106 research outputs found
An Enhancement to the IEEE 802.11e EDCA Providing QoS Guarantees
One of the challenges that must be overcome to realize the practical benefits of ad hoc networks is quality of service (QoS). However, the IEEE 802.11 standard, which undeniably is the most widespread wireless technology of choice for WLANs and ad hoc networks, does not address this issue. In order to support applications with QoS requirements, the upcoming IEEE 802.11e standard enhances the original IEEE 802.11 MAC protocol by introducing a new coordination function which has both contention-based and contention-free medium access methods. In this paper, we consider the contention-based medium access method, the EDCA, and propose an extension to it such that it can be used to provide QoS guarantees in WLANs operating in ad hoc mode. Our solution is fully distributed, uses admission control to regulate the usage of resources and gives stations with high-priority traffic streams an opportunity to reserve time for collision-free access to the medium
Micro mobility and internet access performance for TCP connections in Ad Hoc networks
In ad hoc mobile networks nodes typically communicate over wireless channels and are capable of movement. These are networks that support multihop communication and can be formed on a temporary basis. This paper evaluates a solution that allows mobile nodes to access the wired Internet and roam from base station to base station. The solution is based on the extension of Mobile IP capabilities to the ad hoc network while a micro-mobility protocol is adapted to support local migration. We evaluate the performance of this solution with regard to reliable transport layer connections. It is shown that a high throughput is possible to achieve for high mobility speeds. It is also observed that, as the number of hops between a mobile node and the base station increases, the throughput is decreased because of the characteristics of the wireless environment and the medium access layer protocol
Load Reduction for Timely Applications in the Cloud
In many IoT applications, sensor data is sent remotely to be processed, but only the freshest result is of interest. In this paper we investigate a feedback mechanism that aborts the processing of stale data at the remote end to reduce the load and save costs. The process is approximated by an M/M/∞ queueing system with a feedback loop. We find the exact expression of the average computational time saved and show that with the feedback loop in place the computation time per CPU can be cut up to 25%, making the technique very promising
Author Correction: Limited capacity of tree growth to mitigate the global greenhouse effect under predicted warming.
An amendment to this paper has been published and can be accessed via a link at the top of the paper
Dispersion-Theoretical Analysis of the Nucleon Electromagnetic Formfactors
Dispersion relations allow for a coherent description of the nucleon
electromagnetic form factors measured over a large range of momentum transfer,
GeV. Including constraints from unitarity and
perturbative QCD, we present a novel parametrisation of the absorptive parts of
the various isoscalar and isovector nucleon form factors. Using the current
world data, we obtain results for the electromagnetic form factors, nucleon
radii and meson couplings. We stress the importance of measurements at large
momentum transfer to test the predictions of perturbative QCD.Comment: 33 pp, RevTEX or plain LaTeX, 7 figures (in ffig.uu
On the strong energy dependence of the e^+e^- to ppbar amplitude near threshold
We study the energy dependence of the e^+e^- to ppbar cross section close to
the two-nucleon threshold, recently reported by the BaBar collaboration. Our
analysis also includes the pbarp to e^+e^- data collected by PS170
collaboration and the e^+e^- to NNbar data from the FENICE collaboration. We
show that the near-threshold enhancement in the e^+e^- to ppbar cross section
can be explained by the final-state interaction between proton and antiproton
in the 3S1 partial wave, utilizing the Juelich nucleon-antinucleon model. As a
consequence, the strong dependence of the proton electromagnetic form factors
on the momentum transfer close to the two-nucleon threshold is presumably also
driven by this final-state interaction effect. This result is in line with our
previous studies of the near-threshold enhancement of the ppbar invariant mass
spectrum seen in the J/Psi to gamma ppbar decay by the BES collaboration and in
the B+ to ppbar K+ decay by the BaBar collaboration.Comment: 7 pages, 2 figures, Fig. 2 modified, conclusions unchange
Plasticity in dendroclimatic response across the distribution range of Aleppo pine (Pinus halepensis)
We investigated the variability of the climate-growth relationship of Aleppo pine across its distribution range in the Mediterranean Basin. We constructed a network of tree-ring index chronologies from 63 sites across the region. Correlation function analysis identified the relationships of tree-ring index to climate factors for each site. We also estimated the dominant climatic gradients of the region using principal component analysis of monthly, seasonal, and annual mean temperature and total precipitation from 1,068 climatic gridpoints. Variation in ring width index was primarily related to precipitation and secondarily to temperature. However, we found that the dendroclimatic relationship depended on the position of the site along the climatic gradient. In the southern part of the distribution range, where temperature was generally higher and precipitation lower than the regional average, reduced growth was also associated with warm and dry conditions. In the northern part, where the average temperature was lower and the precipitation more abundant than the regional average, reduced growth was associated with cool conditions. Thus, our study highlights the substantial plasticity of Aleppo pine in response to different climatic conditions. These results do not resolve the source of response variability as being due to either genetic variation in provenance, to phenotypic plasticity, or a combination of factors. However, as current growth responses to inter-annual climate variability vary spatially across existing climate gradients, future climate-growth relationships will also likely be determined by differential adaptation and/or acclimation responses to spatial climatic variation. The contribution of local adaptation and/or phenotypic plasticity across populations to the persistence of species under global warming could be decisive for prediction of climate change impacts across populations. In this sense, a more complex forest dynamics modeling approach that includes the contribution of genetic variation and phenotypic plasticity can improve the reliability of the ecological inferences derived from the climate-growth relationships.This work was partially supported by Spanish Ministry of Education and Science co-funded by FEDER program (CGL2012-31668), the European Union and the National Ministry of Education and Religion of Greece (EPEAEK- Environment – Archimedes), the Slovenian Research Agency (program P4-0015), and the USDA Forest Service. The cooperation among international partners was supported by the COST Action FP1106, STREeSS
Effects of Warming on Shrub Abundance and Chemistry Drive Ecosystem-Level Changes in a Forest–Tundra Ecotone
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