Performance of CSMA in Multi-Channel Wireless Networks

We analyze the performance of CSMA in multi-channel wireless networks, accounting for the random nature of traffic. Specifically, we assess the ability of CSMA to fully utilize the radio resources and in turn to stabilize the network in a dynamic setting with flow arrivals and departures. We prove that CSMA is optimal in ad-hoc mode but not in infrastructure mode, when all data flows originate from or are destined to some access points, due to the inherent bias of CSMA against downlink traffic. We propose a slight modification of CSMA, that we refer to as flow-aware CSMA, which corrects this bias and makes the algorithm optimal in all cases. The analysis is based on some time-scale separation assumption which is proved valid in the limit of large flow sizes

On the stability of flow-aware CSMA

We consider a wireless network where each flow (instead of each link) runs its own CSMA (Carrier Sense Multiple Access) algorithm. Specifically, each flow attempts to access the radio channel after some random time and transmits a packet if the channel is sensed idle. We prove that, unlike the standard CSMA algorithm, this simple distributed access scheme is optimal in the sense that the network is stable for all traffic intensities in the capacity region of the network

Analysis of Large Unreliable Stochastic Networks

In this paper a stochastic model of a large distributed system where users' files are duplicated on unreliable data servers is investigated. Due to a server breakdown, a copy of a file can be lost, it can be retrieved if another copy of the same file is stored on other servers. In the case where no other copy of a given file is present in the network, it is definitively lost. In order to have multiple copies of a given file, it is assumed that each server can devote a fraction of its processing capacity to duplicate files on other servers to enhance the durability of the system. A simplified stochastic model of this network is analyzed. It is assumed that a copy of a given file is lost at some fixed rate and that the initial state is optimal: each file has the maximum number $d$ of copies located on the servers of the network. Due to random losses, the state of the network is transient and all files will be eventually lost. As a consequence, a transient $d$-dimensional Markov process $(X(t))$ with a unique absorbing state describes the evolution this network. By taking a scaling parameter $N$ related to the number of nodes of the network. a scaling analysis of this process is developed. The asymptotic behavior of $(X(t))$ is analyzed on time scales of the type $t\mapsto N^p t$ for $0\leq p\leq d{-}1$. The paper derives asymptotic results on the decay of the network: Under a stability assumption, the main results state that the critical time scale for the decay of the system is given by $t\mapsto N^{d-1}t$. When the stability condition is not satisfied, it is shown that the state of the network converges to an interesting local equilibrium which is investigated. As a consequence it sheds some light on the role of the key parameters $\lambda$, the duplication rate and $d$, the maximal number of copies, in the design of these systems

Etudes sur la flore des Guyanes / 22 : deux Passifloraceae nouvelles et quelques espèces rares en Guyane française

Description de #Passiflora rufostipulata et #P. fanchonae, espèces nouvelles de Guyane française ; premières récoltes en Guyane de #P. acuminata et #P. variolata ; récoltes de l'espèce mal connue #P. citrifolia$. (Résumé d'auteur

Observations sur la limite inférieure de l'étage périglaciaire dans les Pyrénées centrales françaises (Bigorre)

Les limites inférieures de l'étage périglaciaire sont caractérisées dans une partie des Pyrénées centrales françaises par des marqueurs morphologiques. Ces marqueurs sont corrélatifs d'un gel saisonnier et peu profond des sols : petits sols structurés, blocs fluants, terrassettes de gélifluxion. L'altitude de 2 250 m paraît correspondre à un seuil vers lequel ces marqueurs s'associent et se multiplient. À cette altitude, la température annuelle moyenne de l'air est d'environ 2,5°C et la température du mois le plus chaud est de l'ordre de 10°C. Ces limites varient principalement en fonction des caractéristiques propres à la montagne atlantique : vent, neige, humidité et fraîcheur estivale. Leur mobilité répond directement aux fluctuations climatiques. Le repérage des manifestations élémentaires d'une activité périglaciaire actuelle représente un outil privilégié pour enregistrer ces fluctuations