3,309 research outputs found
An admission control scheme for IEEE 802.11e wireless local area networks
Includes bibliographical references (leaves 80-84).Recent times has seen a tremendous increase in the deployment and use of 802.11 Wireless Local Area Networks (WLANs). These networks are easy to deploy and maintain, while providing reasonably high data rates at a low cost. In the paradigm of Next-Generation-Networks (NGNs), WLANs can be seen as an important access network technology to support IP multimedia services. However a traditional WLAN does not provide Quality of Service (QoS) support since it was originally designed for best effort operation. The IEEE 802. 11e standard was introduced to overcome the lack of QoS support for the legacy IEEE 802 .11 WLANs. It enhances the Media Access Control (MAC) layer operations to incorporate service differentiation. However, there is a need to prevent overloading of wireless channels, since the QoS experienced by traffic flows is degraded with heavily loaded channels. An admission control scheme for IEEE 802.11e WLANs would be the best solution to limit the amount of multimedia traffic so that channel overloading can be prevented. Some of the work in the literature proposes admission control solutions to protect the QoS of real-time traffic for IEEE 802.11e Enhanced Distributed Channel Access (EDCA). However, these solutions often under-utilize the resources of the wireless channels. A measurement-aided model-based admission control scheme for IEEE 802.11e EDCA WLANs is proposed to provide reasonable bandwidth guarantees to all existing flows. The admission control scheme makes use of bandwidth estimations that allows the bandwidth guarantees of all the flows that are admitted into the network to be protected. The bandwidth estimations are obtained using a developed analytical model of IEEE 802.11e EDCA channels. The admission control scheme also aims to accept the maximum amount of flows that can be accommodated by the network's resources. Through simulations, the performance of the proposed admission control scheme is evaluated using NS-2. Results show that accurate bandwidth estimations can be obtained when comparing the estimated achievable bandwidth to actual simulated bandwidth. The results also validate that the bandwidth needs of all admitted traffic are always satisfied when the admission control scheme is applied. It was also found that the admission control scheme allows the maximum amount of flows to be admitted into the network, according the network's capacity
The ethics of Research Excellence
We here analyse the ethical dimensions of the UK's ‘Research Excellence Framework’ (REF), the latest (2014) version of an exercise which assesses the quality of university research in the UK every seven or so years. We find many of the common objections to this exercise unfounded, such as that it is excessively expensive by comparison with alternatives such as various metrics, or that it turns on the subjective judgement of the assessors. However there are grounds for concern about the crude language in which for example all relevant scholarship becomes called ‘research’ and publications become ‘outputs’. The focus on the impact of research, which was a new feature of the most recent exercise, is particularly problematic, creating as it does a tendency to what Aristotle called alazony, self-aggrandisement, on the part of academics. We conclude that the REF is a mixed good from an ethical point of view, and that more could be done to mitigate its more unfortunate features
The Larson-Tinsley Effect in the UV: Interacting vs. `Normal' Spiral Galaxies
We compare the UV-optical colors of a well-defined set of optically-selected
pre-merger interacting galaxy pairs with those of normal spirals. The shorter
wavelength colors show a larger dispersion for the interacting galaxies than
for the spirals. This result can best be explained by higher star formation
rates on average in the interacting galaxies, combined with higher extinctions
on average. This is consistent with earlier studies, that found that the star
formation in interacting galaxies tends to be more centrally concentrated than
in normal spirals, perhaps due to gas being driven into the center by the
interaction. As noted in earlier studies, there is a large variation from
galaxy to galaxy in the implied star formation rates of the interacting
galaxies, with some galaxies having enhanced rates but others being fairly
quiescent.Comment: To appear in the Astronomical Journal. 22 pages, 22 figure
The SINFONI Nearby Elliptical Lens Locator Survey: discovery of two new low-redshift strong lenses and implications for the initial mass function in giant early-type galaxies
We present results from a blind survey to identify strong gravitational lenses among the population of low-redshift early-type galaxies. The SINFONI Nearby Elliptical Lens Locator Survey (SNELLS) uses integral-field infrared spectroscopy to search for lensed emission line sources behind massive lens candidates at z 300 km s− 1) and α-element abundances ([Mg/Fe] > 0.3). From the lensing configurations we derive total J-band mass-to-light ratios of 1.8 ± 0.1, 2.1 ± 0.1 and 1.9 ± 0.2 within the ∼2 kpc Einstein radius. Correcting for estimated dark matter contributions, and comparing to stellar population models with a Milky Way (Kroupa) initial mass function (IMF), we determine the ‘mass excess factor’, α. Assuming the lens galaxies have ‘old’ stellar populations (10 ± 1 Gyr), the average IMF mass factor is 〈α〉 = 1.10 ± 0.08 ± 0.10, where the first error is random and the second is systematic. If we instead fit the stellar populations from 6dF optical survey spectra, all three galaxies are consistent with being old, but the age errors are 3–4 Gyr, due to limited signal-to-noise ratio. The IMF constraints are therefore looser in this case, with 〈α〉 = 1.23+0.16−0.13±0.10. Our results are thus consistent with a Kroupa IMF (α = 1.00) on average, and strongly reject very heavy IMFs with α ≳ 2. A Salpeter IMF (α = 1.55) is inconsistent at the 3.5σ level if the galaxies are old, but cannot be excluded using age constraints derived from the currently available optical spectra
Stars that Move Together Were Born Together
It is challenging to reliably identify stars that were born together outside
of actively star-forming regions and bound stellar systems. However, co-natal
stars should be present throughout the Galaxy, and their demographics can shed
light on the clustered nature of star formation and the dynamical state of the
disk. In previous work we presented a set of simulations of the Galactic disk
that followed the clustered formation and dynamical evolution of 4 billion
individual stars over the last 5 Gyr. The simulations predict that a high
fraction of co-moving stars with physical and 3D velocity separation of pc and km s are co-natal. In this
\textit{Letter}, we use \textit{Gaia} DR2 and LAMOST DR4 data to identify and
study co-moving pairs. We find that the distribution of relative velocities and
separations of pairs in the data is in good agreement with the predictions from
the simulation. We identify 111 co-moving pairs in the Solar neighborhood with
reliable astrometric and spectroscopic measurements. These pairs show a strong
preference for having similar metallicities when compared to random field
pairs. We therefore conclude that these pairs were very likely born together.
