Section: New Results
Performance analysis and networks protocols
Participants : Anthony Busson [correspondant] , Thomas Begin, Isabelle Guérin Lassous.
 Modeling and optimization of CSMA/CA in VANET [7] .

We propose a simple theoretical model to compute the maximum spatial reuse feasible in a VANET. We focus on the ad hoc mode of the IEEE 802.11p standard. Our model offers simple and closedform formulas on the maximum number of simultaneous transmitters, and on the distribution of the distance between them. It leads to an accurate upper bound on the maximum capacity. In order to validate our approach, results from the analytical models are compared to simulations performed with the network simulator NS3. We take into account different traffic distributions (traffic of vehicles), and study the impact of this traffic on capacity. An application of this work is the parameterization of the CSMA/CA mechanism.
 Fast and accurate approximate performance analysis of multiserver facilities [4] .

Systems with multiple servers are common in many areas and their correct dimensioning is in general a difficult problem under realistic assumptions on the pattern of user arrivals and service time distribution. We present an approximate solution for the underlying $Ph/Ph/c/N$ queueing model. Our approximation decomposes the solution of the Ph/Ph/c/N queue into solutions of simpler $M/Ph/c/N$ and $Ph/M/c/N$ queues. To further mitigate dimensionality issues, for larger numbers of servers and/or service time phases, we use a reduced state approximation to solve the $M/Ph/c/N$ queue. The proposed approach is conceptually simple, easy to implement and produces generally accurate results for the mean number in the system, as well as the loss probability. Typical relative errors for these two quantities are below 5%. A very significant speed advantage compared to the numerical solution of the full $Ph/Ph/c/N$ queue can be gained as the number of phases representing the arrival process and/or the number of servers increases.
 Interference and throughput in spectrum sensing cognitive radio networks using point processes .

Spectrum sensing is vital for secondary unlicensed nodes to coexist and avoid interference with the primary licensed users in cognitive wireless networks. In this paper, we develop models for bounding interference levels from secondary network to the primary nodes within a spectrum sensing framework. Instead of classical stochastic approaches where Poisson point processes are used to model transmitters, we consider a more practical model which takes into account the medium access control regulations and where the secondary Poisson process is judiciously thinned in two phases to avoid interference with the secondary as well as the primary nodes. The resulting process will be a modified version of the Matérn point process. For this model, we obtain bounds for the complementary cumulative distribution function of interference and present simulation results which show the developed analytical bounds are quite tight. Moreover, we use these bounds to find the operation regions of the secondary network such that the interference constraint is satisfied on receiving primary nodes. We then obtain theoretical results on the primary and secondary throughputs and find the throughput limits under the interference constraint.
 Modeling of IEEE 802.11 Multihop Wireless Chains with Hidden Nodes [11] .

We follow up an existing modeling framework to analytically evaluate the performance of multihop flows along a wireless chain of four nodes. The proposed model accounts for a nonperfect physical layer, handles the hidden node problem, and is applicable under workload conditions ranging from flow(s) with low intensity to flow(s) causing the network to saturate. Its solution is easily and quickly obtained and delivers estimates for the expected throughput and for the datagram loss probability of the chain with a good accuracy.
 Anticipation of ETX Metric to manage Mobility in Ad Hoc Wireless Networks [19] .

When a node is moving in a wireless network, the routing metrics associated to its wireless links may reflect link quality degradations and help the routing process to adapt its routes. Unfortunately, an important delay between the metric estimation and its inclusion in the routing process makes this approach inefficient. In this paper, we introduce an algorithm that predicts metric values a few seconds in advance, in order to compensate the delay involved by the link quality measurement and their dissemination by the routing protocol. We consider classical metrics, in particular ETX (Expected Transmission Count) and ETT (Expected Transmission Time), but we combine their computations to our prediction algorithm. Extensive simulations show the route enhancement as the Packet Delivery Ratio (PDR) is close to 1 in presence of mobility.