24,019 research outputs found
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Towards scalable end-to-end QoS provision for VoIP applications
The growth of the Internet and the development of its new applications have increased the demand for providing a certain level of resource assurance and service support. The concept of ensuring quality of service (QoS) has been introduced in order to provide the support and assurance for these services. Different QoS mechanisms, such as integrated services (IntServ) and differentiated services (DiffServ), have been developed and introduced to provide different levels of QoS provision. However, IntServ can suffer from scalability issues that make it infeasible for large-scale network implementations. On the other hand, the aggregated-based per-flow technique of DiffServ does not provide such an end-to-end QoS guarantee. Recently, the IETF have proposed a new QoS architecture that implements IntServ over DiffServ in order to provide an end-to-end QoS for scalable networks. Hence, it became possible to provide and support a certain level of QoS for some delay sensitive and bandwidth-demanding applications such as voice over Internet Protocol (VoIP). With regard to VoIP applications, delay, jitter and packet loss are crucial issues that have to be taken into consideration for any VoIP system design and such parameters need a distinct level of QoS support
Closed ideals in some algebras of analytic functions
We obtain a complete description of closed ideals of the algebra
\mathcal{D}\cap \mathrm{lip}_\alpha}, where
is the Dirichlet space and \mathrm{lip}_\alpha} is the algebra
of analytic functions satisfying the Lipschitz condition of order Comment: 19 page
Two Body Relaxation in Simulated Cosmological Haloes
This paper aims at quantifying discreetness effects, born of finite particle
number, on the dynamics of dark matter haloes forming in the context of
cosmological simulations. By generalising the standard calculation of two body
relaxation to the case when the size and mass distribution are variable, and
parametrising the time evolution using established empirical relations, we find
that the dynamics of a million particle halo is noise-dominated within the
inner percent of the final virial radius. Far larger particle numbers (~ 10^8)
are required for the RMS perturbations to the velocity to drop to the 10 %
level there. The radial scaling of the relaxation time is simple and strong:
t_relax ~ r^2, implying that numbers >> 10^8 are required to faithfully model
the very inner regions; artificial relaxation may thus constitute an important
factor, contributing to the contradictory claims concerning the persistence of
a power law density cusp to the very centre. The cores of substructure haloes
can be many relaxation times old. Since relaxation first causes their expansion
before recontraction occurs, it may render them either more difficult or easier
to disrupt, depending on their orbital parameters. It may thus modify the
characteristics of the subhalo distribution and effects of interactions with
the parent. We derive simple closed form formulas for the characteristic
relaxation times, as well as for the weak N-scaling reported by Diemand et al.
when the main contribution comes from relaxing subhaloes (abridged).Comment: 11 Pages, 7 figs, Monthly Notices styl
Growth and optical characterisation of multilayers of InGaN quantum dots
We report on the growth (using metal-organic vapour phase epitaxy) and optical
characterization of single and multiple layers of InGaN quantum dots (QDs), which were formed by
annealing InGaN epilayers at the growth temperature in nitrogen. The size and density of the
nanostructures have been found to be fairly similar for uncapped single and three layer QD samples if
the GaN barriers between the dot layers are grown at the same temperature as the InGaN epilayer. The
distribution of nanostructure heights of the final QD layer of three is wider and is centred around a
larger size if the GaN barriers are grown at two temperatures (first a thin layer at the dot growth
temperature, then a thicker layer at a higher temperature). Micro-photoluminescence studies at 4.2 K
of capped samples have confirmed the QD nature of the capped nanostructures by the observation of
sharp emission peaks with full width at half maximum limited by the resolution of the spectrometer.
We have also observed much more QD emission per unit area in a sample with three QD layers, than in
a sample with a single QD layer, as expected
A Hopf Index Theorem for foliations
We formulate and prove an analog of the Hopf Index Theorem for Riemannian
foliations. We compute the basic Euler characteristic of a closed Riemannian
manifold as a sum of indices of a non-degenerate basic vector field at critical
leaf closures. The primary tool used to establish this result is an adaptation
to foliations of the Witten deformation method.Comment: 26 page
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Distributions of brominated organic compounds in the troposphere and lower stratosphere
A comprehensive suite of brominated organic compounds was measured from whole air samples collected during the 1996 NASA Stratospheric Tracers of Atmospheric Transport aircraft campaign and the 1996 NASA Global Tropospheric Experiment Pacific Exploratory Mission-Tropics aircraft campaign. Measurements of individual species and total organic bromine were utilized to describe latitudinal and vertical distributions in the troposphere and lower stratosphere, fractional contributions to total organic bromine by individual species, fractional dissociation of the long-lived species relative to CFC-11, and the Ozone Depletion Potential of the halons and CH3Br. Spatial differences in the various organic brominated compounds were related to their respective sources and chemical lifetimes. The difference between tropospheric mixing ratios in the Northern and Southern Hemispheres for halons was approximately equivalent to their annual tropospheric growth rates, while the interhemispheric ratio of CH3Br was 1.18. The shorter-lived brominated organic species showed larger tropospheric mixing ratios in the tropics relative to midlatitudes, which may reflect marine biogenic sources. Significant vertical gradients in the troposphere were observed for the short-lived species with upper troposphere values 40-70% of the lower troposphere values. Much smaller vertical gradients (3-14%) were observed for CH3Br, and no significant vertical gradients were observed for the halons. Above the tropopause, the decrease in organic bromine compounds was found to have some seasonal and latitudinal differences. The combined losses of the individual compounds resulted in a loss of total organic bromine between the tropopause and 20 km of 38-40% in the tropics and 75-85% in midlatitudes. The fractional dissociation of the halons and CH3Br relative to CFC-11 showed latitudinal differences, with larger values in the tropics. Copyright 1999 by the American Geophysical Union
An Explicative and Predictive Study of Employee Attrition using Tree-based Models
We develop tree-based models to estimate the probability of an employee leaving a firm during a job transition from a dataset of anonymously submitted resumes through Glassdoorâs online portal. Dataset construction and summary statistics are first summarized followed by a more in depth examination through four exploratory studies. Insights provided by these studies are then used to engineer features that serve as input into subsequent attrition related predictive models. We finally perform a thorough search through several dozen binary classification techniques in the cases of an original and extended feature set. We find tree-based methods including random forests and light gradient boosted trees provide the overall strongest predictive performance. Finally, we summarize ROC curves for several such models and describe future potential research directions
Nature and Nurture in Dark Matter Halos
Cosmological simulations consistently predict specific properties of dark
matter halos, but these have not yet led to a physical understanding that is
generally accepted. This is especially true for the central regions of these
structures. Recently two major themes have emerged. In one, the dark matter
halo is primarily a result of the sequential accretion of primordial structure
(ie `Nature'); while in the other, dynamical relaxation (ie `Nurture')
dominates at least in the central regions. Some relaxation is however required
in either mechanism. In this paper we accept the recently established
scale-free sub-structure of halos as an essential part of both mechanisms.
Consequently; a simple model for the central relaxation based on a self-similar
cascade of tidal interactions, is contrasted with a model based on the
accretion of adiabatically self-similar, primordial structure. We conclude that
a weak form of this relaxation is present in the simulations, but that is
normally described as the radial orbit instability.Comment: 25 pages, 3 figures, fig with parts 1 to d, fig 3 with parts a to
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