Página 18 dos resultados de 2591 itens digitais encontrados em 0.032 segundos

A scheme for synchronizing clocks connected by a packet communication network

dos Santos, R. V.; Monteiro, Luiz Henrique Alves
Fonte: ELSEVIER SCIENCE BV; AMSTERDAM Publicador: ELSEVIER SCIENCE BV; AMSTERDAM
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
35.69%
Consider a communication system in which a transmitter equipment sends fixed-size packets of data at a uniform rate to a receiver equipment. Consider also that these equipments are connected by a packet-switched network, which introduces a random delay to each packet. Here we propose an adaptive clock recovery scheme able of synchronizing the frequencies and the phases of these devices, within specified limits of precision. This scheme for achieving frequency and phase synchronization is based on measurements of the packet arrival times at the receiver, which are used to control the dynamics of a digital phase-locked loop. The scheme performance is evaluated via numerical simulations performed by using realistic parameter values. (C) 2011 Elsevier By. All rights reserved.; CNPq; CNPq

Estruturas de dados eficientes para algoritmos evolutivos aplicados a projeto de redes; Efficient Data Structures to Evolutionary Algorithms Applied to Network Design Problems.

Soares, Telma Woerle de Lima
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 22/05/2009 Português
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Problemas de projeto de redes (PPRs) são muito importantes uma vez que envolvem uma série de aplicações em áreas da engenharia e ciências. Para solucionar as limitações de algoritmos convencionais para PPRs que envolvem redes complexas do mundo real (em geral modeladas por grafos completos ou mesmo esparsos de larga-escala), heurísticas, como os algoritmos evolutivos (EAs), têm sido investigadas. Trabalhos recentes têm mostrado que estruturas de dados adequadas podem melhorar significativamente o desempenho de EAs para PPRs. Uma dessas estruturas de dados é a representação nó-profundidade (NDE, do inglês Node-depth Encoding). Em geral, a aplicação de EAs com a NDE tem apresentado resultados relevantes para PPRs de larga-escala. Este trabalho investiga o desenvolvimento de uma nova representação, baseada na NDE, chamada representação nó-profundidade-grau (NDDE, do inglês Node-depth-degree Encoding). A NDDE é composta por melhorias nos operadores existentes da NDE e pelo desenvolvimento de novos operadores de reprodução possibilitando a recombinação de soluções. Nesse sentido, desenvolveu-se um operador de recombinação capaz de lidar com grafos não-completos e completos, chamado EHR (do inglês, Evolutionary History Recombination Operator). Foram também desenvolvidos operadores de recombinação que lidam somente com grafos completos...

Tecnologias WEB aplicadas aos sistemas elétricos de potência; Web technologies applied to electrical power systems

Silva, Lazaro Eduardo da
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 09/08/2010 Português
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A utilização de equipamentos digitais e de meios de comunicação de dados, na conexão entre os dispositivos de uma subestação de energia elétrica oportunizam o uso de tecnologias para aprimorar e elevar as potencialidades de controle e supervisão do sistema elétrico de potência. As concessionárias de energia elétrica demandam altos investimentos na construção e implantação de sistemas de supervisão, controle e aquisição de dados. A partir destes sistemas é possível visualizar em um mapa da rede elétrica, os pontos de instalação dos relés e os status do seu funcionamento, possibilitando a concretização de um diagnóstico e o controle destes equipamentos. A World Wide Web se tornou um meio conveniente para acesso às informações devido ao fato dos browsers serem capazes de integrar diferentes serviços de rede em uma única e amigável interface com o usuário. A implementação de um sistema de supervisão e controle utilizando tecnologias web gratuitas pode, com custo competitivo, agregar dados que estejam dispostos geograficamente, em uma plataforma computacional ergonômica, coesa e flexível, provendo acesso aos mesmos a partir de qualquer dispositivo conectado a Internet. Como resultado deste trabalho...

Self-presentation on on-line communication platforms

Jorge, Filipa; Antunes, Maria João
Fonte: Universidade de Aveiro Publicador: Universidade de Aveiro
Tipo: Conferência ou Objeto de Conferência
Português
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The emergence of the Internet and, more recently, on-line social networks has brought a new form of self-presentation, governed by the specific characteristics of the new medium. The so-called Computer Mediated Communication (CMC), which eliminates some of the constraints of face-to-face communication, and special features, such as easy maintenance of anonymity, introduced changes in how the individuals present themselves. Web 2.0 and its services have changed the process of content production, providing the possibility of former receptors to publish their content on Web, mostly in formats like text, static image or video. As in the off-line context, the way that users manage the content that they publish in the on-line platforms demonstrates the impression that they wish to give of themselves to the others. This paper aims to gain an understanding of the issues related to the particularities of different publication formats, under self-presentation of individuals on their Facebook profiles. To do this, we examined a sample of twenty Facebook profiles - 10 males and 10 females, aged between 18 and 48 years - during fifteen consecutive days, and particular attention was given to the audiovisual content published. The choice of the sample aimed to understand the differences in self-presentation between the different genres and ages. Data analysis shows that the video format is chosen mainly to demonstrate the users’ specific interests...

