## 3 Types of Underwater Communication Systems
### Underwater Wireless Communication Protocols  
To account for the variability in underwater channels, robust communication protocols are implemented to ensure communication link stability [60]. A well-designed protocol must provide robustness, efficiency, and low latency [60]. Robustness enables signalling techniques aimed at resiliency to high bit error rates and multipath events to maintain communication links [5; 60; 108]. Efficiency refers to the optimal utilisation of limited resources and low latency refers to timely signalling, which are critical for data transmission coordination [60]. Various types of protocols are currently used in underwater communication systems. [PERSON] et al. [60] reported the following classes of protocols: Medium Access Control (MAC), routing, transport, and cross-layer networking protocols. MAC protocols make efficient use of available bandwidth; Aloha-based MAC protocols improve collision avoidance. These features make them suitable for underwater acoustic sensor networks [60]. Two Aloha-based MAC protocols are discussed in [60]: Aloha with Carrier Sense (CS) (Aloha-CS) and Aloha with Advanced Notification (AN) (Aloha-AN). Underwater Medium AccessControl (UW) (UW-MAC) is a distributed single-carrier solution utilising Code-Division Multiple Access (CDMA) to enable multiple access to limited underwater bandwidth, keeping low complexity of resource-limited transceivers. In [60], three categories of routing protocols are identified for general applications, namely proactive, reactive, and geographical, to meet specific resiliency needs. Examples of proactive routing protocols are Destination-Sequenced Distance Vector (DSDV), and Optimised Link State Routing Protocol (OLSR), which are unsuitable for underwater applications due to unscalability [60]. As for reactive routing, Ad-hoc On-demand Distance Vector (AODV), and Dynamic Source Routing (DSR) are listed; however, these are advised against due to high latency [60]. The geographical routing approach, namely Greedy Face-Greedy (GFG), and Partial-Topology Knowledge Forwarding (PTKF), is scalable and requires limited signalling [60]. Both GFG and PTKF protocols work by node geographical coordinates to establish forwarding decisions and are suitable for large-scale underwater network applications due to reduced energy consumption, reduced delay, and throughput optimisation features [60]. With regards to transport-layer protocols, they are relied on for the provision of reliable data transport, flow and congestion control, and end-to-end communication management across the nodal networks [60]. Furthermore, ref. [60] states that Transmission Control Protocol (TCP) is not a suitable protocol applicable to the underwater environment due to its requirement to have an accurate estimate of the Round Trip Time (RTT), recommending new designs that offer adaptation, control, and reliability. TCP is one of the core protocols of the Internet Protocol Suite (TCP/IP). TCP responsible for providing reliable, ordered, and error-checked delivery of data streams between applications running on different hosts. On the other hand, RTT is a crucial metric in TCP communication. RTT measures the time taken for a packet to travel from the sender to the receiver and back. RTT measurements include the transmission time, propagation delay, and processing time at both ends. To mitigate the RTT limitations of the TCP, [PERSON] et al. [111] introduced the Optimal Re-transmission Timeout (RTO) Interval stop-and-wait Transmission (ORIT) protocol. The ORIT protocol incorporates a reduced RTO, shorter than the RTT to increase transport efficiency [111]. The ORIT's interval stop-and-wait transmission mechanism is designed to ensure re-transmission is stabilised based on preceding transmission results to increase data delivery assurance in a narrow acoustic channel [111].