE 802.15.4 standard defines a MAC association procedure that allows devices of a Wireless Personal Area Network (WPAN) to associate one each other in accordance to parent-child relationships. This standard can be adopted in Wireless Sensor Networks (WSNs) used to monitor environmental phenomena and to collect data in specific nodes named sinks. The ZigBee Alliance, with the Zig-Bee Specification, defines upper layers of a WSN architecture based on the IEEE 802.15.4. Tree shaped multi-sink networks can be formed by adopting in conjunction these two standards. A key aspect to be evaluated for these kind of networks is how their performance are affected by constraining some topological characteristics. In this paper, in accordance to the ZigBee Specification, we constraint the tree depth of a WSN and evaluate some performance metrics that have a remarkable relevance for WSNs. Moreover, we propose a new strategy to join an existing WPAN that allows a node to select a suitable coordinator to connect with. Performance results show the trade-off in the selection of the maximum tree depth. While benefits of having reduced tree depths can be measured during traffic delivery, due to a reduction of the energy consumption, a counter-party is the energy spent in the network formation that deeply depends on this topological parameter. Moreover, the maximum tree depth has also a great impact on the percentage of nodes that are able to join a network
Performance and Flexibility of Open Source Routing Software
No full textRouting protocols are a critical component in IP networks. Beside dedicated hardware, a great interest on routing systems based on open software is raising among Internet Service Providers. Many open source implementations of this protocol have been developed, among which Quagga and Xorp are the most used in PC-based router. In this paper we evaluate the OSPF performance of Quagga and Xorp routing software according to the test methodologies defined within the Internet Engineering Task Force. Moreover we describe a set of changes made on Quagga code in order to optimize some processes, whose algorithms were not efficient. In order to show the flexibility of an Open Source Routing Software, we have also implemented in Quagga an incremental algorithm for the evaluation of the shortest path. The realized implementation allows the shortest path computation time to be reduced of about the 97 © 2007 Springer
Cross-layer network formation for energy-efficient IEEE 802.15.4/ZigBee Wireless Sensor Networks
No full textIn IEEE 802.15.4/ZigBee Wireless Sensor Networks (WSNs) a specific node (called the PAN coordinator or sink) controls the whole network. When the network operates in a multi-hop fashion, the position of the PAN coordinator has a significant impact on the performance: it strongly affects network energy consumption for both topology formation and data routing. The development of efficient self-managing, self-configuring and self-regulating protocols for the election of the node that coordinates and manages the IEEE 802.15.4/ZigBee WSN is still an open research issue. In this paper we present a cross-layer approach to address the problem of PAN coordinator election on topologies formed in accordance with the IEEE 802.15.4. Our solution combines the network formation procedure defined at the MAC layer by the IEEE 802.15.4 standard with a topology reconfiguration algorithm operating at the network layer. We propose a standard-compliant procedure (named PAN coordinator ELection - PANEL) to self-configure a IEEE 802.15.4/ZigBee WSN by electing, in a distributed way, a suitable PAN coordinator. A protocol implementing this solution in IEEE 802.15.4 is also provided. Performance results show that our cross-layer approach minimizes the average number of hops between the nodes of the network and the PAN coordinator allowing to reduce the data transfer delay and determining significant energy savings compared with the performance of the IEEE 802.15.4 standard. (C) 2011 Elsevier B.V. All rights reserved
A distributed procedure for IEEE 802.15.4 PAN coordinator election in emergency scenarios
No full textIn an emergency scenario, the presence of a reliable communication infrastructure is fundamental. In response to massive catastrophes, such as earthquakes, floods, fires, etc., public authorities are expected to undertake actions to control and limit damages for people and for buildings. To this end, security agents are dispatched to the emergency area and they need to communicate to the people from a remote centre and they are responsible for emergency management. In this context, the use of IEEE 802.15.4 wireless personal area networks (WPANs) to allow communications among security agents seems particularly appropriate, thanks to their characteristics of self-configurability, adaptability, scalability, and low cost. Moreover, this kind of network can be used, if necessary, in a pervasive mode for data collection in the emergency area. In this work, we present an algorithm to self-configure an IEEE 802.15.4 WPAN by electing, in a distributed manner, a suitable node for the WPAN coordination. Our approach achieves a reduction of the network depth and a better distribution of nodes at different levels of the network
Autonomic network configuration in IEEE 802.15.4: a standard-compliant solution
No full textIn the autonomic networking framework, particular attention deserves the application of this paradigm to Wireless Personal Area Networks (WPANs). In this context, network algorithms need to be adaptive, robust and scalable, with fully distributed and self-organizing architectures. We study automation and self-management of the IEEE 802.15.4 WPANs formation, with the aim of having robust and energy efficient topologies that can be used for low-rate, low-cost and low-power communications. Specific attention is given to the proposal of a standard-compliant procedure to reconfigure the network coordination. Beside the description of the proposed implementation issues, we provide a performance evaluation of the benefits, in terms of energy consumption, of our reconfiguration procedure
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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