Efficient mac protocol for provisioning fairness in vehicle to roadside communications

Abstract

Vehicular ad hoc network (VANET) is an important component of intelligent transports system (ITS) that facilitates variety of safety and non-safety applications. It has some unique characteristics, such as high geographically dynamic topology, predictable two direction mobility, varying vehicle density etc. Medium access control (MAC) protocol plays a vital role to share the common wireless channel efficiently among the vehicles in VANET. However, ensuring fairness is a challenging issue to design MAC protocols of VANET. Due to small residence time, existing IEEE 802.11 DCF protocol provides less opportunity for the vehicles with high velocity to communicate with road side unit (RSU), consequently allowing less amount of data be transferred compared to the vehicles low velocity. Existing MAC protocols take into account only velocities and cannot ensure the data transmission rate to be proportional to residence time while providing minimum amount of data transmission for all vehicles irrespective of velocity. Therefore, an efficient MAC protocol, which takes position, direction and velocity of vehicles into consideration for ensuring minimum opportunity for all the vehicles, is yet to be proposed. In this research work, an efficient MAC protocol based on IEEE 802.11 DCF is developed which minimizes the unfairness problem of VANET. Unfairness problem has two aspects: higher velocity vehicles cannot transmit a minimum number of packets and lower velocity vehicles cannot transmit above a maximum number of packets. To address the above mentioned issues, the proposed MAC protocol adjusts the transmission probability for each vehicle according to its residence time by changing the value of MAC parameters (minimum contention window size, maximum backoff stage, retransmission limit, etc ) dynamically. An analytical model is developed to analyze the performance of the proposed protocol. Analytical results show that the proposed scheme overcomes the limitations of existing MAC protocols by ensuring that packet transmission rate remains proportional to the residence time of the vehicles.