## 3 Principals of Routing Protocols and Their Breakdowns
### Energy Aware Multi-Path Routing Protocols  
The energy consumption of each node depends on the mode of communication and the processing load of the signal. Three trivial factors determine the energy consumption of a node. Firstly, the distance between nodes is considered as sending signals to a node further away from the source requires more energy; secondly, the surroundings of the node are considered as harsher conditions would require tremendous energy to pass the signal from the source to the destination; and lastly, the capacity of the battery is also considered. All these are considered, and an equation is formed for the sender:  
\[E_{tx}=\begin{cases}1e_{elec}+l\epsilon_{fs}d^{2},&d<d_{0}\\ l\epsilon_{elec}+l\epsilon_{fs}d^{4},&d\geq d_{0}\end{cases} \tag{1}\]  
In Equation (1), \(l\) represents the bit number of a single packet; the distance between the receiver and the transmitter is denoted by \(d\). \(d_{0}\) represents the minimum distance for data transmission, \(\epsilon_{lcc}\) symbolizes the energy needed for radio transmission, \(mp\) is the multi-path transmitter amplifier coefficient, and \(f_{s}\) is the free space coefficient.  
However, the terrestrial WSN network defers from underwater wireless sensor networks, and the overall energy model is inadequate for UWSNs since an acoustic network's energy consumption differs from that of a radio signal. To calculate the acoustic signals' energy consumption, amplifier coefficients (\(a(f^{d})\)) of both free-space and multi-paths were used. The distance between the receiver and sender is denoted by \(d\), and the signal frequency is denoted as \(f\). Unfortunately, because the energy consumption of an acoustic signal differs from that of a radio signal, the energy consumption model of a terrestrial WSN network is not adequate for UWSNs. Both free-space and multi-path models in UWSNs utilize the amplifier coefficient, which is defined as \(a(f^{d})\), where a is the absorption coefficient, \(d\) is the distance between the transmitter and receiver, and \(f\) is the acoustic signal frequency. [PERSON]'s empirical formula can be used to approximate the value of \(a(f)\). If the frequency is 1000 Hz, for example,  
\[\log_{a}(f)=0.011\frac{f^{2}}{1+f^{2}}+4.4\frac{f^{2}}{4100+f^{2}}+2.75*10^{-}5 f^{2}+0.003. \tag{2}\]  
As a result, we can compute the energy consumption definition in UWSNs as follows. The transmission of data packets in an underwater environment, where the distance between the two nodes is \(d\) and the frequency \(f\), is transformed as follows:  
\[E_{tx}=\begin{cases}le_{elec}+la(f)^{d}d^{2},&d<d_{o}\\ le_{elec}+la(f)^{d}d^{4},&d\geq d_{0}\end{cases}, \tag{3}\]  
The following equation expresses the energy required to receive data packets:  
\[E_{rx}=le_{elec} \tag{4}\]  
The network layer aims to achieve sensor-to-observer connection, data routing, and cooperative sensing. The routing protocols in UWSNs must be set to achieve the required criteria in terms of energy efficiency, scalability, stability, and convergence. The primary goal of these protocols is to offer a reliable and energy-efficient route for nodes and enhance the lifetime of UWSNs. The energy consumption of a routing system is influenced by several aspects, including neighborhood finding, communication, and computing expenses. Energy-aware routing protocols are further divided into eight subcategories as shown in Figure 3.  
* Reactive Routing Protocols: When sending a packet, the reactive protocol starts a route-searching approach to find the path to the destination. Reactive routing protocols are also known as on-demand routing protocols. They do not hold a routing table. By doing this, they act as a bandwidth-efficient routing mechanism. UWSN adopted reactive routing protocols to provide bandwidth efficiency by using an incremental search method for new path discovery and the surrounding repair method for maintenance.