## 4 Classification of MAC Protocols Based on Channel
### Random Selection Based
#### 4.11.2 Self-Sorting-Based MAC  
This protocol is based on self-sorting the channel allocation based on TDMA. It provides collision-free transmission with reduced packet loss and delays in message delivery in even dense scenarios. The un-ordered random access is altered by ordered self-sorting-based access. Vehicles access channels in a queue via a TDMA-based mechanism. Nodes in the same queue compete to access the channel to minimize access collision via random channel access. The self-sorting process occurs in three steps: self-sorting, channel reservation, and data transmission [101]. Figure 16 depicts the mechanism of the protocol for sorting the message in queue, reservation of the channel, and the transmission of data. As soon as the capacity of a queue reaches its threshold, the members or nodes of the queue start to transmit in the channel in the order in which they joined the queue. An M/G/1/\(\infty\) queuing model was used in the analysis for the computation of average service time, delay, and packet delivery ratio. Markov chain modeling was used to estimate the probability of queuing successfully. The protocol overcomes the overhead of maintaining a schedule table, unlike other slotted protocols, as the nodes are self-sorted. It effectively handles delays and PDR. Figure 16: Mechanism of self-sorting [101]. However, adapting the dynamic scenarios, handling hidden terminals, and various other requirements of delay and PDR are still challenges. No analysis is given based on merging collision. An extensive study of the MAC layer protocols for VANETs was conducted in the survey under various categories. These protocols aim to build an efficient mechanism for data dissemination. There are, however, numerous issues to be addressed in the future. The future directions include but are not limited to prioritizing safety messages for guaranteed reliability; ensuring fairness, in which a robust MAC protocol should also meet different QoS requirements, prioritizing services; handling network loads with dynamic channel access and proper channel utilization not limited to a single CCH; and minimizing interference by adopting a multi-channel access mechanism, where a centralized MAC protocol should be implemented under the premise of unreliable link and high vehicle mobility since RSUs and CHs can support direct multi-hop transmissions by adjusting to high dynamic networks. To conclude, the research direction includes designing an optimized MAC scheme providing QoS, adapting appropriate channel mechanisms based on the scenario (dense/sparse), and adapting clustering mechanisms for efficiency. A chronological diagram of the above discussed MAC protocols is depicted in Figure 17, and a summary of the overall comparison is shown in Table 3.  
Figure 17: Chronological diagram of MAC protocols survey. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**SR. No. ** & **Protocols** & **Comparison** & **Objective** & **Methodology** & **Simulator** & **Classification** & **Pros** & **Cons** \\ \hline \multirow{3}{*}{1} & \multirow{3}{*}{eRTS-SA [69]} & \multirow{3}{*}{VeMAC [53], RTS-SA [102]} & Avoids interference, reduces overhead, and eliminates the hidden terminal problem & Micro-base station, physical interference, and SINR & Hybrid and TDMA & \begin{tabular}{c} High \\ throughput and \\ fair channel \\ \end{tabular}  & Scalability and energy efficiency \\ \hline \multirow{3}{*}{2} & \multirow{3}{*}{PT MAC [70]} & ADHOC [47], Even Odd MAC [47] & Collision avoidance, minimized overheads, and high delivery ratio & Potential collision prediction, and potential collision elimination & MATLAB, and SUMO & \begin{tabular}{c} Prediction based, \\ TDMA, contention \\ free, and \\ decentralized \\ \end{tabular}  & \begin{tabular}{c} Minimized \\ delay and \\ collision \\ \end{tabular}  & Overhead and energy consumption \\ \hline \multirow{3}{*}{3} & \multirow{3}{*}{I MAC [104]} & \multirow{3}{*}{ \begin{tabular}{c} CF-MAC [53], \\ DTB MAC [105] \\ \end{tabular}