Emmanuel Chaput

Simulation of vehicular ad-hoc networks: Challenges, review of tools and recommendations

Vehicular communications are considered the next step in increasing transportation safety and comfort. The prohibitive price of operational test beds means that computer simulations are the only viable solution for analysing the performance of different protocols and architectures. However, simulation frameworks used in vehicular ad-hoc networks research are still highly heterogeneous and, as a consequence, many of the proposed ideas can not be compared and validated. In this paper, we focus on the challenges faced when modelling the vehicular environment and the solutions adopted in the main simulation tools. As the research community is concerned with many different problems, from safety related issues to traffic efficiency and from intersection management to Internet access, we consider that every study should chose the appropriate simulator based on its requirements. Consequently, we make some recommendations which take into account the scope of the simulated scenario and the properties of the simulation frameworks.

Enhancements of IEEE 802.11p Protocol for Access Control on a VANET Control Channel

Adding communication capabilities to vehicles and road infrastructure has become a major goal in the intelligent transportation systems industry. The IEEE 802.11p amendment has specially been conceived for the Wireless Access in Vehicular Environments (WAVE) architecture. In this paper we study the performance of this standard by the means of extensive simulations and we argue that the current version of the protocol can not cope with high vehicular densities. We propose a simple but efficient modification of the back off mechanism which has an important impact on the quality of communications on the control channel.

Properties of the MAC layer in safety vehicular Ad Hoc networks

With intervehicle communications becoming a more and more popular research topic recently, the medium access control layer of the vehicular network has also received a considerable amount of attention. However, this increased interest has not always translated into a careful analysis of the properties exhibited by the MAC protocol when used by vehicular safety applications. This article tries to fill this gap by providing a comprehensive discussion on a number of important characteristics of the link layer in vehicular communications.

Local density estimation for contention window adaptation in vehicular networks

The medium access control protocol of a future vehicular ad-hoc network is expected to cope with highly heterogeneous conditions. An essential parameter for protocols issued from the IEEE 802.11 family is the minimum contention window used by the backoff mechanism. While its impact has been thoroughly studied in the case of wireless local area networks, the importance of the contention window has been somehow neglected in the studies focusing on vehicle-to-vehicle communication. In this paper we show that the adjustment of the minimum contention window depending on the local node density can notably improve the performance of the IEEE 802.11 protocol. Moreover, we compare through simulation in a realistic framework five different methods for estimating the local density in a vehicular environment, presenting the advantages and the shortcomings of each of them.

Physical Carrier Sense in Vehicular Ad-Hoc Networks

Enhancing road safety using vehicle-to-vehicle communication has become an important goal for the automotive industry. A lot of effort has been put in the design of an efficient medium access control protocol, capable to function correctly even under heavy congestion. The solutions proposed so far focus on data rate or transmission power control. In this paper, we argue that the most important parameter for congestion control in vehicular ad hoc networks is the carrier sense threshold. We support this theory with analytical and simulation results and we demonstrate that the optimal threshold depends on the vehicular density. Furthermore, we propose an adaptive mechanism for physical carrier sense control and analyse its performance, showing an important increase in message reception probability.

Broadcast communication in Vehicular Ad-Hoc Network safety applications

Contention-based protocols for Medium Access Control (MAC) in Vehicular Ad-Hoc Networks (VANET) are currently under development in several standardization organizations. The availability and maturity of the IEEE 802.11 technology makes it the first choice for the future vehicle-to-vehicle communications. At the same time, safety applications in a vehicular environment are expected to intensively use broadcast messages. However, the IEEE 802.11 standard has not been designed for broadcast communication and a number of problems arise from this. In this paper, we analyze the impact of the minimum Contention Window (CW) on the MAC layer performance in a realistic vehicular environment and we propose a simple solution for adapting CW to the network density in order to improve the reception probability of broadcast messages.

Congestion control in CSMA-based vehicular networks: Do not forget the carrier sensing

Inter-vehicular communications are considered to be an efficient proactive approach for reducing the number and the consequences of road accidents. After a series of remarkable standardisation efforts, one of the last points needing to be addressed in order for safety vehicular networks to become a reality is the scalability problem of the CSMA-based medium access control layer. With node densities that can range from very sparse to several hundred contending stations, the MAC protocol needs the capacity to adapt to the state of the vehicular network without compromising the performance of the safety applications. While previous studies focused on individual mechanisms for data rate selection or transmission power control from a global point of view, this paper proposes a complete congestion control framework aiming to increase the message reception probability in the immediate neighbourhood under heavy congestion conditions. We propose a new concept for physical carrier sensing, which takes into account the location of the transmitter, and we combine it with transmission power control and a recently proposed backoff mechanism to obtain an important improvement over the original protocol. Several implementation problems are discussed, showing the feasibility of the solution using existing hardware, and a simulation study confirms the performance of this enhanced channel access method.

Fuzzy logic based layers 2 and 3 handovers in IEEE 802.16e network

In outdoor wireless communication networks such as mobile WiMAX (802.16e), mobile stations move all the time, thus they need to be handed off to different base stations based on certain criteria. In this paper, a fuzzy logic based scheme for fast selection of best base station and of handover technique at the handover time is presented in order to minimize the delay during handover for sensitive multimedia traffic. The scheme considers several parameters such as receiver power levels, handover type, traffic type, base station load and mobile station speed for making the handover decision by the mobile station. Through simulations, we compare our proposed handover scheme based on fuzzy logic modeling with classical handover decision. And finally, we present results based on Quality of Service (QoS) criteria to confirm the validity of the proposed approach.

Enhancing TCP based communications in mobile satellite scenarios: TCP PEPs issues and solutions

Networking has experienced tremendous changes. Once dedicated to one single type of services, technologies are now offering integrated services: we want to have access to any services anywhere. One challenge is the mobility, with 3G terrestrial offers beginning to be successful. In such a context, satellite systems appear as a real opportunity since they may fill the gap between dense connected areas (city) and low populated areas. Nevertheless, Internet-like communications over satellite segments needs TCP Performance Enhancing Proxies (PEPs) to alter classical TCP on satellite part for a better behaviour. As a drawback, PEPs often induce connection splitting and thus hinder mobility. This paper tackles this issue. After a brief introduction on existing satellite mobile systems and on PEPs mechanisms, we focus on mobility scenarios and propose some architecture elements. A detailed case is settled to illustrate context transfer issues.

Why VANET Beaconing Is More Than Simple Broadcast

The use of inter-vehicle communication is considered the next step to be taken in order to reduce the number of traffic accidents. The design of a versatile and efficient protocol that would manage the access to the control channel reserved for safety applications would represent a significant progress towards a generally accepted technology. However, the solutions proposed hitherto rarely took into account the broadcast nature of the messages produced by the safety applications. Furthermore, the specific properties of periodic vehicular beaconing are yet to be considered by any technical committee or research study. We present the specificities of this type of messages and discuss their impact on the performance of the medium access control layer, using both analytical and simulation tools.