## 3 MAC Classification in VANETs Based on Contention Mechanism
The clustering algorithms are classified into three broad categories: intelligence-based strategies (machine learning algorithms, fuzzy logic algorithms, and hybrid algorithms), mobility-based strategies (NEMO algorithm and mobility algorithm), and multi-hop-based strategies (2-hop algorithms and 2+-hop algorithms) [62]. Some other clustering-based algorithms are proposed in [63; 64]. The SDN-based network is one of the technologies used recently in most fields for reliable and efficient communication; it splits the control plane from the data plane by providing centralized access to network resources. Reference [65] presents a concise layer \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Protocols** & **Mechanism** & **Objective** & **Disadvantages** \\ \hline ALOHA [41] & TDMA based & Collision avoidance & Throughput reduction \\ \hline \multirow{4}{*}{S-AIOHA [42]} & TDMA and CSMA sense the & \multirow{4}{*}{Collision avoidance and improved } & Unable to avoid collision in \\  & channel before transmission, and & & \\  & CSMA/CD used for collision & throughput & \\  & detection & & \\ \hline MACA [43], & Handshake method by RTS/CTS & Overcomes hidden terminal & Performance degradation in dense \\ MACAW [44] & and ACK & problem & scenarios \\ \hline \multirow{2}{*}{IEEE 802.11p [51]} & DCF, CSMA/CA, WAVE, and & Reliable, adapted to frequent \\  & DSRC-based & changing VANET environments, & \\  & and alleviates collision & & \\ \hline ADHOC- & Dynamic TDMA & \begin{tabular}{c} Contention-free access, and \\ multi-hop and one-hop broadcast \\ with QoS \\ \end{tabular} & \begin{tabular}{c} Transmission overhead, inefficient \\ real-time traffic control, and \\ transmission delay \\ \end{tabular} \\ \hline ACFM [52] & TDMA, and dynamic slot & \multirow{2}{*}{Alleviate collision} & Lacks real-time control, and QoS \\  & assignment & & degrades in dynamic scenarios \\ \hline VeMAC [53] & TDMA based, and one-hop and & Handles hidden/exposed \\  & multi-hop broadcasts & terminals & \\ \hline VeSOMAC [54] & TDMA-based distributed & Improved throughput in highway & Channel utilization is inefficient in \\  & scheduling & scenarios & high densities \\ \hline CAH MAC [55] & Distributed TDMA & \begin{tabular}{c} Enhanced throughput (non-safety \\ application) \\ \end{tabular} & Access collision problem \\ \hline A-ADHOC [56] & Adaptive frame length & \
\begin{tabular}{c} Proper channel utilization and \\ real-time application \\ \end{tabular} & Less throughput due to overhead \\ \hline HER MAC [58] & Multi-channel access and & Channel split and three-way \\  & adaptive broadcasting & handshake & Collision problems and delay \\ \hline HTC MAC [59] & Hybrid TDMA/CSMA & \multirow{2}{*}{Overcome collision} & Hidden/exposed terminal \\  & multi-channel & & problems and communication \\  & & & overhead \\ \hline SOFT MAC [57] & SDMA, OFDMA, TDMA, and & Proper channel and resource \\  & CSMA mechanisms & allocation & Highly complex with overheads \\ \hline DM MAC [60] & Dedicated multi-channel, & Proper broadcasting and &
\begin{tabular}{c} Performance degradation in \\ changing topologies due to \\ random slot assignment \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of traditional MAC protocols. structure of VANETs along with an SDN controller. The application of SDN-VANETs on various parameters in IoT and wireless communication is presented. The open challenges and research directions faced by the latest work, including VANET integration with SDN; the recent and emerging technologies; as well as the use cases are demonstrated in [66]. With the new standards described in Section 2, it is possible to transition from \"legacy\" IEEE 802.11p systems smoothly. By implementing the same frequency channel worldwide, IEEE 802.11 bd takes advantage of existing deployments and infrastructure without interfering with current ITS applications. IEEE 802.11 bd achieves interoperability through a compatible waveform structure.