Research On Medium Access Control Protocol In VANET

As an important part of Intelligent Transportation System, VANET can make driving much safer and more comfortable. The network performance of VANET depends on Medium Access Control protocol(MAC). However, with the characteristics of high node mobility speed and frequently changed network topology, The MAC protocol with Carrier Sense Multiple Access/Collision Avoidance in IEEE 802.11 p is difficult to satisfy the requirements of safety applications. It is, therefore, necessary to improve existing MAC protocols for an efficient broadcasting of safety messages in VANET.Firstly, this paper proposed performance modeling of safety messages dissemination in VANET with priority, then the paper determined average transmission delay of emergency messages in the presence of status messages. In addition, the numerical analysis is validated by simulation, which indicate the model is helpful to select appropriate network parameters to reduce the delay of emergency messages transmission and study other performance in VANET.Secondly, aiming at the problem that the MAC protocol in IEEE 802.11 p and existing improved protocols show poor performance in the presence of significant traffic density, this thesis study a MAC protocol based on Multiple-Data-Rates(MDR-MAC). The protocol eases channel congestion by reducing the message transmission time through the use of multiple data rates. The efficient propagation of beacon is ensured by reducing message transmission collision through centralized slots allocation. Simulation results show that the performance of beacon is better than existed protocols.Thirdly, this thesis is dedicated to optimize the efficiency and reliability of emergency messages dissemination. On one hand, the transmission efficiency of emergency messages is improved by reducing channel access time through slot reservation mechanism. On the other hand, cooperation transmission mechanism improves the transmission reliability of emergency messages through the procedure of identification with the failed transmission, selection of helpers and retransmission emergency messages. Simulation results show that fast and reliable transmission of emergency messages can be guaranteed.