Bridge the Gap: Measuring and Analyzing Technical Data for Social Trust between Smartphones

Trapp, Sebastian; Wählisch, Matthias; Schiller, Jochen
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 14/05/2012 Português
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Mobiles are nowadays the most relevant communication devices in terms of quantity and flexibility. Like in most MANETs ad-hoc communication between two mobile phones requires mutual trust between the devices. A new way of establishing this trust conducts social trust from technically measurable data (e.g., interaction logs). To explore the relation between social and technical trust, we conduct a large-scale survey with more than 217 Android users and analyze their anonymized call and message logs. We show that a reliable a priori trust value for a mobile system can be derived from common social communication metrics.

Power and Delay Aware On-Demand Routing For Ad Hoc Networks

Jagadev, Alok Kumar; Pattanayak, Binod Kumar; Mishra, Manoj Kumar; Nayak, Manojranjan
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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Wide implementation of IEEE 802.11 based networks could lead to deployment of localized wireless data communication environments with a limited number of mobile hosts, called ad hoc networks. Implementation of a proper routing methodology in ad hoc networks makes it efficient in terms of performance. A wide spectrum of routing protocols has been contributed by several researchers. Real time applications have been most popular among the applications, run by ad hoc networks. Such applications strictly adhere to the Quality of Service (QoS) requirements such as overall throughput, end-toend delay and power level. Support of QoS requirements becomes more challenging due to dynamic nature of MANETs, where mobility of nodes results in frequent change in topology. QoS aware routing protocols can serve to the QoS support, which concentrate on determining a path between source and destination with the QoS requirements of the flow being satisfied. We propose a protocol, called Power and Delay aware Temporally Ordered Routing Algorithm (PDTORA), based on Temporally Ordered Routing Algorithm (TORA) Protocol, where verification of power and delay requirements is carried out with a query packet at each node along the path between source and destination. Simulations justify better performance of the proposed new protocol in terms of network lifetime...

Tree-based Coarsening and Partitioning of Complex Networks

Glantz, Roland; Meyerhenke, Henning; Schulz, Christian
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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Many applications produce massive complex networks whose analysis would benefit from parallel processing. Parallel algorithms, in turn, often require a suitable network partition. For solving optimization tasks such as graph partitioning on large networks, multilevel methods are preferred in practice. Yet, complex networks pose challenges to established multilevel algorithms, in particular to their coarsening phase. One way to specify a (recursive) coarsening of a graph is to rate its edges and then contract the edges as prioritized by the rating. In this paper we (i) define weights for the edges of a network that express the edges' importance for connectivity, (ii) compute a minimum weight spanning tree $T^m$ with respect to these weights, and (iii) rate the network edges based on the conductance values of $T^m$'s fundamental cuts. To this end, we also (iv) develop the first optimal linear-time algorithm to compute the conductance values of \emph{all} fundamental cuts of a given spanning tree. We integrate the new edge rating into a leading multilevel graph partitioner and equip the latter with a new greedy postprocessing for optimizing the maximum communication volume (MCV). Experiments on bipartitioning frequently used benchmark networks show that the postprocessing already reduces MCV by 11.3%. Our new edge rating further reduces MCV by 10.3% compared to the previously best rating with the postprocessing in place for both ratings. In total...

Limiting Byzantien Influence in Multihop Asynchronous Networks

Maurer, Alexandre; Tixeuil, Sébastien
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/01/2012 Português
Relevância na Pesquisa
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We consider the problem of reliably broadcasting information in a multihop asyn- chronous network that is subject to Byzantine failures. That is, some nodes of the network can exhibit arbitrary (and potentially malicious) behavior. Existing solutions provide de- terministic guarantees for broadcasting between all correct nodes, but require that the communication network is highly-connected (typically, 2k + 1 connectivity is required, where k is the total number of Byzantine nodes in the network). In this paper, we investigate the possibility of Byzantine tolerant reliable broadcast be- tween most correct nodes in low-connectivity networks (typically, networks with constant connectivity). In more details, we propose a new broadcast protocol that is specifically designed for low-connectivity networks. We provide sufficient conditions for correct nodes using our protocol to reliably communicate despite Byzantine participants. We present experimental results that show that our approach is especially effective in low-connectivity networks when Byzantine nodes are randomly distributed.; Comment: 18 pages

