The Intelligent Transportation System (ITS) is a cooperative system that relies on reliable and robust communication schemes among vehicles and between vehicles and their surroundings. The main objective of the ITS is to ensure the safety of vehicle drivers and pedestrians. It provides an efficient and reliable transportation system that enhances traffic management, reduces congestion time, enables smooth traffic re-routing, and avoids economic losses. An essential part of the ITS is the Vehicular Ad hoc Network (VANET). VANET enables the setup of Vehicle-to-Vehicle (V2V) as well as Vehicle-to-Infrastructure (V2I) communication platforms: the two key components in the ITS. The de-facto standard used in wireless V2V and V2I communication applications is the Dedicated Short Range Communication (DSRC). The protocol that defines the specifications for the Medium Access Control (MAC) layer and the physical layer in the DSRC is the IEEE 802.11p protocol. The IEEE 802.11p protocol and its Enhanced Distributed Channel Access (EDCA) mechanism are the main focus of this thesis. Our main objective is to develop new IEEE 802.11p-based protocol for V2V and V2I communication systems, to improve the performance of safety-related applications. These applications are of paramount importance in ITS, because their goal is to decrease the rate of vehicle collisions, and hence reduce the enormous costs associated with them. In fact, large percentage of vehicle collisions can be easily avoided with the exchange of relevant information between vehicles and the Road Side Units (RSUs) installed on the sides of the roads. In this thesis, we propose various enhancements to the IEEE 802.11p protocol to improve its performance by lowering the average end-to-end delay and increasing the average network throughput. We introduce multiple adaptive algorithms to promote the QoS support across all the Access Categories (AC) in IEEE 802.11p. We propose two adaptive backoff algorithms and two algorithms that adaptively change the values of the Arbitrary Inter-Frame Space (AIFS). Then we extend our model to be applied in a large-scale vehicular network. In this context, a multi-layer cluster-based architecture is adopted, and two new distributed time synchronization mechanisms are developed.

近年來，無線網路發展的成熟、行車安全問題的重視、全球定位系統的普及，以及車用網路(Vehicular Ad Hoc Networks, VANET)在研究上的前瞻性與未來的高應用價值，使得車用網路的發展也愈來愈受到各方的探討。然而，車用網路的拓撲變動性相當高且規模大，且屬於點對點(Ad-Hoc)的網路架構，因此，對於吞吐量(throughput)和碰撞率(collision)以及延遲時間(delay)皆有一定的需求。此外，由於車輛在道路上的分布容易造成訊號之間的碰撞和干擾，所以我們藉由修改RTS/CTS機制提出一個Detection-Based MAC，其機制可透過封包之間的訊息傳遞，偵測出網路的壅塞狀態，預測網路中鄰近節點的競爭情形，並依此來動態調整競爭視窗大小。最後，將此方法實作在IEEE 802.11p的媒體存取控制層(MAC)中，以解決車用網路節點互相干擾所造成的過多碰撞，並提升整體吞吐量。 / In recent years, the research on Vehicular Ad Hoc Networks (VANET) has been paid much attention due to the development of wireless network becoming mature, people think highly of traffic safety issues, the popularity of Global Positioning System (GPS), promising applications of VANET being exploited. However, VANET has the essential property of rapid change of large scale network topology. Besides, vehicles randomly distributed over the road usually causes serious signal collisions and interferences. Thus, it calls for special concerns to deal with the performances of throughput, collision, and delay. In this paper we propose a Detection-Based MAC mechanism by modifying RTS/CTS to detect network congestions through message exchange, and predict number of competing nodes. Both these information is used to dynamically adjust contention window sizes. Simulations are implemented on the MAC layer of IEEE 802.11p. The results show that the proposed method is able to effectively reduce collisions and increase overall throughput.

