An analytical model for TDMA-based MAC protocols in VANETs

The design of reliable and adaptive Medium Access Control (MAC) protocol is of crucial importance in the actual emergence of vehicular ad hoc networks (VANETs). Time Division Multiple Access (TDMA) based MAC protocol is promising for VANETs as many works have proven that it can provide reliable broadcast services without hidden terminal problem when properly designed. However, the high vehicle mobility, dynamic traffic density and changeable network topology make the TDMA scheduling very difficult. Therefore, the performance of TDMA-based MAC mechanism needs in-depth evaluation. In this paper, three performance metrics for TDMA protocols in VANETs, namely the average reservation delay, slot utility ratio and slot available duration, are introduced and evaluated analytically and by simulations. The expressions of these metrics are derived taking the impact of vehicle mobility and different network parameters such as transmission range, the number of available time slots, the traffic density, vehicle moving speed into account. The proposed analytical model is applied for two existing TDMA based protocols, ADHOC MAC and VeMAC, and the theoretical results are coherent with the simulation results. As a result, some important observations are obtained for the future design and enhancement of TDMA protocols.