Interactive Sensing in Social Networks

Krishnamurthy, Vikram; Poor, H. Vincent
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 30/12/2013 Português
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This paper presents models and algorithms for interactive sensing in social networks where individuals act as sensors and the information exchange between individuals is exploited to optimize sensing. Social learning is used to model the interaction between individuals that aim to estimate an underlying state of nature. In this context the following questions are addressed: How can self-interested agents that interact via social learning achieve a tradeoff between individual privacy and reputation of the social group? How can protocols be designed to prevent data incest in online reputation blogs where individuals make recommendations? How can sensing by individuals that interact with each other be used by a global decision maker to detect changes in the underlying state of nature? When individual agents possess limited sensing, computation and communication capabilities, can a network of agents achieve sophisticated global behavior? Social and game theoretic learning are natural settings for addressing these questions. This article presents an overview, insights and discussion of social learning models in the context of data incest propagation, change detection and coordination of decision making.

Calling patterns in human communication dynamics

Jiang, Zhi-Qiang; Xie, Wen-Jie; Li, Ming-Xia; Podobnik, Boris; Zhou, Wei-Xing; Stanley, H. Eugene
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 30/01/2013 Português
Relevância na Pesquisa
35.7%
Modern technologies not only provide a variety of communication modes, e.g., texting, cellphone conversation, and online instant messaging, but they also provide detailed electronic traces of these communications between individuals. These electronic traces indicate that the interactions occur in temporal bursts. Here, we study the inter-call durations of the 100,000 most-active cellphone users of a Chinese mobile phone operator. We confirm that the inter-call durations follow a power-law distribution with an exponential cutoff at the population level but find differences when focusing on individual users. We apply statistical tests at the individual level and find that the inter-call durations follow a power-law distribution for only 3460 individuals (3.46%). The inter-call durations for the majority (73.34%) follow a Weibull distribution. We quantify individual users using three measures: out-degree, percentage of outgoing calls, and communication diversity. We find that the cellphone users with a power-law duration distribution fall into three anomalous clusters: robot-based callers, telecom frauds, and telephone sales. This information is of interest to both academics and practitioners, mobile telecom operator in particular. In contrast...

The Role of Information Diffusion in the Evolution of Social Networks

Weng, Lilian; Ratkiewicz, Jacob; Perra, Nicola; Gonçalves, Bruno; Castillo, Carlos; Bonchi, Francesco; Schifanella, Rossano; Menczer, Filippo; Flammini, Alessandro
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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Every day millions of users are connected through online social networks, generating a rich trove of data that allows us to study the mechanisms behind human interactions. Triadic closure has been treated as the major mechanism for creating social links: if Alice follows Bob and Bob follows Charlie, Alice will follow Charlie. Here we present an analysis of longitudinal micro-blogging data, revealing a more nuanced view of the strategies employed by users when expanding their social circles. While the network structure affects the spread of information among users, the network is in turn shaped by this communication activity. This suggests a link creation mechanism whereby Alice is more likely to follow Charlie after seeing many messages by Charlie. We characterize users with a set of parameters associated with different link creation strategies, estimated by a Maximum-Likelihood approach. Triadic closure does have a strong effect on link formation, but shortcuts based on traffic are another key factor in interpreting network evolution. However, individual strategies for following other users are highly heterogeneous. Link creation behaviors can be summarized by classifying users in different categories with distinct structural and behavioral characteristics. Users who are popular...

Extremal Properties of Three Dimensional Sensor Networks with Applications

Ravelomanana, Vlady
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 10/11/2004 Português
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In this paper, we analyze various critical transmitting/sensing ranges for connectivity and coverage in three-dimensional sensor networks. As in other large-scale complex systems, many global parameters of sensor networks undergo phase transitions: For a given property of the network, there is a critical threshold, corresponding to the minimum amount of the communication effort or power expenditure by individual nodes, above (resp. below) which the property exists with high (resp. a low) probability. For sensor networks, properties of interest include simple and multiple degrees of connectivity/coverage. First, we investigate the network topology according to the region of deployment, the number of deployed sensors and their transmitting/sensing ranges. More specifically, we consider the following problems: Assume that $n$ nodes, each capable of sensing events within a radius of $r$, are randomly and uniformly distributed in a 3-dimensional region $\mathcal{R}$ of volume $V$, how large must the sensing range be to ensure a given degree of coverage of the region to monitor? For a given transmission range, what is the minimum (resp. maximum) degree of the network? What is then the typical hop-diameter of the underlying network? Next...

