## 3 Triangle Metric Based Multi-Layered Routing Protocol
### Network Architecture  
Figure 1 indicates the popular UWSNs' structure utilized in well-known protocols. Sink nodes are organized on the water top linked by radiofrequency hyperlinks. All regular sensor nodes are deployed randomly underwater from top to bottom at different depths and assumes that those sink nodes are applied with acoustic and radio links. Sensor nodes forward the data packets from the source node toward the sink/destination via forwarding the data packets using sensor nodes positioned closer to the sink. Individually, the sensor nodes deployed at the lowest of the deployment area may forward the data packet in a hop by hop manner toward the sink. On the other hand, the sensor nodes must be available at various depth levels for data transmission. However, due to the node's movement, there is a high chance of expanding a void region in the network, leading to a communication void problem (i.e., there is no available node with positive progress toward the destination to forward the data packets). Furthermore, we agree that reducing the number of information exchange between nodes significantly impacts decreasing network overheads and increasing communication throughput. It is highlighted that the proposal is a step closer to reducing information exchange in underwater networking by focusing on supernode consideration in the networking scenarios. These nodes have more information surrounding their local network environment, resulting in a lesser number of hello packet information exchanges in the overall network environment. This technique further reduces energy consumption toward green computing scenarios in underwater networking environments. The channel mode of TM2 RP is focusing on accurate calculation of signal spreading and total noise loss for underwater networking environment. It has basically extended the channel presented in [43] considering the signal attenuation and absorption loss rate for underwater acoustic signal. The absorption loss \(\alpha(f)\) of an underwater communication channel can be computed as expressed in Equation (1). \[10\log(\alpha(f))=\left\{ \begin{array}{c}\frac{0.11f^{2}}{(1+f^{2})}+\frac {44f^{2}}{(4100+f)}+2.75\times 104f^{2}+0.003,\ f\geq 0.4\\ 0.002+0.11\left(\frac{f}{(1+f)}\right)+0.011f,\ f<0.4\end{array} \right. \tag{1}\]  
Figure 1: Traditional underwater sensor network architecture. where \(f\) represents the frequency of the acoustic channel. The absorption loss is measured in dB/km and channel frequency in kHz. The absorption loss value \(\alpha\) can be computed as \(\alpha=\frac{10^{\alpha(f)}}{10}\). By combining the absorption loss and spreading loss, the total attenuation \(A(l,f)\) can be expressed as follows in Equation (2). \[10\ \log(A(l,f))=k\times 10\log l+l\times 10\log(\alpha(f)) \tag{2}\]  
where \(l\) denotes the distance at which spreading loss is computed as represented by the first part of the right side of this equation, and the second part is denoted by the absorption loss at frequency \(f\). Additionally, the underwater signal propagation geometry is denoted by the spreading coefficient \(k\), which is in the first part of this equation. Therefore, we proposed an improved network architecture than above-mentioned for underwater networking. The supernode has been presented in MRP [42] to identify different layers with different power levels toward the whole architecture. Figure 2 below shows the improved architecture of the proposal with two different sensor nodes called ordinary nodes and supernodes. In this architecture, supernodes are embedded with high energy and high ability packet transmission with different power-level. Supernodes are static and positioned at the bottom level of the water surface. Supernodes have multi-use in our protocol, as discussed in next subsection. This architecture helps reduce the void areas in the network and avoids the communication hole. Moreover, sensor nodes have limited energy. Therefore, the nodes' energy is soon exhausted.