Adaptive medium access control for VoIP services in IEEE 802.11 WLANs

Abstract- Voice over Internet Protocol (VoIP) is an important service with strict Quality-of-Service (QoS) requirements in Wireless Local Area Networks (WLANs). The popular Distributed Coordination Function (DCF) of IEEE 802.11 Medium Access Control (MAC) protocol adopts a Binary Exponential Back-off (BEB) procedure to reduce the packet collision probability in WLANs. In DCF, the size of contention window is doubled upon a collision regardless of the network loads. This paper presents an adaptive MAC scheme to improve the QoS of VoIP in WLANs. This scheme applies a threshold of the collision rate to switch between two different functions for increasing the size of contention window based on the status of network loads. The performance of this scheme is investigated and compared to the original DCF using the network simulator NS-2. The performance results reveal that the adaptive scheme is able to achieve the higher throughput and medium utilization as well as lower access delay and packet loss probability than the original DCF

Constraining the Circumbinary Envelope of Z CMa via imaging polarimetry

Z CMa is a complex binary system, composed of a Herbig Be and an FU Ori star. The Herbig star is surrounded by a dust cocoon of variable geometry, and the whole system is surrounded by an infalling envelope. Previous spectropolarimetric observations have reported a preferred orientation of the polarization angle, perpendicular to the direction of a large, parsec-sized jet associated with the Herbig star. The variability in the amount of polarized light has been associated to changes in the geometry of the dust cocoon that surrounds the Herbig star. We aim to constrain the properties of Z CMa by means of imaging polarimetry at optical wavelengths. Using ExPo, a dual-beam imaging polarimeter which operates at optical wavelengths, we have obtained imaging (linear) polarimetric data of Z CMa. Our observations were secured during the return to quiescence after the 2008 outburst. We detect three polarized features over Z CMa. Two of these features are related to the two jets reported in this system: the large jet associated to the Herbig star, and the micro-jet associated to the FU Ori star. Our results suggest that the micro-jet extends to a distance ten times larger than reported in previous studies. The third feature suggests the presence of a hole in the dust cocoon that surrounds the Herbig star of this system. According to our simulations, this hole can produce a pencil beam of light that we see scattered off the low-density envelope surrounding the system.Comment: Accepted for publication in A\&

Laboratory mid-IR spectra of equilibrated and igneous meteorites. Searching for observables of planetesimal debris

Meteorites contain minerals from Solar System asteroids with different properties (like size, presence of water, core formation). We provide new mid-IR transmission spectra of powdered meteorites to obtain templates of how mid-IR spectra of asteroidal debris would look like. This is essential for interpreting mid-IR spectra of past and future space observatories, like the James Webb Space Telescope. We show that the transmission spectra of wet and dry chondrites, carbonaceous and ordinary chondrites and achondrite and chondrite meteorites are distinctly different in a way one can distinguish in astronomical mid-IR spectra. The two observables that spectroscopically separate the different meteorites groups (and thus the different types of parent bodies) are the pyroxene-olivine feature strength ratio and the peak shift of the olivine spectral features due to an increase in the iron concentration of the olivine

Laboratory-based grain-shape models for simulating dust infrared spectra

Analysis of thermal dust emission spectra for dust mineralogy and physical grain properties depends on laboratory-measured or calculated comparison spectra. Often, the agreement between these two kinds of spectra is not satisfactory because of the strong influence of the grain morphology on the spectra. We investigate the ability of the statistical light-scattering model with a distribution of form factors (DFF model) to reproduce experimentally measured infrared extinction spectra for particles that are small compared to the wavelength. We take advantage of new experimental spectra measured for free particles dispersed in air with accompanying information on the grain morphology. For the calculations, we used DFFs that were derived for aggregates of spherical grains, as well as for compact grain shapes corresponding to Gaussian random spheres. Irregular particle shapes require a DFF similar to that of a Gaussian random sphere with sigma=0.3, whereas roundish grain shapes are best fitted with that of a fractal aggregate of a fractal dimension 2.4-1.8. In addition we used a fitting algorithm to obtain the best-fit DFFs for the various laboratory samples. In this way we can independently derive information on the shape of the grains from their infrared spectra. For anisotropic materials, different DFFs are needed for the different crystallographic axes. This is due to a theoretical problem, which is inherent to all models that are simply averaging the contributions of the crystallographic directions.Comment: 8 pages, 8 figures, accepted by Astronomy and Astrophysic

Dust-grain processing in circumbinary discs around evolved binaries. The RV Tauri spectral twins RU Cen and AC Her