One of the most challenging issues facing vehicular networks lies in the design of an efficient MAC protocol due to mobile nature of nodes, delay constraints for safety applications and interference. In this thesis, I propose an efficient Multichannel QoS Cognitive MAC (MQOG). MQOG assesses the quality of channel prior to transmission employing dynamic channel allocation and negotiation algorithms to achieve significant increase in channel reliability, throughput and delay constraints while simultaneously addressing Quality of Service. The uniqueness of MQOG lies in making use of the free unlicensed bands. To consider fair effective sharing of resources I propose a Mobility Based Dynamic Transmit Opportunity (MoByToP) while modifying the 802.11e TXOP (Transmit Opportunity). The proposed protocols were implemented in OMNET++ 4.1, and extensive experiments demonstrated a faster and more efficient reception of safety messages compared to existing VANet MAC Protocols. Finally, improvements in delay, packet delivery ratios and throughput were noticed.

One of the most challenging issues facing vehicular networks lies in the design of an efficient MAC protocol due to mobile nature of nodes, delay constraints for safety applications and interference. In this thesis, I propose an efficient Multichannel QoS Cognitive MAC (MQOG). MQOG assesses the quality of channel prior to transmission employing dynamic channel allocation and negotiation algorithms to achieve significant increase in channel reliability, throughput and delay constraints while simultaneously addressing Quality of Service. The uniqueness of MQOG lies in making use of the free unlicensed bands. To consider fair effective sharing of resources I propose a Mobility Based Dynamic Transmit Opportunity (MoByToP) while modifying the 802.11e TXOP (Transmit Opportunity). The proposed protocols were implemented in OMNET++ 4.1, and extensive experiments demonstrated a faster and more efficient reception of safety messages compared to existing VANet MAC Protocols. Finally, improvements in delay, packet delivery ratios and throughput were noticed.

Vehicular ad hoc networks (VANETs) are an extreme case of mobile ad hoc networks (MANETs). High speed and frequent network topology changes are the main characteristics of vehicular networks. These characteristics lead to special issues and challenges in the network design, especially at the medium access control (MAC) layer. Due to high speed of nodes and their frequent disconnections, it is difficult to design a MAC scheme in VANETs that satisfies the quality-of-service requirements in all networking scenarios. In this thesis, we provide a comprehensive evaluation of the mobility impact on the IEEE 802.11p MAC performance. The study evaluates basic performance metrics such as packet delivery ratio, throughput, and delay, as well as the impact of mobility factors. The study also presents a relation between the mobility factors and the respective medium access behavior. Moreover, a new unfairness problem according to node relative speed is identified for both broadcast and unicast scenarios. To achieve better performance, we propose two dynamic contention window mechanisms to alleviate network performance degradation due to high mobility. Extensive simulation results show the significant impact of mobility on the IEEE 802.11p MAC performance, an identification of a new unfairness problem in the vehicle-to-vehicle (V2V) communications, and the effectiveness of the proposed MAC schemes.

Vehicular ad hoc networks (VANETs) are an extreme case of mobile ad hoc networks (MANETs). High speed and frequent network topology changes are the main characteristics of vehicular networks. These characteristics lead to special issues and challenges in the network design, especially at the medium access control (MAC) layer. Due to high speed of nodes and their frequent disconnections, it is difficult to design a MAC scheme in VANETs that satisfies the quality-of-service requirements in all networking scenarios. In this thesis, we provide a comprehensive evaluation of the mobility impact on the IEEE 802.11p MAC performance. The study evaluates basic performance metrics such as packet delivery ratio, throughput, and delay, as well as the impact of mobility factors. The study also presents a relation between the mobility factors and the respective medium access behavior. Moreover, a new unfairness problem according to node relative speed is identified for both broadcast and unicast scenarios. To achieve better performance, we propose two dynamic contention window mechanisms to alleviate network performance degradation due to high mobility. Extensive simulation results show the significant impact of mobility on the IEEE 802.11p MAC performance, an identification of a new unfairness problem in the vehicle-to-vehicle (V2V) communications, and the effectiveness of the proposed MAC schemes.

