426 research outputs found
Perturbation analysis of an M/M/1 queue in a diffusion random environment
We study in this paper an queue whose server rate depends upon the
state of an independent Ornstein-Uhlenbeck diffusion process so that
its value at time is , where is some bounded
function and . We first establish the differential system for the
conditional probability density functions of the couple in the
stationary regime, where is the number of customers in the system at
time . By assuming that is defined by for some positive real numbers
, and , we show that the above differential system has a
unique solution under some condition on and . We then show that this
solution is close, in some appropriate sense, to the solution to the
differential system obtained when is replaced with
for sufficiently small . We finally
perform a perturbation analysis of this latter solution for small
. This allows us to check at the first order the validity of the
so-called reduced service rate approximation, stating that everything happens
as if the server rate were constant and equal to \mu(1-\eps\E(X(t)))
Perturbation Analysis of a Variable M/M/1 Queue: A Probabilistic Approach
Motivated by the problem of the coexistence on transmission links of
telecommunication networks of elastic and unresponsive traffic, we study in
this paper the impact on the busy period of an M/M/1 queue of a small
perturbation in the server rate. The perturbation depends upon an independent
stationary process (X(t)) and is quantified by means of a parameter \eps \ll 1.
We specifically compute the two first terms of the power series expansion in
\eps of the mean value of the busy period duration. This allows us to study the
validity of the Reduced Service Rate (RSR) approximation, which consists in
comparing the perturbed M/M/1 queue with the M/M/1 queue where the service rate
is constant and equal to the mean value of the perturbation. For the first term
of the expansion, the two systems are equivalent. For the second term, the
situation is more complex and it is shown that the correlations of the
environment process (X(t)) play a key role
La nano-intégration : un besoin d'adaptation des procédés de fabrication actuels
L'arrivĂ©e des nanomatĂ©riaux parmi les objets du quotidien semble porteuse de fonctions d'usage innovantes, mais pose de nombreux problĂšmes de mise en Ćuvre lors de l'intĂ©gration de particules nanomĂ©triques dans des procĂ©dĂ©s de fabrication standardisĂ©s. Il semble donc impĂ©ratif d'adapter les procĂ©dĂ©s de transformation habituels
An identification problem in an urn and ball model with heavy tailed distributions
We consider in this paper an urn and ball problem with replacement, where
balls are with different colors and are drawn uniformly from a unique urn. The
numbers of balls with a given color are i.i.d. random variables with a heavy
tailed probability distribution, for instance a Pareto or a Weibull
distribution. We draw a small fraction of the total number of balls.
The basic problem addressed in this paper is to know to which extent we can
infer the total number of colors and the distribution of the number of balls
with a given color. By means of Le Cam's inequality and Chen-Stein method,
bounds for the total variation norm between the distribution of the number of
balls drawn with a given color and the Poisson distribution with the same mean
are obtained. We then show that the distribution of the number of balls drawn
with a given color has the same tail as that of the original number of balls.
We finally establish explicit bounds between the two distributions when each
ball is drawn with fixed probability
Inference of Flow Statistics via Packet Sampling in the Internet
We show in this note that by deterministic packet sampling, the tail of the
distribution of the original flow size can be obtained by rescaling that of the
sampled flow size. To recover information on the flow size distribution lost
through packet sampling, we propose some heuristics based on measurements from
different backbone IP networks. These heuristic arguments allow us to recover
the complete flow size distribution
On the Statistical Characterization of Flows in Internet Traffic with Application to Sampling
A new method of estimating some statistical characteristics of TCP flows in
the Internet is developed in this paper. For this purpose, a new set of random
variables (referred to as observables) is defined. When dealing with sampled
traffic, these observables can easily be computed from sampled data. By
adopting a convenient mouse/elephant dichotomy also dependent on traffic, it is
shown how these variables give a reliable statistical representation of the