Optimizing Spread of Influence in Weighted Social Networks via Partial Incentives

Cordasco, Gennaro; Gargano, Luisa; Rescigno, Adele A.; Vaccaro, Ugo
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/12/2015 Português
Relevância na Pesquisa
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A widely studied process of influence diffusion in social networks posits that the dynamics of influence diffusion evolves as follows: Given a graph $G=(V,E)$, representing the network, initially \emph{only} the members of a given $S\subseteq V$ are influenced; subsequently, at each round, the set of influenced nodes is augmented by all the nodes in the network that have a sufficiently large number of already influenced neighbors. The general problem is to find a small initial set of nodes that influences the whole network. In this paper we extend the previously described basic model in the following ways: firstly, we assume that there are non negative values $c(v)$ associated to each node $v\in V$, measuring how much it costs to initially influence node $v$, and the algorithmic problem is to find a set of nodes of \emph{minimum total cost} that influences the whole network; successively, we study the consequences of giving \emph{incentives} to member of the networks, and we quantify how this affects (i.e., reduces) the total costs of starting process that influences the whole network. For the two above problems we provide both hardness and algorithmic results. We also experimentally validate our algorithms via extensive simulations on real life networks.; Comment: An extended abstract of a preliminary version of this paper appeared in: Proceedings of 22nd International Colloquium on Structural Information and Communication Complexity (SIROCCO 2015)...

A Trust Based Fuzzy Algorithm for Congestion Control in Wireless Multimedia Sensor Networks (TFCC)

Chakraborty, Arpita; Ganguly, Srinjoy; Naskar, Mrinal Kanti; Karmakar, Anupam
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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Network congestion has become a critical issue for resource constrained Wireless Sensor Networks (WSNs), especially for Wireless Multimedia Sensor Networks (WMSNs)where large volume of multimedia data is transmitted through the network. If the traffic load is greater than the available capacity of the sensor network, congestion occurs and it causes buffer overflow, packet drop, deterioration of network throughput and quality of service (QoS). Again, the faulty nodes of the network also aggravate congestion by diffusing useless packets or retransmitting the same packet several times. This results in the wastage of energy and decrease in network lifetime. To address this challenge, a new congestion control algorithm is proposed in which the faulty nodes are identified and blocked from data communication by using the concept of trust. The trust metric of all the nodes in the WMSN is derived by using a two-stage Fuzzy inferencing scheme. The traffic flow from source to sink is optimized by implementing the Link State Routing Protocol. The congestion of the sensor nodes is controlled by regulating the rate of traffic flow on the basis of the priority of the traffic. Finally we compare our protocol with other existing congestion control protocols to show the merit of the work.; Comment: 6 pages...

Predicting future conflict between team-members with parameter-free models of social networks

Rovira-Asenjo, Nuria; Gumi, Tania; Sales-Pardo, Marta; Guimera, Roger
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/11/2014 Português
Relevância na Pesquisa
35.7%
Despite the well-documented benefits of working in teams, teamwork also results in communication, coordination and management costs, and may lead to personal conflict between team members. In a context where teams play an increasingly important role, it is of major importance to understand conflict and to develop diagnostic tools to avert it. Here, we investigate empirically whether it is possible to quantitatively predict future conflict in small teams using parameter-free models of social network structure. We analyze data of conflict appearance and resolution between 86 team members in 16 small teams, all working in a real project for nine consecutive months. We find that group-based models of complex networks successfully anticipate conflict in small teams whereas micro-based models of structural balance, which have been traditionally used to model conflict, do not.

Stabilizing Maximal Independent Set in Unidirectional Networks is Hard

Masuzawa, Toshimitsu; Tixeuil, Sébastien
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 18/03/2009 Português
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A distributed algorithm is self-stabilizing if after faults and attacks hit the system and place it in some arbitrary global state, the system recovers from this catastrophic situation without external intervention in finite time. In this paper, we consider the problem of constructing self-stabilizingly a \emph{maximal independent set} in uniform unidirectional networks of arbitrary shape. On the negative side, we present evidence that in uniform networks, \emph{deterministic} self-stabilization of this problem is \emph{impossible}. Also, the \emph{silence} property (\emph{i.e.} having communication fixed from some point in every execution) is impossible to guarantee, either for deterministic or for probabilistic variants of protocols. On the positive side, we present a deterministic protocol for networks with arbitrary unidirectional networks with unique identifiers that exhibits polynomial space and time complexity in asynchronous scheduling. We complement the study with probabilistic protocols for the uniform case: the first probabilistic protocol requires infinite memory but copes with asynchronous scheduling, while the second probabilistic protocol has polynomial space complexity but can only handle synchronous scheduling. Both probabilistic solutions have expected polynomial time complexity.

