Improving Performance of IEEE 802.11p MAC Layer for Emergency Message Dissemination

摘要

Vehicular ad-hoc networking is the most promising subfield of mobile ad-hoc networks, which may become the ad-hoc networking technology in near future for vehicles communicating amongst themselves on road. It uses IEEE 802.11p MAC protocol as wireless networking technology. The IEEE 802.11p MAC protocol has inherent problems in wireless ad-hoc networking environment due its heterogeneous, infrastructureless and highly dynamic nature. The performance of IEEE 802.11p MAC layer for vehicular ad-hoc networking is based on performance of one-hop broadcasting.The performance of IEEE 802.11p one-hop broadcasting is of major concern regarding emergency message dissemination. The CSMA/CA protocol used in IEEE 802.11p is far from optimal solution for emergency message dissemination due to inherent properties of random access, higher delivery delays and retransmissions. Techniques to improve emergency message dissemination delivery rate and minimize time latency of message dissemination, such as, disabling backoff and synchronous transmission, have been mentioned in this thesis out of which one technique such as disabling backoff is being evaluated through simulation results.The goal of this thesis work is to evaluate a technique, modifying the IEEE 802.11p MAC layer protocol using Network Simulator 3 (NS3). The technique is based on introducing a separate EDCA queue and a separate EDCAF function for emergency messages in QoS EDCA priority queues, disabling backoff for emergency messages and giving highest priority to emergency messages in a station having different AC queues seeking for transmission opportunity. Disabling backoff for emergency messages may reduce time latency arising from exponential backoff algorithm. As the backoff is disabled, more than one station may start transmitting emergency message at the same time. So, it can be deduced that such technique could be beneficial for simple emergency applications. The simulation results show that this technique could be useful for emergency applications utilizing a buzz signal for hazardous warnings on road.