## 4 Classification of Energy Efficient MAC Protocols for UWSNs
### Slotted-ALOHA (S-ALOHA)
The performance of arbitrarily selected users, which they refer to as tagged users, that are used for the analyses the system's performance. Authors in [116] had studied the implementation of S-ALOHA using smaller number of packet buffers. While the behavior of the hybrid ALOHA/TDMA protocol with client-side buffers had been investigated in depth by authors in [117]. The authors of [118] proposed the Buffered ALOHA, and had also investigated the influence of buffering packets on P-ALOHA. The authors of this paper presented a formula for calculating ALOHA throughput for a given number of active nodes. They divide the causes of failure into three categories on the basis of dropped data packets [118]. _(vi)_ _Slotted carrier sense ALOHA (Slotted_CS_ALOHA)_  
The authors in [99] highlighted the issue of energy efficiency in UWSNs. For that they had presented the slotted carrier sense ALOHA protocol (Slotted_CS_ALOHA) as a solution. Furthermore, if a packet collides while being sent from source to destination, it must be resent to receive a successful acknowledgement of the packet, which consumes energy. As a result, the sensor node will become ineffective and stop serving after a short period of time, and the data in this field will be wasted. Therefore, the authors in [119] address this issue by introducing a sleep mode whenever a node is not sending data and is inactive, to reduce power consumption. Prior of going into sleep mode, the node was allowed to send more than one packet. These data packets would then be sent to the buffer where the two conditions, slot time and CS would be used to reduce the possibility of collusion. In addition, there is a second buffer that will handle the ALOHA cycle, feeding only one packet to the system at a time. In addition, when that packet successfully reaches its destination, it will send another packet, and so on. It is found by the authors of the aforementioned study that while this strategy reduced process power consumption, and increase throughputs. It did not improve average delay. * _Variable interval ALOHA (VI-ALOHA)_ This protocol is specifically designed to reduce the likelihood of data packet collisions with one another within a network. It divides the broadcast channel used by anchor nodes, into sub channels, hence lengthening the time slot indirectly. For which the authors of [120] developed a variable interval ALOHA (VI-ALOHA) protocol with a randomly changing interval time slot and compared it to equal interval ALOHA (EI-ALOHA). To demonstrate the effect of the two protocols, that how they can reduce collision by increasing randomization in space. Where a variable interval was used to reduce the intersection of a beacon coverage. Secondly, they employee the position random distribution approach to generate a random beacon interval, which increases the randomness of each beacon broadcast, while reducing collisions caused by equal intervals. * _Learning-ALOHA (L-ALOHA)_ According to the authors in [121], two aspects are needed to be taken care off when implementing the learning-ALOHA (L-ALOHA). Firstly, it takes care of the learning algorithm that will be used while communicating between nodes, in which the node sends data packets at random intervals to identify a successful acknowledgement, where no collision between data packets occurs, which in turn will help to avoid data retransmission. The second portion is the steady component, which occurs when the entire network's learning process is stable. Where each node has ready data packets to transmit over the network and simply needs to be able to deliver data packets at a predetermined time period. The authors of the aforementioned research only compare their approach to S-ALOHA and P-ALOHA, where just two metrics (throughput and average end-to-end delay) are compared to demonstrate the magnitude of the differences. * _Saving time slotted carrier sense ALOHA (ST-Slotted-CS_ALOHA)_ Saving time slotted carrier sense ALOHA protocol is regarded as an update to Slotted_CS_ALOHA protocol.