Vehicular ad-hoc networks (VANETs) aim to provide efficient safety and infotainment services. Wireless Access in Vehicular Environments (WAVE), which is a standard designed specifically for VANETs, stipulates seven 10-MHz channels: one control channel (CCH) for safety message transmission and six service channels (SCHs) for service message transmission. However, providing reliable broadcasting of safety messages on the CCH and efficient coordination of vehicles for service message transmission across the multiple SCHs in highly dynamic VANET environment is a nontrivial problem. In this paper, we propose a coordinated, adaptive, and reliable multichannel medium-access control scheme for the VANETs (VCAR-MAC). In VCAR-MAC, a novel time-division multiple access (TDMA)-based scheme that considers real-world environmental conditions is proposed to identify every vehicle quickly so that a time slot on the CCH can be allocated efficiently for reliable safety message transmission. On the SCHs, VCAR-MAC provides a multi-SCH coordination scheme, which adaptively adjusts the SCH reservation period in order to fully utilize the channel bandwidth. Furthermore, a dynamic contention window (CW) mechanism is proposed, in which the initial CW size is adaptively optimized to maximize the number of successful SCH reservations, thereby maximizing the service message throughput. It has been proven via the mathematical analysis and simulation experiment that VCAR-MAC can significantly improve throughput and reduce delay for both safety and service messages.INDEX TERMS MAC scheme, vehicular ad-hoc networks (VANETs), safety and service applications.Recently, the research demand regarding Vehicular Ad-hoc Networks (VANETs) has been arising in industrial and academical fields in view of their great potential to support safety and infotainment services [1] [4]. There are two major components in VANETs: On Board Unit (OBU) and Roadside Unit (RSU) [5]. The OBU is a communication device mounted on a vehicle to operate as a mobile node. The RSU, which is an infrastructure that can be located at any fixed point of interest on the road, provides vehicles with Internet access. Communication between an RSU and an OBU is referred to as Vehicle-to-Infrastructure (V2I) communication, and communication among OBUs is referred to as Vehicle-to-Vehicle (V2V) communication [6].The associate editor coordinating the review of this manuscript and approving it for publication was Mohammad S. Khan.
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