###### Abstract  
Traffic density around the globe is increasing on a day-to-day basis, resulting in more accidents, congestion, and pollution. The dynamic vehicular environment induces challenges in designing an efficient and reliable protocol for communication. Timely delivery of safety and non-safety messages is necessary for traffic congestion control and for avoiding road mishaps. For efficient resource sharing and optimized channel utilization, the media access control (MAC) protocol plays a vital role. An efficient MAC protocol design can provide fair channel access and can delay constraint safety message dissemination, improving road safety. This paper reviews the applications, characteristics, and challenges faced in the design of MAC protocols. A classification of the MAC protocol is presented based on contention mechanisms and channel access. The classification based on contention is oriented as contention-based, contention-free, and hybrid, whereas the classification based on channel access is categorized as distributed, centralized, cluster-based, cooperative, token-based, and random access. These are further sub-classified as single-channel and multi-channel, based on the type of channel resources they utilize. This paper gives an analysis of the objectives, mechanisms, advantages/disadvantages, and simulators used in specified protocols. Finally, the paper concludes with a discussion on the future scope and open challenges for improving the MAC protocol design. MAC classification; vehicular networks; VANETs; distributed MAC; centralized MAC; multi-channel MAC; single-channel MAC; spectrum allocation +
Footnote †: journal: 17  
(V2I) communication [2], where infrastructure is the electronic control units within the vehicles. Two mechanisms of communication in an ITS are mobile ad hoc networks (MANETs) and VANETs [3]. There is no central administration or fixed infrastructure in MANETs; nodes self-configure themselves and interact. Unlike MANETs, VANETs follows predictable patterns along a road. Moreover, the processing and storage capacity of VANETs is better than that of MANETs. VANETs have unique features such as high mobility and the constraint of a dynamic road network topology, unpredictable network size, and infrastructure support that differentiates it from MANETs. A basic model diagram of VANETs includes vehicles and other infrastructures communicating via V2V, V2I, Road Side Units (RSUs), and Onboard Units (OBUs). These communications play a vital role in the transportation system to improve traffic efficiency and safety. A schematic representation of the communication is depicted in Figure 1.  
VANET applications are divided into categories such as life-critical applications, safety, warning applications, e-toll collection, group communication, traffic management, and user applications, as in [4]. Most papers have broadly classified VANET applications into two categories: safety-based applications (handling real-time traffic and avoiding accidents) and non-safety-based applications (infotainment services, parking availability, GPS tracking of nearby places, etc.) [5; 6]. The salient characteristics of VANETs include the dynamic topology, unpredictable network size, high scalability, recurrent information exchange, time-critical communication, wireless medium of communication, energy efficiency, and real-time applications [7; 8]. Intelligent transportation systems (ITSs) in smart cities have improved road safety and minimized the risk of mishaps on the road. The use of DSRC [9] (IEEE 802.11p [10]), WAVE (IEEE 1609 protocol stack [11]), and 5G LTE have regulated communication in VANETs extensively. Different countries of the world have adopted dedicated frequency bands for vehicular communication by ITS [12]. Based on the division of a spectrum, the MAC protocols are divided into single-channel MAC and multi-channel MAC.