One of the most challenging issues facing vehicular networks lies in the design of an efficient MAC protocol due to mobile nature of nodes, delay constraints for safety applications and interference. In this thesis, I propose an efficient Multichannel QoS Cognitive MAC (MQOG). MQOG assesses the quality of channel prior to transmission employing dynamic channel allocation and negotiation algorithms to achieve significant increase in channel reliability, throughput and delay constraints while simultaneously addressing Quality of Service. The uniqueness of MQOG lies in making use of the free unlicensed bands. To consider fair effective sharing of resources I propose a Mobility Based Dynamic Transmit Opportunity (MoByToP) while modifying the 802.11e TXOP (Transmit Opportunity). The proposed protocols were implemented in OMNET++ 4.1, and extensive experiments demonstrated a faster and more efficient reception of safety messages compared to existing VANet MAC Protocols. Finally, improvements in delay, packet delivery ratios and throughput were noticed.

One of the most challenging issues facing vehicular networks lies in the design of an efficient MAC protocol due to mobile nature of nodes, delay constraints for safety applications and interference. In this thesis, I propose an efficient Multichannel QoS Cognitive MAC (MQOG). MQOG assesses the quality of channel prior to transmission employing dynamic channel allocation and negotiation algorithms to achieve significant increase in channel reliability, throughput and delay constraints while simultaneously addressing Quality of Service. The uniqueness of MQOG lies in making use of the free unlicensed bands. To consider fair effective sharing of resources I propose a Mobility Based Dynamic Transmit Opportunity (MoByToP) while modifying the 802.11e TXOP (Transmit Opportunity). The proposed protocols were implemented in OMNET++ 4.1, and extensive experiments demonstrated a faster and more efficient reception of safety messages compared to existing VANet MAC Protocols. Finally, improvements in delay, packet delivery ratios and throughput were noticed.

