## 3 Medium Access Control (MAC) Layered UWAC Protocols
### Contention-Based
#### 3.3.1 Hand-Shaking (HS)
The DA-CSMA UWAC protocol is compared to the traditional HD-CSMA-UWAC protocol, as shown in Figure 4. The DA-CSMA protocol outperforms conventional HD-CSMA by providing higher throughputs, lower energy consumption, exhibiting low end-to-end delays under heavy network load. * **FS-MACa**. The FD FS-MACA protocol [37], based on the future scheduling (FS) handshaking (HS) mechanism scheme, is proposed as a new handshaking protocol [37]. The HS scheme ensures data transmission by scheduled procedures and future negotiations between the sender and receiving segment. The physical channel is divided into two parts, one for real data transmission and the other for control packet negotiation, resulting in higher throughput and less interference while achieving FD communication. This protocol was tested in a network of 100 unsystematically distributed nodes in an underwater environment with a 2 \(\times\) 2 square km of surface area, and it achieved a data rate of 100 kbps with packet lengths ranging from 400 to 1000 bytes. Despite the fact that the mentioned protocol above is capable of managing high traffic loads while preventing collisions without the use of time synchronization or ACK, there are still open issues that need to be addressed. Since it does not use time synchronization, this protocol is unable to manage long propagation delays and experiences severe throughput degradation; however, this may be an important topic for future study. * **FDCA-MAC**. The FDCA-MAC UWAC protocol [34] is based on the idea of transmitting and receiving on the same frequency at the same time, reducing hidden and exposed terminal problems and allowing FD-UWAC modems to achieve high throughput. Recent research shows that using FD underwater acoustic modems, the SI effect can be cancelled easily [11; 29; 33], i.e., the HS mechanism can be used to predict transmission scheduling by passively receiving local information about neighboring node propagation delays. The proposed protocol is based on this concept, and it achieves large throughputs [34]. Furthermore, FD modems are used to implement multiple HS with their neighbors and to increase throughputs while achieving temporal and spatial reuse. It will be necessary to test its success in both a lack and a sea environment in the future. * **TFH-MAC**. A novel TFH-MAC protocol is proposed, which significantly reduces the SLI issue, which is a common problem with FD UWAC systems, while achieving FD communication [69]. The TFH-MAC protocol is implemented on a node that acts as a centralized node among multiple nodes. The centralized node connects the  
Figure 4: (**a**) Throughput and (**b**) end-to-end delay comparison of full-duplex DA-CSMA and traditional CSMA MAC protocols. multiple nodes, using the TFH-MAC protocol to ensure that communication between the nodes is secure. The TFH-MAC protocol significantly reduces SLI, allowing for higher throughput while using less power. However, we discovered that this protocol is unable to cope with long propagation delays, resulting in reduced performance when used in a large network. As shown in Figure 5, we compare the throughputs of various HS-FD-MAC layered-based protocols [35; 36]. The IBFD-MAC protocol [35] outperformed the full-duplex FS-MACA and HS-HD-MACA protocols [21] by demonstrating a substantial throughput gain. * **E-FD-MAC**. An enhanced E-FD-MAC protocol is proposed in [38], that allows the FD channel to simultaneously exchange data packets in a minimum possible time, as shown in Figure 6. This protocol allows the source and destination to exchange data using sensed information without being affected by unexpected collisions, which greatly increases throughput and reduces propagation delays. Due to its ability to transmit and receive control and data packets, the proposed E-FD-MAC receives active transmissions earlier than the current HD-MAC protocol. The way it works is shown in Figure 6. The UW sensor nodes detect the data and set a back-off timer based on the transmission priority.