Design of a medium access control protocol that exploits the structures of vehicular networks

Vehicular networks (VANETs) have distinctive structures that distinguish them in comparison to general mobile ad hoc networks. In this thesis we identify the relevant special structures that do exist, and show how they can be exploited to design a MAC layer that provides better performance than a MAC layer designed without taking such structures into account. Particular important examples of the structures in VANETs are: (i) Periodic broadcast is the common transmission pattern due to the requirements of vehicular safety applications; (ii) GPS is available on nodes; (iii) Node mobility is constrained along one-dimensional roadways, and traffic can only move in one of the two directions of each roadway; and (iv) Traffic moving on one lane in one direction of a road has group mobility. These structures offer unique exploitable opportunities for building specific protocols tailored to VANETs which perform better than other general purpose protocols designed for structureless networks. We will show that, at the MAC layer, these structures can be thoroughly exploited to design a protocol based on dynamic TDMA that is well suited for VANETs. We present the Dynamic Channel Partition and Reservation (DCPR) Protocol, which is a specific VANET MAC protocol designed for operating a dynamic TDMA mechanism in a way robust to vehicular mobility, by dynamically partitioning the set of TDMA channels for the use of vehicles according to their different velocities. DCPR also enhances the basic dynamic TDMA mechanism with features for proper operation in a wireless environment with fading. We evaluate DCPR with simulations via ns-2 and VanetMobiSim, using the Intelligent Driver Model with Lane Changing. Our simulation results show that DCPR achieves packet loss rates which are one to two orders of magnitude lower than those obtained by IEEE 802.11p at various vehicle densities and in different vehicular traffic environments. Moreover, DCPR increases goodput by a factor as high as nine in a wireless environment with fading compared to a basic dynamic TDMA protocol. With regard to variations in vehicular traffic, DCPR maintains stable performance across all levels of traffic asymmetry in a highway scenario. Based on the systematic exploitation of group mobility, we are led to suggest an overarching design based on the Velocity Differentiated Dynamic Channel Partition (VDDCP) Mechanism. In the scenarios we have evaluated, DCPR equipped with the VDDCP mechanism performs as well as the original DCPR, and further evaluation is required to determine if there are other scenarios of vehicular traffic geometries and intensities in which the VDDCP mechanism delivers significantly better performance. To summarize, DCPR shows the possibility of building a MAC layer for VANETs that provides high packet delivery rate, supporting the development of vehicular safety applications that demand reliability.