Le développement du transport partout dans le monde a fourni un grand nombre d'avantages pour de nombreux aspects de la vie humaine. Les systèmes de transport intelligents (ITS) sont des applications avancées qui visent à rendre les réseaux de transport plus sûrs, plus pratiques et plus intelligents. Selon leurs usages, ils peuvent être classés en deux types d'applications ITS, qui sont des applications de sûreté et des applications non-sûreté. Le réseau de véhicules ad hoc (VANET) est un élément clé des systèmes ITS, car il permet la communication entre les unités de transport. Ces communications prennent en charge différentes applications ITS avec différentes propriétés. Parmi les deux types d'applications, nous nous intéressons aux applications de sûreté qui ont des contraintes de qualité de service et des contraintes de sécurité plus strictes. Selon le scénario considéré et l'application de sûreté donnée, les informations échangées entre les véhicules doivent être diffusé localement dans une communication à un seul saut et / ou également notifiées aux véhicules à large dimension. L'objectif principal de cette thèse est d'améliorer les performances des applications de sûreté en termes de qualité de service et de sécurité, à la fois dans une communication à un saut et dans une communication multi-sauts. Nous nous intéressons à la fiabilité, la connectivité et le déni de service (DoS). Nous étudions et proposons des solutions techniques provenant de couches inférieures (Physique, Liaison et Réseaux) qui jouent un rôle fondamental dans l'atténuation des défis créés par la nature de l'environnement des véhicules. Tout d'abord, nous introduisons une nouvelle méthode efficace pour fiabiliser la radiodiffusion. Dans notre système, les messages de sécurité sont rediffusés lorsque l'expéditeur est sollicité. Cela augmente le pourcentage de véhicules qui reçoivent les messages alors que le nombre de messages dupliqués reste limité. En second lieu, en tenant compte de la fragmentation du réseau, nous étudions des solutions qui permettent de pallier la déconnexion temporaire du réseau pour apporter l'information de sécurité aux destinataires. Basé sur les propriétés sociales des réseaux de véhicules, nous proposons un protocole de transfert basé sur des relations sociales pour relayer la communication entre les véhicules et des points d'intérêt qui fournissent des services de sécurité avec des contraintes de temps plus souples, telles que la recherche et le sauvetage. Troisièmement, nous étudions l'attaque de brouillage, une sorte d'attaques DoS, qui est cruciale pour les applications de sûreté et qui et facilement réalisable au niveau des couches inférieures. Nous modélisons l'attaque de brouillage afin d'étudier la dégradation causée par l'attaque sur les performances du réseau. La dégradation à un certain niveau dans les performances du réseau est une indication de présence d'attaques de brouillage dans le réseau; donc les résultats de cette analyse nous permettent de déterminer les seuils de performance du réseau pour distinguer entre les scénarios normaux et les scénarios attaqués. Toutefois, selon cette analyse, le procédé utilisant la dégradation comme une indication pour détecter une attaque de brouillage est impossible pour des applications temps réel. Par conséquent, nous proposons des nouvelles méthodes afin de détecter les attaques de brouillage temps réel. Nos méthodes permettent la détection en temps réel avec une grande précision, non seulement chez le moniteur central mais aussi au niveau de chaque véhicule. Par conséquent, les véhicules sont avertis sur l'attaque assez tôt pour récupérer la communication et réagir à ces attaques. / The development of transportation all over the world has been providing a lot of benefits for many aspects of human life. Intelligent Transportation Systems (ITS) are advanced applications that aim to make the transport networks safer, more convenient and smarter. According to their usages, they can be classified into two types of ITS applications, which are safety applications and non-safety applications. Vehicular ad hoc network (VANET) is a key component of ITS since it enables communications among transportation units. These communications support different ITS applications with various properties. Between two types of applications, we are interested in safety applications which have tighter quality and security constraints. Depending on an applied scenario of a given safety application, the exchanged information among vehicles must be broadcast locally within one-hop communication and/or also be notified to vehicles in large range. The main objective of this thesis is to improve the performance of safety applications in term of the quality of service and security, in both one-hop communication and multi-hop communication. We focus on reliability, connectivity and Denial of Services (DoS) attack. We study and propose technical solutions coming from lower layers (Physical, MAC and network layers) which play a fundamental role in mitigation to challenges created by the nature of the vehicular environment. Firstly, we introduce a reliable scheme to achieve the reliability for broadcasting. In our scheme, the safety messages are rebroadcast when the sender is solicited. This increases the percentage of vehicles receiving the messages while duplicated messages are limited. Secondly, with consideration of the fragmentation of the network, we study solutions that overcome the temporary disconnection in the network to bring the safety information to the recipients. Based on the social properties of vehicular networks, we propose a social-based forwarding protocol to support the communication between vehicles to points of interest that provide safety services with looser time constraints, such as search and rescue. Thirdly, we investigate jamming attack, a kind of DoS attacks, which is crucial for safety applications because of the adequate condition of the attack at the lower layers. We model jamming attack on broadcasting in order to study the degradation caused by the attack on network performance. The degradation at a certain level in network performance is an indication of a jamming attack presence in the network; therefore results from this analysis will allow us to determine network performance thresholds to distinguish between normal and attacked scenarios. However, according to our analysis, the method using the degradation as an indication to detect a jamming attack is not feasible for real-time applications. Hence, we propose methods to detect jamming attacks in real-time. Our methods allow real-time detection with high accuracy, not only at the central monitor but also at each vehicle. Therefore, vehicles are noticed about the attack soon enough to recover the communication and react to these attacks.

