## 4 Classification of Energy Efficient MAC Protocols for UWSNs
### Slotted-ALOHA (S-ALOHA)  
The slotted-ALOHA (S-ALOHA) protocol was developed to increase the performance of pure ALOHA by preventing data packet collisions during communication between nodes in a UWSNs. The whole time window is broken into smaller sub time slots in \begin{table}
\begin{tabular}{c c c}
**Features** & **CSMA** & **ALOHA** \\ \hline Presentation in WSNs & Unchanging & Unchanging \\ Presentation in UWSNs & Not unchanging & Unchanging \\ Utilization of channel & High & Low \\ Limitations of optimization & The transporter intellect is & adjust the unpleasant \\  & starting point which is attuned & back-off time \\ Energy ingesting & Low & High \\ Rate of Collision & Low & High \\ Transmission delay & Actual high in Underwater & High \\ Left over nodes of the network & Average & Lesser \\ \end{tabular}
\end{table} 
Table 7: The main differences between CSMA and ALOHA protocols. order to avoided data collision [105; 106]. With this sub time slotting method, each node must send a data packet in its assigned time slot otherwise, it must wait for all other nodes to deliver their data in their allotted sub time slots. During transmission and acknowledgements, this approach ensures that no data packets clash [107]. The (S-ALOHA) approach will take longer to accomplish a task as a result of the preferred way of sub item slotting [82; 106]. _(ii)_ _Pure-ALOHA (P-ALOHA)_  
If we increase the size of the sub time slots discussed earlier in S-ALOHA), it will be converted into pure-ALOHA (P_ALOHA). When P-Aloha is studied for communication in UWSNs, the performance is similar to that of RF networks since P-Aloha does not have a collision control system and frame collision is random [108]. Both these properties made the performance of the P-ALOHA poor as it will drastically drops the throughput of the system. The end-to-end delay will also be increased. In addition, as the control system works randomly in P-ALOHA, it will take more time to receive and send acknowledgements about data packets. Which in turn will make the system to use more energy [109]. _(iii)_ _ALOHA with carrier sense (ALOHA-CS)_  
The ALOHA-CS protocol simply observe the network that weather its half-duplex node is receiving any data packets from any node that is trying to communicate with it in a UWSNs. Due to which the sensor nodes would never send any new data packets while listening for a data packet within the network, regardless of whether it is the intended recipient or not [110; 111]. In the underwater acoustic environment, this protocol has taken advantage of a long propagation delay. Furthermore, when compared to pure ALOHA, this protocol delivers a significant increase in network throughput ratio, where the data packet size is large and the network has few nodes. Otherwise, throughput rapidly declines. _(iv)_ _ALOHA with advantage notification (ALOHA-AN)_  
ALOHA-AN is based on a concept similar to ALOHA with collision avoidance (ALOHA-CA). The goal of ALOHA-CA is to overcome the limitations of ALOHA-CS, while it is widely acknowledged that ALOHA-CS has the advantage of preventing data packets from being sent, when listening to another sensor node in the same network [110]. Furthermore, the listening procedure carried out by a sensor node can occasionally be assisted in reducing the likelihood of collisions of data packets [110]. ALOHA-AN has to collect and store more data, which necessitates the use of resources other than ALOHA-CA [112]. When the packet size is big and there are few nodes in the network, this protocol gives a significant increase in network throughput compared to pure ALOHA. _(v)_ _Buffered ALOHA protocol_  
Several research groups have used the buffers to improve the ALOHA protocol's performance [113; 114]. In [115] the authors developed an approximation approach for analyzing the S-ALOHA technique, which is based on a small user group with restricted storage capacity. The assumption of channel asymmetry was the main proof of concept of their method.