number of packets transmitted by large flows during successive time intervals
with an appropriate duration. A mathematical framework is developed to estimate
the accuracy of the method. As an application, it is shown how one can estimate
the number of large TCP flows when only sampled traffic is available. The
algorithm proposed is tested against experimental data collected from different
types of IP networks
Micro-Nano : des comportements différents, des procédés à adapter
6 pagesL'arrivĂ©e de nanomatĂ©riaux parmi les diffĂ©rents matĂ©riaux utilisĂ©s en pyrotechnie pose de nombreux problĂšmes de mise en Ćuvre. En effet, leurs propriĂ©tĂ©s texturales particuliĂšres ne permettent pas de les utiliser comme des matĂ©riaux classiques. Il est impĂ©ratif d'adapter les procĂ©dĂ©s de mise en Ćuvre habituels. Pour obtenir l'adĂ©quation de ces nouveaux matĂ©riaux et de nos procĂ©dĂ©s, des Ă©tudes spĂ©cifiques du comportement des nanomatĂ©riaux au cours des procĂ©dĂ©s de mise en Ćuvre par coulĂ©e ont Ă©tĂ© rĂ©alisĂ©s. L'observation des Ă©carts de comportement des nanomatĂ©riaux par rapport aux "micro-matĂ©riaux" pour une mĂȘme nature chimique et des conditions identiques de mise en Ćuvre permet d'Ă©tablir des lois de comportement spĂ©cifiques, fonction de la taille des particules manipulĂ©es. Les mentalitĂ©s, comme les procĂ©dĂ©s, doivent Ă©voluer et prendre en compte l'Ă©lĂ©vation de surface spĂ©cifique qu'offre le passage Ă l'Ă©chelle nanomĂ©trique. Ainsi un chargement moindre en nanomatĂ©riaux ne sera pas forcement synonyme de lacunes sur les caractĂ©ristiques du produit fini (propriĂ©tĂ©s mĂ©caniques ou pyrotechniques). Deux questions peuvent alors se poser : faut-il adapter les formulations aux procĂ©dĂ©s, en prenant en compte les caractĂ©ristiques des matiĂšres premiĂšres ou bien adapter des procĂ©dĂ©s devant supporter des taux de chargement Ă©levĂ©s en nanomatĂ©riaux ? Le manque de connaissance et le recul sur le comportement des nanomatĂ©riaux sont un frein Ă leur intĂ©gration dans des composĂ©s pyrotechniques mais les propriĂ©tĂ©s exhibĂ©es laissent prĂ©sagĂ©es d'un grand intĂ©rĂȘt en vue de l'optimisation des propriĂ©tĂ©s des composĂ©s fabriquĂ©s
A Flow Time Model for Melt-Cast Insensitive Explosive Process
International audienceDiphasic flows of concentrated suspensions of melt-cast insensitive explosives exhibit specific rheological properties. In order to limit the handling of pyrotechnical products presenting a risk with respect to the mechanical and thermal shocks, a lot of work has been undertaken for many years in the civil engineering sector. The objective of this study is to propose a predictive model of the flow time of a concentrated suspension through a nozzle located at the bottom of a tank. Similar to our industrial process, the suspension is made out of insensitive energetic materials and flows under gravity. Experimental results are compared to three models (Quemada, Krieger-Dougherty, and Mooney) predicting the viscosity ÎŒ of a suspension as a function of the solid volume fraction Ï, the maximum packing density Ïm and the viscosity ÎŒ0 of the interstitial liquid. De Larrard's model is used to calculate Ïm. The value of viscosity measured for the pure liquid is close to the one predicted by the Bernoulli theorem, where liquids are considered as incompressible and inviscid. Finally, it was found that the Quemada's model gives a fair agreement between predictions and experiments
Adaptive algorithms for identifying large flows in IP traffic
We propose in this paper an on-line algorithm based on Bloom filters for
identifying large flows in IP traffic (a.k.a. elephants). Because of the large
number of small flows, hash tables of these algorithms have to be regularly
refreshed. Recognizing that the periodic erasure scheme usually used in the
technical literature turns out to be quite inefficient when using real traffic
traces over a long period of time, we introduce a simple adaptive scheme that
closely follows the variations of traffic. When tested against real traffic
traces, the proposed on-line algorithm performs well in the sense that the
detection ratio of long flows by the algorithm over a long time period is quite
high. Beyond the identification of elephants, this same class of algorithms is
applied to the closely related problem of detection of anomalies in IP traffic,
e.g., SYN flood due for instance to attacks. An algorithm for detecting SYN and
volume flood anomalies in Internet traffic is designed. Experiments show that
an anomaly is detected in less than one minute and the targeted destinations
are identified at the same time
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