The most important standard in wireless local area networks (WLANs) is IEEE 802.11. For this reason, much of the research work for the enhancement of WLANs is generally based on the behaviour of the IEEE 802.11 standard. This standard is divided into several layers. One of the important layers is the medium access control (MAC) layer. It plays an important role in accessing the transmission medium and data transmission of wireless stations. However, it still presents many challenges related to the performance metrics of quality of service (QoS), such as system throughput and access delay. Modelling and performance analysis of the MAC layer are also extremely important. Thus, the performance modelling and analysis have become very important in the design and enhancement of wireless networks. Therefore, this research work is devoted to evaluate and enhance the performance modelling of IEEE 802.11 MAC-distributed coordination function (DCF), which can lead to the improvement of the performance metrics of QoS. In order to more accurately evaluate the system performance for IEEE 802.11 DCF, a new analytical model to compute a packet transmission probability for IEEE 802.11 DCF has been proposed based on difference probabilities in transmission mechanism. The performance saturated throughput is then evaluated with the proposed analytical model. In addition, a new analytical model for estimating the MAC layer packet delay distribution of IEEE 802.11 DCF is also proposed. The performance results highlight the importance of considering the different probabilities between events in transmission mechanism for an accurate performance evaluation model of IEEE 802.11 DCF in terms of throughput and delay. To enhance the effectiveness of IEEE 802.11 DCF, a new dynamic control backoff time algorithm to enhance both the delay and throughput performances of the IEEE 802.11 DCF is proposed. This algorithm considers the distinction between high and low traffic loads in order to deal with unsaturated traffic load conditions. In particular, the equilibrium point analysis (EPA) model is used to represent the algorithm under various traffic load conditions. Results of extensive simulation experiments illustrate that the proposed algorithm yields better performance throughput and a better average transmission packet delay than related algorithms.

Covert channels are becoming more popular as security risks grow in networks. One area that is promising for covert channels is wireless networks, since many use a collision avoidance scheme such as carrier sense multiple access with collision avoidance (CSMA/CA). These schemes often introduce randomness in the network, which provides good cover for a covert timing channel. In this thesis, we use the 802.11 standard as an example to demonstrate a wireless covert channel. In particular, most 802.11 configurations use a distributed coordinated function (DCF) to assist in communications. This DCF uses a random backoff to avoid collisions, which provides the cover for our covert channel. Our timing channel provides great improvements on other recent covert channels in the field of throughput, while maintaining high accuracy. We are able to achieve throughput over 8000 bps using Covert DCF, or by accepting a throughput of 1800 bps we can achieve higher covertness and 99% accuracy as well.

Le protocole IEEE 802.11, basé sur les principes CMSA/CA, est largement déployé dans les communications sans fil actuelles, principalement en raison de sa simplicité et sa mise en œuvre à faible coût. Une utilisation intéressante de ce protocole peut être trouvée dans les réseaux sans fil multi-sauts, où les communications entre les nœuds peuvent impliquer l'emploi de nœuds relais. Une topologie simple de ces réseaux impliquant une source et une destination est communément connue en tant que chaîne. Dans cette thèse, un modèle hiérarchique, composé de deux niveaux, est présenté dans le but d'analyser la performance associée à ces chaînes. Le niveau supérieur modélise la topologie de la chaîne et le niveau inférieur modélise chacun de ses nœuds. On estime les performances de la chaîne, en termes de débit obtenu et de pertes de datagrammes, en fonction de différents modes de qualité du canal. En termes de précision, le modèle offre, en général, des résultats justes. Par ailleurs, le temps nécessaire à sa résolution reste très faible. Le modèle proposé est ensuite appliqué aux chaînes avec deux, trois et quatre nœuds, en présence de stations cachées potentielles, de tampons finis et d'une couche physique non idéale. Par ailleurs, l'utilisation du modèle proposé permet de mettre en évidence certaines propriétés inhérentes à ces réseaux. Par exemple, on peut montrer que la chaîne présente un maximum de performance (en ce qui concerne le débit atteint) en fonction du niveau de charge de du système, et que cette performance s'effondre par l'augmentation de cette charge. Cela représente un comportement non trivial des réseaux sans fil et il ne peut pas être facilement identifié. Cependant, le modèle capture cet effet non évident. Finalement, certains impacts sur les performances des chaînes occasionnés par les mécanismes IEEE 802.11 sont analysés et détaillés. La forte synchronisation entre les nœuds d'une chaîne et comment cette synchronisation représente un défi pour la modélisation de ces réseaux sont décrites. Le modèle proposé permet de surmonter cet obstacle et d'assurer une évaluation facile des performances de la chaîne / The IEEE 802.11 protocol, based on the CMSA/CA principles, is widely deployed in current communications, mostly due to its simplicity and low cost implementation. One common usage can be found in multi-hop wireless networks, where communications between nodes may involve relay nodes. A simple topology of these networks including one source and one destination is commonly known as a chain. In this thesis, a hierarchical modeling framework, composed of two levels, is presented in order to analyze the associated performance of such chains. The upper level models the chain topology and the lower level models each of its nodes. It estimates the performance of the chain in terms of the attained throughput and datagram losses, according to different patterns of channel degradation. In terms of precision, the model delivers, in general, accurate results. Furthermore, the time needed for solving it remains very small. The proposed model is then applied to chains with 2, 3 and 4 nodes, in the presence of occasional hidden nodes, finite buffers and non-perfect physical layer. Moreover, the use of the proposed model allows us to highlight some inherent properties to such networks. For instance, it is shown that a chain presents a performance maximum (with regards to the attained throughput) according to the system workload level, and this performance collapses with the increase of the workload. This represents a non-trivial behavior of wireless networks and cannot be easily identified. However, the model captures this non-trivial effect. Finally, some of the impacts in chains performance due to the IEEE 802.11 mechanisms are analyzed and detailed. The strong synchronization among nodes of a chain is depicted and how it represents a challenge for the modeling of such networks. The proposed model overcomes this obstacle and allows an easy evaluation of the chain performance

