Les exigences de Qualité de Service (QoS) des applications VANET varient selon la nature et le type de l’application. Par conséquent, un protocole de communication VANET doit pouvoir répondre aux diverses exigences de QoS selon le type du trafic. Dans VANET, le canal de transmission est partagé par tous les véhicules en utilisant une même fréquence radio. Une mauvaise exploitation du canal peut donc conduire à des collisions et peut aussi engendrer un gaspillage de la bande passante. Un protocole MAC doit être alors conçu pour partager le canal entre les différents noeuds d’une manière efficace et équitable.Dans cette thèse nous présentons les contributions suivantes :1- Analyse et amélioration de la diffusion dans la norme IEEE 802.11.2- Optimisation de la technique CSMA pour des réseaux 1D et 2D.3- Développement d’un algorithme CSMA de transmission adaptatif qui met à jour le taux de détection de la porteuse en fonction d’une valeur de référence.4- Étude du gain obtenu par l’utilisation d’antennes directionnelles pour Aloha, Aloha non-slotté, et CSMA. / Quality of Service (QoS) requirements for VANET applications vary depending on the nature and type of the application. Therefore, a communication protocol in VANETs must be able to meet various QoS requirements according to the type of traffic. In VANET, the transmission channel is shared by all the vehicles using the same radio frequency. A poor exploitation of the channel can therefore lead to collisions and wasted bandwidth. A MAC protocol must therefore be designed to share the channel between the different nodes in an efficient and fair way.In this thesis we present the following contributions:1- Analysis and improvement of diffusion in the IEEE 802.11 standard.2- Optimization of the CSMA technique for 1D and 2D networks.3- Design of an adaptive transmission algorithm that updates the Carrier Sense threshold to reach a target value.4- Study the gain obtained by the use of directional antennas for Aloha, non-slotted Aloha, and CSMA.

This thesis explores the combined subjects of keep alive forwarding, denial of service attacks and decentralized congestion control. In a society where the technical requirements on vehicles constantly escalate; inter-vehicle communication has risen as a potential springboard for new technologies. The ETSI ITS-G5 standard is a vehicular ad-hoc network standard that offers manufacturers the possibility to include a feature called keep alive forwarding. This feature lets the nodes forward messages on a pre-defined time interval even if the original broadcaster is no longer present. As this feature might provide exploits for potential people with malicious intent it has been evaluated how resilient the standard might be in such a situation. Also included in this thesis is the decentralized congestion control, a feature required by the standard. To evaluate these features a series of simulations has been performed where vehicles in a highway scenario have been exposed to a denial of service attack where the attacker uses the keep-alive forwarding as an exploit. The findings are that decentralized congestion control does mitigate some of the direct consequences of such an attack. Although it does not eliminate these entirely and new problems are introduced. Finally, alternative methods to perform keep alive forwarding are suggested to enhance this feature.