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Vehicular Ad-hoc Networks (VANET) offer promising solutions towards infrastructureless collaborative active safety of the road vehicles. Active safety related data dissemination in vehicle-to-vehicle (V2V) communication requires reliable and real-time transmission. Broadcasting and relaying safety messages to alert the surrounding vehicles about critical driving situations is considered to be timely reaction. Real-time, collision-free and cooperative media access control (MAC) protocols need to be enhanced to guarantee single and multi-hop data dissemination in the vehicular networks. In this paper, media access mechanism for efficient multihop broadcasting is presented. A relaying metric is embedded in the vehicular nodes to assure self-organizing, fast reacting and collision-free medium access mechanism. Two different MAC protocols are studied. The first protocol uses periodical beaconing messages exchanged among the individual nodes and the second protocol does not require those beaconing messages. For both of the cases, the distributed relay-competition and adaptive back-off schemes are simulated on multilane dense traffic. Network simulator (ns-3) simulation results are used to compare the two protocols with and without beaconing messages versus their success rates and network connectivity. To enhance applicability of the protocols, positioning of the vehicular nodes are assumed to be subject to GPS drift and instead of precise position information, sector value is considered.

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