Véhicule-à-tous (V2X) se réfère à un Système de Transport Intelligent (ITS) où les véhicules et l'infrastructure sont interconnectés. Cette connectivité permet une connaissance précise des conditions de circulation sur l'ensemble du réseau routier, ce qui contribue à améliorer la sécurité routière, réduire les temps d'encombrement et éviter les pertes économiques. Cette communication permet aussi une variété de nouvelles applications pour la sécurité routière et l'infodivertissement. Bien que la communauté de scientifique ait réalisé de grands progrès dans l'étude sur le V2X, il reste encore des défis à surmonter et des problèmes clés qui doivent être étudiés plus en profondeur. Cette thèse considère deux des questions les plus importantes; fiabilité et sécurité des communications V2X. Du point de vue de la fiabilité, nous proposons d'abord une méthode basée sur la modélisation de l'utilisateur pour évaluer la capacité de la norme DSRC IEEE 802.11p à répondre aux exigences de Qualité De Service (QoS) de la diffusion des messages de sécurité. La nouveauté de la méthode réside dans son application qui élimine le problème de la définition d'un modèle Markovien par la détermination des moments d'équilibre du processus de retard. Cette méthode fournit des informations importantes sur les paramètres de conception IEEE 802.11p et sur ses fonctionnalités, ce qui permet d'améliorer la configuration proposée. De plus, nous proposons un modèle Régénératif pour résoudre le problème de la caractérisation des processus de trafic interconnecté dans les réseaux V2X hybrides à grande échelle. Ce dernier est une préoccupation majeure pour parvenir à une opérabilité efficace et adéquate pour les réseaux de véhicules à grande échelle. Du point de vue de la sécurité, nous introduisons une nouvelle méthodologie d'optimisation. Notre méthodologie lie les exigences de QoS des différentes classes d'application avec le paramètre de conception de base du mécanisme de résolution de contention dans le protocole MAC IEEE 802.11p. En outre, un nouvel algorithme de détection d'attaque de brouillage dans l'environnement véhiculaire est proposé. L'algorithme utilise la méthodologie d'optimisation développée pour définir un seuil de détection et intègre la méthode séquentielle de détection pour détecter les attaques de brouillage à chaque fois que la valeur seuil est franchie. Des expérimentations analytiques et de simulation approfondies ont été effectuées pour chaque contribution afin de montrer la validité des méthodes/modèles proposés et de prouver leur efficacité. / Vehicle-to-anything (V2X) refers to an Intelligent Transportation System (ITS) where the vehicles and infrastructure systems are all interconnected with each other. This connectivity provides precise knowledge of the traffic situations across the entire road network which in turn helps to enhance traffic safety, reduce congestion time, avoid economic losses, in addition to enable a variety of novel ITS applications for road safety and passenger infotainment. V2X communications is based on two technologies; Dedicated Short-Range Communications (DSRC) which is an essential technology for realizing V2X and cellular networks which provide an o_-the-shelf potential solution for V2X communications. Although the research community has achieved much great progress on V2X study, there are still some challenges that need to be overcome and some key issues that need to be further investigated. This thesis considers two of the most prominent issues; reliability and security of V2X communications. From the reliability perspective, we first propose User Model-based Method to evaluate the capacity of IEEE 802.11p-based DSRC standard to meet the Quality-of-Service (QoS) requirements of safety messages dissemination. The novelty of the method lies in its application which avoids the problem of defining a Markovian model by determining the steady state moments of the induced delay process. This applicability feature provides important insights about IEEE 802.11p design parameters and its functionality leading to proposed reconfigurations for enhanced performance. Moreover, we propose Regenerative model, that we believe to be the first to address the problem of interconnected-traffic process characterization in large-scale hybrid V2X networks. The latter is a primary concern in achieving efficient and adequate operability for large-scale vehicular networks. From the security perspective, we introduce a new optimization methodology which ties the QoS requirements of different application classes with the basic design parameters of the contention resolution mechanism in IEEE 802.11p MAC protocol. In addition, a novel detection algorithm for jamming attacks in the vehicular environment is proposed. The algorithm utilizes the developed optimization methodology to de_ne a detection threshold. By integrating the sequential detection of change method it traces and detects jamming attacks whenever the threshold value is crossed. Analytical and simulation experimentations have been performed for each contribution to show the validity of the proposed methods/models and to prove their efficiency.