Context: We study the structure and evolution of circumstellar discs around evolved binaries and their impact on the evolution of the central system. Aims: To study in detail the binary nature of RUCen and ACHer, as well as the structure and mineralogy of the circumstellar environment. Methods: We combine multi-wavelength observations with a 2D radiative transfer study. Our radial velocity program studies the central stars, while our Spitzer spectra and broad-band SEDs are used to constrain mineralogy, grain sizes and physical structure of the circumstellar environment. Results: We determine the orbital elements of RUCen showing that the orbit is highly eccentric with a rather long period of 1500 days. The infrared spectra of both objects are very similar and the spectral dust features are dominated by Mg-rich crystalline silicates. The small peak-to-continuum ratios are interpreted as being due to large grains. Our model contains two components with a cold midplain dominated by large grains, and the near- and mid-IR which is dominated by the emission of smaller silicates. The infrared excess is well modelled assuming a hydrostatic passive irradiated disc. The profile-fitting of the dust resonances shows that the grains must be very irregular. Conclusions: These two prototypical RVTauri pulsators with circumstellar dust are binaries where the dust is trapped in a stable disc. The mineralogy and grain sizes show that the dust is highly processed, both in crystallinity and grain size. The cool crystals show that either radial mixing is very efficient and/or that the thermal history at grain formation has been very different from that in outflows. The physical processes governing the structure of these discs are similar to those observed in protoplanetary discs around young stellar objects.Comment: 11 pages, 12 figures, accepted for publication by A&

Direct and Indirect Detection of Neutralino Dark Matter and Collider Signatures in an SO(10)SO(10) Model with Two Intermediate Scales

We investigate the detectability of neutralino Dark Matter via direct and indirect searches as well as collider signatures of an $SO(10)$ model with two intermediate scales. We compare the direct Dark Matter detection cross section and the muon flux due to neutralino annihilation in the Sun that we obtain in this model with mSUGRA predictions and with the sensitivity of current and future experiments. In both cases, we find that the detectability improves as the model deviates more from mSUGRA. In order to study collider signatures, we choose two benchmark points that represent the main phenomenological features of the model: a lower value of $|\mu|$ and reduced third generation sfermion masses due to extra Yukawa coupling contributions in the Renormalization Group Equations, and increased first and second generation slepton masses due to new gaugino loop contributions. We show that measurements at the LHC can distinguish this model from mSUGRA in both cases, by counting events containing leptonically decaying $Z^0$ bosons, heavy neutral Higgs bosons, or like--sign lepton pairs.Comment: 21 pages, 16 figure

The color dependent morphology of the post-AGB star HD161796

Context. Many protoplanetary nebulae show strong asymmetries in their surrounding shell, pointing to asymmetries during the mass loss phase. Questions concerning the origin and the onset of deviations from spherical symmetry are important for our understanding of the evolution of these objects. Here we focus on the circumstellar shell of the post-AGB star HD 161796. Aims. We aim at detecting signatures of an aspherical outflow, as well as to derive the properties of it. Methods. We use the imaging polarimeter ExPo (the extreme polarimeter), a visitor instrument at the William Herschel Telescope, to accurately image the dust shell surrounding HD 161796 in various wavelength filters. Imaging polarimetry allows us to separate the faint, polarized, light from circumstellar material from the bright, unpolarized, light from the central star. Results. The shell around HD 161796 is highly aspherical. A clear signature of an equatorial density enhancement can be seen. This structure is optically thick at short wavelengths and changes its appearance to optically thin at longer wavelengths. In the classification of the two different appearances of planetary nebulae from HST images it changes from being classified as DUPLEX at short wavelengths to SOLE at longer wavelengths. This strengthens the interpretation that these two appearances are manifestations of the same physical structure. Furthermore, we find that the central star is hotter than often assumed and the relatively high observed reddening is due to circumstellar rather than interstellar extinction.Comment: Accepted for publication in A&

The Gluon Spin in the Chiral Bag Model

We study the gluon polarization contribution at the quark model renormalization scale to the proton spin, $\Gamma$, in the chiral bag model. It is evaluated by taking the expectation value of the forward matrix element of a local gluon operator in the axial gauge $A^+=0$. It is shown that the confining boundary condition for the color electric field plays an important role. When a solution satisfying the boundary condition for the color electric field, which is not the conventionally used but which we favor, is used, the $\Gamma$ has a positive value for {\it all} bag radii and its magnitude is comparable to the quark spin polarization. This results in a significant reduction in the relative fraction of the proton spin carried by the quark spin, which is consistent with the small flavor singlet axial current measured in the EMC experiments.Comment: Corrections to figure