As users become more comfortable using IEEE 802.11 Wireless Local Area Networks, the need for quality of service is becoming more important because of the lack of support in current standards and the increase of multimedia traffic over the Internet. The IEEE 802.11 working group has recognized this fact proposing the Enhanced Distributed Channel Access (EDCA), a priority-based distributed scheme meant to provide service differentiation. EDCA relies on either different Arbitrary Interframe Space (AIFS), or Contention Window (CW) parameters, or both to provide service differentiation. In this thesis, a performance evaluation of the EDCA using five different combinations of the above mentioned parameters is included and compared to the current DCF (Distributed Coordination Function) standard, which is used as the base case. Simulation results show that simpler schemes based on one parameter alone can provide good average service differentiation. However, only multiparameter schemes provide the average and instantaneous high throughput and low delay values needed to support streaming applications. Starvation is a problem spanning all these schemes. It is especially more pronounced in schemes using combinations of parameters. In this thesis, a measurement-based admission control mechanism is proposed to overcome the above stated problems. The admission control mechanism uses an algorithm that admits a flow depending on the jitter values for high priority traffic and the throughput of the low priority traffic. It also allows the administrator to set the bandwidth sharing policy between the high priority traffic and low priority traffic. Results show that the admission control mechanism not only protects existing high priority flows from jitter and low priority flows from starvation, but also improves upon the network utilization.

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Long time ago, there were developed methods which can allow fast exchange of information at the longest distance possible. Until recent time, the possibilities of long way communications were very limited. There were technological and financial limitations mainly. With telegraph, telephone and the newest – computer networks invention, the telecommunication services became cheaper and much more comprehensive. With accession and high scale spread of internet, the role of communications is much more important. The most actual trend is mobile internet and connected multimedia networks and their instant accessibility from anywhere. On the first side there are classical telecommunication networks as GSM, UMTS, on the other side there is very strong alternative in shape of WiMAX and WiFi networks combination. For the usage of multimedia services in that networks is very important to handle their prefferization over other traffic, which is very difficult task even on wireless media. For the finding of new processes and methods, which can allow it, there is, at fist, the need for highly accurate and authentic mathematical models. In this doctoral thesis is mapped actual state of the art and proposed the new mathematical model of Distribution Coordination Function, which is much-frequent used as access method in 802.11 networks, Wi-Fi.

La maîtrise de la QoS dans 802.11e EDCA (Enhanced Distributed Coordination Function) ne peut être assurée que par un mécanisme de contrôle d'admission qui empêche le réseau d'atteindre un état de saturation critique et par la même garantit les besoins de QoS des applications voix/vidéo.<br />Ce mécanisme de contrôle d'admission a besoin pour sa prise de décision de prédire les métriques de performances si un nouveau flux est admis. Dans le but de rendre les décisions efficaces, nous choisissons d'utiliser une méthode de prédiction basée sur un modèle analytique. Ce dernier doit remplir deux conditions : 1) fournir une bonne précision de prédiction et 2) avoir une complexité numérique faible et un temps de réponse limité. Vu que la majorité des modèles analytiques de la littérature ne satisfont pas à ces deux conditions, nous développons un nouveau modèle analytique pour EDCA qui est capable de prédire le débit et le délai d'accès des différentes Access Category (AC) d'EDCA.<br />Ainsi, après la modélisation analytique du temps de transmission des ACs en prenant en compte le paramètre de différentiation TXOPLimit, nous développons un modèle analytique pour EDCA sous la forme d'une chaîne de Markov à quatre dimensions. Celui-ci est développé d'abord dans les conditions de saturation puis étendu aux conditions générales de trafic.<br />Pour finir, nous proposons un algorithme de contrôle d'admission à implémenter au sein du point d'accès et qui utilise le modèle analytique proposé. Nous proposons un abaque de solution d'optimisation des paramètres d'accès d'EDCA. Le but étant d'améliorer les performances du mécanisme de contrôle d'admission par l'utilisation optimale des ressources du réseau.