Providing safety applications is one of the principal motivations behind deploying vehicular ad hoc networks (VANETs), where each vehicle is equipped with a wireless transmitter and receiver. These applications require fair (i.e., all vehicles get equal fraction of time allocation for their transmissions) and reliable (i.e., transmissions are received with high probably by the intended receivers) broadcasting of relevant driving data, such as position, speed and direction of a vehicle. In this thesis we compare the performance of IEEE 802.11p and a recent time-division based medium access control protocol, Dynamic Channel Reservation (DCR) in realistic high-density traffic scenarios. We focus on the communication requirements that allow vehicles to receive safety messages well enough in advance to warn the driver in a timely manner and avoid crashes. We observe performance degradation in both schemes as we examine them in congested environments. Previous work confirms our observation on the performance of 802.11p. In DCR, on the other hand, some vehicles may face starvation (i.e., they do not get a chance to transmit in a long time) in dense scenarios, where all channels have been pre-reserved by other vehicles. In order to avoid this situation, we propose a modified version of DCR, fDCR, in which channels can be occupied by several vehicles, thus fostering a fair channel reservation scheme. Our channel reservation scheme is designed in a way that minimizes packet collisions when transmitter and receiver are close to each other. Furthermore, to enhance the probability of reception in nearby vehicles, which is one of the main communication requirements of safety applications, we propose a low-overhead transmission power control scheme. Our fully distributed power control scheme leverages on the extra transmitted information by DCR to estimate the number of vehicles in its transmission range, and accordingly adjust the transmission power. Experimental results show significant performance gains in cases of both cross-through and non-cross-through traffic for our proposed scheme in comparison with 802.11p
