539 research outputs found
Delays in IP routers, a Markov model
Delays in routers are an important component of end-to-end delay and therefore have a significant impact on quality of service. While the other component, the propagation time, is easy to predict as the distance divided by the speed of light inside the link, the queueing delays of packets inside routers depend on the current, usually dynamically changing congestion and on the stochastic features of the flows. We use a Markov model taking into account the distribution of the size of packets and self-similarity of incoming flows to investigate their impact on the queueing delays and their dynamics
Persistence time of SIS infections in heterogeneous populations and networks
For a susceptible-infectious-susceptible (SIS) infection model in a
heterogeneous population, we present simple formulae giving the leading-order
asymptotic (large population) behaviour of the mean persistence time, from an
endemic state to extinction of infection. Our model may be interpreted as
describing an infection spreading through either (i) a population with
heterogeneity in individuals' susceptibility and/or infectiousness; or (ii) a
heterogeneous directed network. Using our asymptotic formulae, we show that
such heterogeneity can only reduce (to leading order) the mean persistence time
compared to a corresponding homogeneous population, and that the greater the
degree of heterogeneity, the more quickly infection will die out
Diffusion Models for Double-ended Queues with Renewal Arrival Processes
We study a double-ended queue where buyers and sellers arrive to conduct
trades. When there is a pair of buyer and seller in the system, they
immediately transact a trade and leave. Thus there cannot be non-zero number of
buyers and sellers simultaneously in the system. We assume that sellers and
buyers arrive at the system according to independent renewal processes, and
they would leave the system after independent exponential patience times. We
establish fluid and diffusion approximations for the queue length process under
a suitable asymptotic regime. The fluid limit is the solution of an ordinary
differential equation, and the diffusion limit is a time-inhomogeneous
asymmetric Ornstein-Uhlenbeck process (O-U process). A heavy traffic analysis
is also developed, and the diffusion limit in the stronger heavy traffic regime
is a time-homogeneous asymmetric O-U process. The limiting distributions of
both diffusion limits are obtained. We also show the interchange of the heavy
traffic and steady state limits
Bayesian prediction of the transient behaviour and busy period in short and long-tailed GI/G/1 queueing systems
Bayesian inference for the transient behavior and duration of a busy period in a single server queueing
system with general, unknown distributions for the interarrival and service times is investigated. Both
the interarrival and service time distributions are approximated using the dense family of Coxian distributions. A suitable reparameterization allows the definition of a non-informative prior and Bayesian
inference is then undertaken using reversible jump Markov chain Monte Carlo methods. An advantage of
the proposed procedure is that heavy tailed interarrival and service time distributions such as the Pareto
can be well approximated. The proposed procedure for estimating the system measures is based on
recent theoretical results for the Coxian/Coxian/1 system. A numerical technique is developed for every
MCMC iteration so that the transient queue length and waiting time distributions and the duration of
a busy period can be estimated. The approach is illustrated with both simulated and real data
Meter-scale spark X-ray spectrumstatistics
X-ray emission by sparks implies bremsstrahlung from a population of
energetic electrons, but the details of this process remain a mystery. We
present detailed statistical analysis of X-ray spectra detected by multiple
detectors during sparks produced by 1 MV negative high-voltage pulses with 1
s risetime. With over 900 shots, we statistically analyze the signals,
assuming that the distribution of spark X-ray fluence behaves as a power law
and that the energy spectrum of X-rays detectable after traversing 2 m of
air and a thin aluminum shield is exponential. We then determine the parameters
of those distributions by fitting cumulative distribution functions to the
observations. The fit results match the observations very well if the mean of
the exponential X-ray energy distribution is 86 7 keV and the spark X-ray
fluence power law distribution has index -1.29 0.04 and spans at least 3
orders of magnitude in fluence
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