1,720,957 research outputs found
Graph Models of Information Spreading in Wireless Networks
Full text linkThis thesis investigates the structural properties of graph models of wireless networks, where autonomous agents communicate using radios in order to accomplish a predefined task. Ad hoc, sensor, and vehicle networks are perhaps the most familiar examples. The goal of this thesis is the analytical characterization of information spreading in graph models of wireless networks, since this fundamental process is a primitive needed to accomplish more complex tasks.
The well-established graph-based approaches adopted when analyzing
the behavior of “classical” distributed systems (e.g., P2P networks, computing clusters, etc.) fail to generalize to wireless networks, due to several causes, including the stricter physical constraints governing the operation of these systems (e.g., interference on the physical channel or scarce energy/computational resources) and the fact that the topology of the network might be unknown at design time or it might evolve over time.
This thesis shows how to tackle these problems by suitably defining
and rigorously analyzing graph models and graph processes capturing
the structure, evolution and operation of these networks. We present two
reference scenarios.
In the first one we study a family of random graphs known as Bluetooth Topology, which closely model the connectivity of a network built by
the device discovery phase of Bluetooth-like protocols, largely employed
in wireless networks. Formally, the Bluetooth Topology generalizes the
well-known Random Geometric Graph model, introducing a distributed
pruning of the edge set. We investigate the expansion and the diameter of
these graphs, as they quantify the bandwidth and the latency of a wireless
network. We give tight bounds on the expansion and, leveraging on these,
we prove nearly-tight bounds on the diameter. Our results show that the
Bluetooth Topology features the same global level of connectivity of the Random Geometric Graph but requires maintaining much fewer communication links.
Motivated by the recent and rapidly growing interest in mobile systems,
in the second part of the thesis we turn our attention to the dynamics of
information dissemination between agents performing random walks on
a planar grid and communicating over short distances. This setting can
also be employed to study phenomena like the spreading of a disease,
where infections are the result of local interactions between agents. We
prove that, for a sufficiently sparse system, the broadcast time of a message
is independent from the transmission radius; indeed, we show that the
broadcast time is dominated by the time needed for many agents to meet.
Our findings nicely complements previous results that dealt with dense
systems, where there is dependency from the transmission radius. Moreover,
our analysis techniques extend to similar mobility-communication models,
suggesting some interesting further research directions.Questa tesi studia le proprieta' strutturali di alcuni modelli a grafo di reti
di agenti autonomi che comunicano via radio per completare un prefissato compito. Reti ad hoc, di sensori e veicolari sono forse gli esempi piu' immediati. Lo scopo di questa tesi e caratterizzare la diffusione dell’infor-
mazione in questi modelli a grafo di reti wireless, considerata l’importanza
di questo processo come primitiva fondamentale per realizzare protocolli piu' complessi.
Gli approcci basati su tecniche combinatorie adottati per l’analisi di
sistemi distribuiti “classici”, come le reti P2P o i cluster di calcolo, non
possono essere estesi alle reti wireless, per varie ragioni: ad esempio a
causa dei vincoli fisici che governano il funzionamento di questi sistemi
(interferenza sul canale radio, scarse risorse energetiche/computazionali,
ecc.) e per il fatto che la topologia della rete puo' essere ignota in fase di
progettazione o puo' evolvere nel tempo.
Questa tesi suggerisce come sia possibile affrontare tali problemi tramite
l’opportuna definizione e l’analisi rigorosa di modelli a grafo (o processi
su grafi) che catturino l’evoluzione e il funzionamento delle reti wireless.
Mostriamo come sia possibile applicare quest’approccio a due scenari di
riferimento.
Innanzitutto studiamo una famiglia di grafi random nota come Bluetooth
Topology, che ben rappresenta la connettivita' della rete creata dalla fase di device discovery in protocolli simili al Bluetooth, largamente utilizzati nelle reti wireless. Dal punto di vista formale, la Bluetooth Topology generalizza il ben noto modello Random Geometric Graph, introducendovi una selezione distribuita degli archi. Studiamo l’espansione e il diametro di questi grafi, poiche' quantificano la banda e la latenza della rete. Dimostriamo limiti stretti all’espansione e, sfruttando questa caratterizzazione, diamo dei limiti quasi stretti al diametro. I nostri risultati provano che la Bluetooth Topology presenta lo stesso livello globale di connettivita' del Random Geometric Graph, pur richiedendo molti meno link di comunicazione.
Graph, pur richiedendo molti meno link di comunicazione.
Motivati dal recente crescente interesse verso i sistemi mobili, nella
seconda parte della tesi concentriamo la nostra attenzione sulle dinamiche
di disseminazione dell’informazione tra agenti che effettuano random walk
su una griglia planare e che comunicano su brevi distanze. Questo scenario puo' essere utilizzato per studiare fenomeni come la diffusione di malattie, dove le infezioni sono il risultato di interazioni locali tra gli agenti. Proviamo che, per un sistema sufficientemente sparso, il tempo di broadcast di un messaggio e indipendente dal raggio di trasmissione, dimostrando che esso e' dominato dal tempo necessario affinche' molti agenti si incontrino. I nostri risultati completano l’analisi, apparsa in lavori precedenti, di sistemi densi, dove viceversa vi e' dipendenza del tempo di broadcast dal raggio di trasmissione. Inoltre le nostre tecniche di analisi possono essere estese a modelli di mobilita'-comunicazione simili, suggerendo alcune interessanti linee di ulteriore ricerca
On the Expansion and Diameter of Bluetooth-like Topologies
Full text linkThe routing capabilities of an interconnection network are
strictly related to its bandwidth and latency characteristics, which are
in turn quantifiable through the graph-theoretic concepts of expansion
and diameter. This paper studies expansion and diameter of a family
of subgraphs of the random geometric graph, which closely model the
topology induced by the device discovery phase of Bluetooth-based ad
hoc networks. The main feature modeled by any such graph, denoted
as BT (r(n), c(n)), is the small number c(n) of links that each of the n
devices (vertices) may establish with those located within its communi-
cation range r(n). First, tight bounds are proved on the expansion of
BT (r(n), c(n)) for the whole set of functions r(n) and c(n) for which
connectivity has been established in previous works. Then, by leveraging
on the expansion result, tight (up to a logarithmic additive term) upper
and lower bounds on the diameter of BT (r(n), c(n)) are derived
A Novel Resource-Driven Job Allocation Scheme for Desktop Grid Environments
No full textIn this paper we propose a novel framework for the dynamic allocation of jobs in grid-like environments, in which such jobs are dispatched to the machines of the grid by a centralized scheduler. We apply a new, full resource-driven approach to the scheduling task: jobs are allocated and (possibly) relocated on the basis of the matching between their resource requirements and the characteristics of the machines in the grid. We provide experimental evidence that our approach effectively exploits the computational resources at hand, successfully keeping the completion time of the jobs low, even without having knowledge of the actual running times of the jobs
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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