271 research outputs found
Effects of variations of load distribution on network performance
This paper is concerned with the characterization of the relationship between
topology and traffic dynamics. We use a model of network generation that allows
the transition from random to scale free networks. Specifically, we consider
three different topological types of network: random, scale-free with \gamma =
3, scale-free with \gamma = 2. By using a novel LRD traffic generator, we
observe best performance, in terms of transmission rates and delivered packets,
in the case of random networks. We show that, even if scale-free networks are
characterized by shorter characteristic-path- length (the lower the exponent,
the lower the path-length), they show worst performances in terms of
communication. We conjecture this could be explained in terms of changes in the
load distribution, defined here as the number of shortest paths going through a
given vertex. In fact, that distribu- tion is characterized by (i) a decreasing
mean (ii) an increas- ing standard deviation, as the networks becomes
scale-free (especially scale-free networks with low exponents). The use of a
degree-independent server also discriminates against a scale-free structure. As
a result, since the model is un- controlled, most packets will go through the
same vertices, favoring the onset of congestion.Comment: 4 pages, 4 figures, included in conference proceedings ISCAS 2005,
Kobe Japa
Communication models with distributed transmission rates and buffer sizes
The paper is concerned with the interplay between network structure and
traffic dynamics in a communications network, from the viewpoint of end-to-end
performance of packet transfer. We use a model of network generation that
allows the transition from random to scale-free networks. Specifically, we are
able to consider three different topologycal types of networks: (a) random; (b)
scale-free with \gamma=3; (c) scale free with \gamma=2. We also use an LRD
traffic generator in order to reproduce the fractal behavior that is observed
in real world data communication. The issue is addressed of how the traffic
behavior on the network is influenced by the variable factors of the
transmission rates and queue length restrictions at the network vertices. We
show that these factors can induce drastic changes in the throughput and
delivery time of network performance and are able to counter-balance some
undesirable effects due to the topology.Comment: 4 pages, 5 figures, IEEE Symposium on Circuits and Systems, Island of
Kos, Greece, 200
Robustness of Trans-European Gas Networks
Here we uncover the load and fault-tolerant backbones of the trans-European
gas pipeline network. Combining topological data with information on
inter-country flows, we estimate the global load of the network and its
tolerance to failures. To do this, we apply two complementary methods
generalized from the betweenness centrality and the maximum flow. We find that
the gas pipeline network has grown to satisfy a dual-purpose: on one hand, the
major pipelines are crossed by a large number of shortest paths thereby
increasing the efficiency of the network; on the other hand, a non-operational
pipeline causes only a minimal impact on network capacity, implying that the
network is error-tolerant. These findings suggest that the trans-European gas
pipeline network is robust, i.e., error tolerant to failures of high load
links.Comment: 11 pages, 8 figures (minor changes
An overview of research programmes and prospective technology in the development of more secure supply chains: The Case of Shipping Containers
The development of new, more secure, container systems should consider the main techno-economic items and devise a solution that, not only provides increased tamper-resistance, but also contains economically beneficial buy-in features that will motivate the adoption of new container models by the shipping trade. This report provides an overview of these aspects within the context of EU policy and R&D programmes in this area.JRC.G.5-European laboratory for structural assessmen
Congestion dependencies in the European gas pipeline network during crises.
Conflicts, geo-political crises, terrorist attacks, or natural disasters can turn large parts of energy distribution networks off-line, creating unexpected congestion in the remaining infrastructure. Given the importance of the security of natural gas supply, we need models that enable the management of network congestion, especially during crises. We develop a decentralized model of congestion control to explore the effects of removing supply or transit countries from the network. Recently, in R. Carvalho et. al. PLoS ONE, Vol. 9, no. 3, 2014, we evaluated how cooperation between countries helps to mitigate the effect of crises. Here, we extend our previous results by exploring the structure of downstream and upstream congestion dependencies between countries
Asymptotic Behavior of the Einstein-Yang-Mills-Dilaton System for a Closed Friedmann-Lemaitre Universe
We study the coupled Einstein-Yang-Mills-Dilaton (EYMD) equations for a
Fried\-mann-Le\-mai\-tre universe with constant curvature . Our detailed
analysis is restricted to the case where the dilaton potential and the
cosmological constant vanish. Also assuming a static gauge field, we present
analytical and numerical results on the behavior of solutions of the EYMD
equations. For different values of the dilaton coupling constant we analyze the
phase portrait for the time evolution of the dilaton field and give the
behavior of the scale factor. It turns out that there are no inflationary
stages in this model.Comment: 18 pages, Uuencoded gzip compressed tar file containing a latex file
and 12 figures. The epsfig.sty is neede
ASYMPTOTIC BEHAVIOR OF COMPLEX SCALAR FIELDS IN A FRIEDMAN-LEMAITRE UNIVERSE
We study the coupled Einstein-Klein-Gordon equations for a complex scalar
field with and without a quartic self-interaction in a curvatureless
Friedman-Lema\^{\i}\-tre Universe. The equations can be written as a set of
four coupled first order non-linear differential equations, for which we
establish the phase portrait for the time evolution of the scalar field. To
that purpose we find the singular points of the differential equations lying in
the finite region and at infinity of the phase space and study the
corresponding asymptotic behavior of the solutions. This knowledge is of
relevance, since it provides the initial conditions which are needed to solve
numerically the differential equations. For some singular points lying at
infinity we recover the expected emergence of an inflationary stage.Comment: uuencoded, compressed tarfile containing a 15 pages Latex file and 2
postscipt figures. Accepted for publication on Phys. Rev.
Magnesium-catalysed nitrile hydroboration
A β-diketiminato n-butylmagnesium complex is presented as a selective precatalyst for the reductive hydroboration of organic nitriles with pinacolborane (HBpin). Stoichiometric reactivity studies indicate that catalytic turnover ensues through the generation of magnesium aldimido, aldimidoborate and borylamido intermediates, which are formed in a sequence of intramolecular nitrile insertion and inter- and intramolecular B–H metathesis events. Kinetic studies highlight variations in mechanism for the catalytic dihydroboration of alkyl nitriles, aryl nitriles bearing electron withdrawing (Ar(EWG)CN) and aryl nitriles bearing electron donating (Ar(EDG)CN) substitution patterns. Kinetic isotope effects (KIEs) for catalysis performed with DBpin indicate that B–H bond breaking and C–H bond forming reactions are involved in the rate determining processes during the dihydroboration of alkyl nitriles and Ar(EDG)CN substrates, which display divergent first and second order rate dependences on [HBpin] respectively. In contrast, the hydroboration of Ar(EWG)CN substrates provides no KIE and HBpin is not implicated in the rate determining process during catalysis. Irrespective of these differences, a common mechanism is proposed in which the rate determining steps are deduced to vary through the establishment of several pre-equilibria, the relative positions of which are determined by the respective stabilities of the dimeric and monomeric magnesium aldimide and magnesium aldimidoborate intermediates as a result of adjustments to the basicity of the nitrile substrate. More generally, these observations indicate that homogeneous processes performed under heavier alkaline earth catalysis are likely to demonstrate previously unappreciated mechanistic diversity
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