## 3 MAC Classification in VANETs Based on Contention Mechanism
Sync intervals (SIs) typically consist of CCH intervals (CCHIs) and SCH intervals (SCHIs), separated by guard intervals. Adaptive collision-free MAC (ACFM) [52] is a TDMA contention-based protocol with dynamic slot allocation. In ACFM protocols, unused slots are avoided during sparse traffic, and additional spaces are allocated in dense traffic. The VeMAC [53] protocol supports multi-hop broadcasts and one-hop broadcasts on the control channel while reducing collisions caused by node mobility on the access channel. Vehicles moving the opposite way are assigned disjoint time slots and roadside units to reduce collisions during merging. Two transceivers are used per node, tuned to the control and service channels, and synced by the GPS signal. The VeSOMAC protocol [54] is a self-organizing, DSRC-based MAC protocol used in multimedia applications. The TDMAslot information is exchanged in-band during distributed MAC scheduling. In highway scenarios, this enables fast reconfiguration of TDMA slots without depending on roadside infrastructure. VeSOMAC operates both synchronously and asynchronously. Through cooperative TDMA MACs [55], non-safety applications can achieve greater throughput. This protocol overcomes problems associated with poor channel conditions, which lead to transmission failures. If a packet fails to be transmitted, the neighboring nodes call helper nodes to relay the packet in a time slot. It is important to note that the helper nodes use the available time slots, which may lead to access collisions between the vehicles and the helper nodes. An enhancement of ADHOC MAC, the adaptive real-time distributed MAC (A-ADHOC) [56] protocol, intends for real-time application in large-scale wireless vehicle networks that provide adaptive frame lengths. Both the channel resource utilization and response time of A-ADHOC were better than those of ADHOC MAC, and A-ADHOC avoided network failure regardless of traffic density. Some of the hybrid protocols (TDMA + CSMA/CA) include HER-MAC (hybrid efficient and reliable MAC), TDMA/CSMA MAC (HTC MAC), SOFT-MAC (space orthogonal frequency-time MAC), and DMMAC (dedicated multi-channel MAC with adaptive broadcasting). The SDMA, OFDMA, TDMA, and CSMA techniques were incorporated into SOFT-MAC [57], with GPS to locate sub-carriers shared between vehicles belonging to that particular cell. The HER MAC [58] is a multi-channel MAC with adaptive broadcasting. Every vehicle in the CCH transmits safety/alert messages using a half-duplex transceiver without colliding with another vehicle. HTC MAC [59] alleviates the collision and enhances the throughput of HER MAC by broadcasting the announcement packet (ANC) and reservation period (RP). DMMAC [60] is also a dedicated multi-channel MAC with access time split into intervals (CCHI and SCHI), again split as contention-based reservation period (CRP) and adaptive broadcast frame (ABF). This discussion presents some of the traditional MAC protocols classified based on contention mechanism as contention-based, contention-free, and hybrid presented in Figure 2. Furthermore, the next section provides a brief classification of contemporary MAC protocols in vehicular communication. A summary of different MAC protocols is shown in Table 2.  
Figure 2: Classification of MAC protocols based on the contention mechanism. In recent years, MAC protocols with dynamic interval schemes [61] have been proposed for the optimization of channel control intervals. A MAC protocol for dynamic adaptation due to changing VANETs topology is a challenge as the static service channel and the control channel cannot adapt to the versatile VANETs environment. Protocols designed with a dynamic interval scheme provide maximized performance with less end-to-end delay and minimized collision. Clustering is a prime concept in VANETs for efficient group communication. Some VANETs integrates machine learning and fuzzy logic algorithms to stabilize and make clusters more efficient.