Vehicular Ad hoc NETworks (VANETs) are the key to the future of Intelligent Transportation Systems (ITS). In VANETs, event driven safety messages, generated for example when an abnormal condition or an imminent danger is detected, have stringent requirements on delay. However, due to the inherent drawbacks of CSMA which is used in VANETs, such messages will not be able to meet the timing constraints in dense traffic scenarios. In this paper, we introduce a Context Aware Variable Interval MAC (CAVI-MAC) protocol for IEEE 802.11p/WAVE VANETs which utilizes multichannel access scheduling in a novel way. We differentiate between event driven and periodic messages by assigning priorities and also suggest a probabilistic model for addressing interference from hidden terminals. We also use a novel atomic model to analyze the propagation of event driven messages. Extensive simulations over lower Manhattan city scenario for a variety of metrics indicate that CAVI-MAC provides better performance (packet delivery ratio) than any of the existing protocols.
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