The simulations predict that co-natal pairs originate preferentially from
high-mass and relatively young ( Gyr) star clusters. \textit{Gaia} will
eventually deliver well-determined metallicities for the brightest stars,
enabling the identification of thousands of co-natal pairs due to disrupting
star clusters in the solar neighborhood.Comment: 6 pages, 4 figures, 1 table. Submitted to ApJL. Catalog here:
http://harshilkamdar.github.io/2019/04/03/pairs.htm
Population Parameters of Intermediate-Age Star Clusters in the Large Magellanic Cloud. III. Dynamical Evidence for a Range of Ages Being Responsible for Extended Main Sequence Turnoffs
We present new analysis of 11 intermediate-age (1-2 Gyr) star clusters in the
Large Magellanic Cloud based on Hubble Space Telescope imaging data. Seven of
the clusters feature main sequence turnoff (MSTO) regions that are wider than
can be accounted for by a simple stellar population, whereas their red giant
branches indicate a single value of [Fe/H]. The star clusters cover a range in
present-day mass from about 1E4 to 2E5 solar masses. We compare radial
distributions of stars in the upper and lower parts of the MSTO region, and
calculate cluster masses and escape velocities from the present time back to a
cluster age of 10 Myr. Our main result is that for all clusters in our sample
with estimated escape velocities > 15 km/s at an age of 10 Myr, the stars in
the brightest half of the MSTO region are significantly more centrally
concentrated than the stars in the faintest half AND more massive red giant
branch and asymptotic giant branch stars. This is not the case for clusters
with escape velocities < 10 km/s at an age of 10 Myr. We argue that the wide
MSTO region of such clusters is mainly caused by to a 200 - 500 Myr range in
the ages of cluster stars due to extended star formation within the cluster
from material shed by first-generation stars featuring slow stellar winds.
Dilution of this enriched material by accretion of ambient interstellar matter
is deemed plausible if the spread of [Fe/H] in this ambient gas was very small
when the second-generation stars were formed in the cluster.Comment: 11 pages (in emulateapj format), 6 figures, accepted for publication
in Ap
A theoretical framework for combining techniques that probe the link between galaxies and dark matter
We develop a theoretical framework that combines measurements of
galaxy-galaxy lensing, galaxy clustering, and the galaxy stellar mass function
in a self-consistent manner. While considerable effort has been invested in
exploring each of these probes individually, attempts to combine them are still
in their infancy despite the potential of such combinations to elucidate the
galaxy-dark matter connection, to constrain cosmological parameters, and to
test the nature of gravity. In this paper, we focus on a theoretical model that
describes the galaxy-dark matter connection based on standard halo occupation
distribution techniques. Several key modifications enable us to extract
additional parameters that determine the stellar-to-halo mass relation and to
simultaneously fit data from multiple probes while allowing for independent
binning schemes for each probe. In a companion paper, we demonstrate that the
model presented here provides an excellent fit to galaxy-galaxy lensing, galaxy
clustering, and stellar mass functions measured in the COSMOS survey from z=0.2
to z=1.0. We construct mock catalogs from numerical simulations to investigate
the effects of sample variance and covariance on each of the three probes.
Finally, we analyze and discuss how trends in each of the three observables
impact the derived parameters of the model. In particular, we investigate the
various features of the observed galaxy stellar mass function (low-mass slope,
plateau, knee, and high-mass cut-off) and show how each feature is related to
the underlying relationship between stellar and halo mass. We demonstrate that
the observed plateau feature in the stellar mass function at Mstellar~2x10^10
Msun is due to the transition that occurs in the stellar-to-halo mass relation
at Mhalo ~ 10^12 Msun from a low-mass power-law regime to a sub-exponential
function at higher stellar mass.Comment: 21 pages. Accepted to Ap
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