5,526 research outputs found
Traffic Profiling for Mobile Video Streaming
This paper describes a novel system that provides key parameters of HTTP
Adaptive Streaming (HAS) sessions to the lower layers of the protocol stack. A
non-intrusive traffic profiling solution is proposed that observes packet flows
at the transmit queue of base stations, edge-routers, or gateways. By analyzing
IP flows in real time, the presented scheme identifies different phases of an
HAS session and estimates important application-layer parameters, such as
play-back buffer state and video encoding rate. The introduced estimators only
use IP-layer information, do not require standardization and work even with
traffic that is encrypted via Transport Layer Security (TLS). Experimental
results for a popular video streaming service clearly verify the high accuracy
of the proposed solution. Traffic profiling, thus, provides a valuable
alternative to cross-layer signaling and Deep Packet Inspection (DPI) in order
to perform efficient network optimization for video streaming.Comment: 7 pages, 11 figures. Accepted for publication in the proceedings of
IEEE ICC'1
Are Gamma Ray Bursts due to Rotation Powered High Velocity Pulsars in the Halo ?
The BATSE experiment has now observed more than 1100 gamma-ray bursts. The
observed angular distribution is isotropic, while the brightness distribution
of bursts shows a reduced number of faint events. These observations favor a
cosmological burst origin. Alternatively, very extended Galactic Halo (EGH)
models have been considered. In the latter scenario, the currently favored
source of gamma-ray bursts involves high velocity pulsars ejected from the
Galactic disk. To be compatible with the observed isotropy, most models invoke
a sampling distance of 300 kpc, a turn-on delay of 30 Myrs, and a source life
time of about 1 Gyr. We consider the global energy requirements of such models
and show that the largest known resource. rotational kinetic energy, is
insufficient by orders of magnitude to provide the observed burst rate. More
exotic energy sources or differently tuned pulsar models may be able to get
around the global energy constraint but at the cost of becoming contrived.
Thus, while extended halo models are not ruled out, our arguments place a
severe obstacle for such models and we encourage proponents of EGH models to
clearly address the issue of global energetics.Comment: 18 pages, with 2 figures included. Postscript. ApJ, in pres
Periodic substorm activity in the geomagnetic tail
On 19 May 1978 an anusual series of events is observed with the Quadrispherical LEPEDEA on board the ISEE-1 satellite in the Earth's geomagnetic tail. For 13 hours periodic bursts of both ions and electrons are seen in all the particle detectors on the spacecraft. On this day periodic activity is also seen on the ground, where multiple intensifications of the electrojets are observed. At the same time the latitudinal component of the interplanetary magnetic field shows a number of strong southward deflections. It is concluded that an extended period of substorm activity is occurring, which causes repeated thinnings and recoveries of the plasma sheet. These are detected by ISEE, which is situated in the plasma sheet boundary layer, as periodic dropouts and reappearances of the plasma. Comparisons of the observations at ISEE with those at IMP-8, which for a time is engulfed by the plasma sheet, indicate that the activity is relatively localized in spatial extent. For this series of events it is clear that a global approach to magnetospheric dynamics, e.g., reconnection, is inappropriate
Constraints on the small-scale power spectrum of density fluctuations from high-redshift gamma-ray bursts
Cosmological models that include suppression of the power spectrum of density
fluctuations on small scales exhibit an exponential reduction of high-redshift,
non-linear structures, including a reduction in the rate of gamma ray bursts
(GRBs). Here we quantify the constraints that the detection of distant GRBs
would place on structure formation models with reduced small-scale power. We
compute the number of GRBs that could be detectable by the Swift satellite at
high redshifts (z > 6), assuming that the GRBs trace the cosmic star formation
history, which itself traces the formation of non-linear structures. We
calibrate simple models of the intrinsic luminosity function of the bursts to
the number and flux distribution of GRBs observed by the Burst And Transient
Source Experiment (BATSE). We find that a discovery of high-z GRBs would imply
strong constraints on models with reduced small-scale power. For example, a
single GRB at z > 10, or 10 GRBs at z > 5, discovered by Swift during its
scheduled two-year mission, would rule out an exponential suppression of the
power spectrum on scales below R_c=0.09 Mpc (exemplified by warm dark matter
models with a particle mass of m_x=2 keV). Models with a less sharp suppression
of small-scale power, such as those with a red tilt or a running scalar index,
n_s, are more difficult to constrain, because they are more degenerate with an
increase in the power spectrum normalization, sigma_8, and with models in which
star-formation is allowed in low-mass minihalos. We find that a tilt of \delta
n_s ~ 0.1 is difficult to detect; however, an observed rate of 1 GRB/yr at z >
12 would yield an upper limit on the running of the spectral index, alpha =
d(n_s)/d(ln k) > -0.05.Comment: 10 pages, 6 figures; Minor changes to match version published in Ap
Lightweight Asynchronous Snapshots for Distributed Dataflows
Distributed stateful stream processing enables the deployment and execution
of large scale continuous computations in the cloud, targeting both low latency
and high throughput. One of the most fundamental challenges of this paradigm is
providing processing guarantees under potential failures. Existing approaches
rely on periodic global state snapshots that can be used for failure recovery.
Those approaches suffer from two main drawbacks. First, they often stall the
overall computation which impacts ingestion. Second, they eagerly persist all
records in transit along with the operation states which results in larger
snapshots than required. In this work we propose Asynchronous Barrier
Snapshotting (ABS), a lightweight algorithm suited for modern dataflow
execution engines that minimises space requirements. ABS persists only operator
states on acyclic execution topologies while keeping a minimal record log on
cyclic dataflows. We implemented ABS on Apache Flink, a distributed analytics
engine that supports stateful stream processing. Our evaluation shows that our
algorithm does not have a heavy impact on the execution, maintaining linear
scalability and performing well with frequent snapshots.Comment: 8 pages, 7 figure
Cosmic-ray Acceleration and Propagation
The origin of cosmic rays (CRs) has puzzled scientists since the pioneering
discovery by Victor Hess in 1912. In the last decade, however, modern
supercomputers have opened a new window on the processes regulating
astrophysical collisionless plasmas, allowing the study of CR acceleration via
first-principles kinetic simulations. At the same time, a new-generation of
X-ray and -ray telescopes has been collecting evidence that Galactic
CRs are accelerated in the blast waves of supernova remnants (SNRs). I present
state-of-the-art particle-in-cells simulations of non-relativistic shocks, in
which ion and electron acceleration efficiency and magnetic field amplification
are studied in detail as a function of the shock parameters. I then discuss the
theoretical and observational counterparts of these findings, comparing them
with predictions of diffusive shock acceleration theory and with
multi-wavelength observations of young SNRs. I especially outline some major
open questions, such as the possible causes of the steep CR spectra inferred
from -ray observations of SNRs and the origin of the knee in the
Galactic CR spectrum. Finally, I put such a theoretical understanding in
relation with CR propagation in the Galaxy in order to bridge the gap between
acceleration in sources and measurements of CRs at Earth.Comment: 24 pages, 7 figures, Invited Review Talk at the 34th International
Cosmic Ray Conference, The Hague, The Netherland
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