Optimal CSMA-based Wireless Communication with Worst-case Delay and Non-uniform Sizes

Li, Hongxing; Vaidya, Nitin
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 15/01/2014 Português
Relevância na Pesquisa
35.7%
Carrier Sense Multiple Access (CSMA) protocols have been shown to reach the full capacity region for data communication in wireless networks, with polynomial complexity. However, current literature achieves the throughput optimality with an exponential delay scaling with the network size, even in a simplified scenario for transmission jobs with uniform sizes. Although CSMA protocols with order-optimal average delay have been proposed for specific topologies, no existing work can provide worst-case delay guarantee for each job in general network settings, not to mention the case when the jobs have non-uniform lengths while the throughput optimality is still targeted. In this paper, we tackle on this issue by proposing a two-timescale CSMA-based data communication protocol with dynamic decisions on rate control, link scheduling, job transmission and dropping in polynomial complexity. Through rigorous analysis, we demonstrate that the proposed protocol can achieve a throughput utility arbitrarily close to its offline optima for jobs with non-uniform sizes and worst-case delay guarantees, with a tradeoff of longer maximum allowable delay.

Practical implementation and performance analysis on security of sensor networks

Verma, Nidhi
Fonte: Rochester Instituto de Tecnologia Publicador: Rochester Instituto de Tecnologia
Tipo: Tese de Doutorado Formato: 975972 bytes; application/pdf
Português
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A wireless sensor network (WSN) is a network made of thousands of sensing elements called as nodes with wireless capabilities. Their application is varied and diverse ranging from military to domestic and household. As the world of self-organizing sensor networks tip to the edge of maximum utilization, their wider deployment is adding pressure on the security front. Powerful laptops and workstations make it more challenging for small sensors. In addition, there are many security challenges in WSN, e.g- confidentiality, authentication, freshness, integrity etc. Contributions of this work are as follows: “Symmetric” security implementation: This thesis work designs a symmetric-key based security in sensor hardware in the Link layer of sensor network protocols. Link Layer security can protect a wireless network by denying access to the network itself before a user is successfully authenticated. This prevents attacks against the network infrastructure and protects the network from devastating attacks. “Public key” implementation in sensor hardware: Asymmetric key techniques are attractive for authentication data or session keys. Traditional schemes like RSA require considerable amounts of resources which in the past has limited their use. This thesis has implemented Elliptic Curve Cryptography (ECC) in Mica2 hardware...

Permutation Routing in All-Optical Product Networks

Liang, Weifa; Shen, Xiaojun
Fonte: Institute of Electrical and Electronics Engineers (IEEE Inc) Publicador: Institute of Electrical and Electronics Engineers (IEEE Inc)
Tipo: Artigo de Revista Científica
Português
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In this brief, we study the permutation routing for an all-optical product network. We show a lower bound on the number of wavelengths needed to implement any permutation with one round. We also present efficient routing algorithms for two models, the wavelength non-conversion and conversion models, respectively.

A new fault tolerant architecture for time sensitive automation networks

Kleineberg, Oliver
Fonte: University of Limerick Publicador: University of Limerick
Tipo: info:eu-repo/semantics/doctoralThesis; all_ul_research; ul_published_reviewed; ul_theses_dissertations
Português
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peer-reviewed; Networked control systems have been widely used in Industrial Automation since the latter part of the 20st century. Such networks have evolved from small systems with proprietary and slow Fieldbus technology to a much larger scope. Today, Ethernet-based automation systems encompass whole factories and carry large amounts of data traffic with differing latency requirements. Proprietary real-time Ethernet technologies such as Profinet IRT (Isochronous Real-Time) have demonstrated the feasibility of real-time Ethernet for even the most demanding applications such as functional safety and motion control. The drawback of these vendor-specific solutions is their dependence on proprietary ASICs (Application-Specific Integrated Circuit) and their very limited compatibility to standard Ethernet and Bridging as defined by IEEE (Institute of Electrical and Electronics Engineers) 802.3 and IEEE 802.1. Up to the present day, this has limited the widespread success of Ethernet as a real-time network solution. With the emerging AVB (Audio- and Video Bridging) technology, IEEE 802.1 has defined the first vendor-neutral real-time Ethernet, enabling the possibility of a standardised solution for Industrial Automation. However...