## 5 Proposed Reliable, Energy Optimized and Void Aware Routing Protocol
### Strategic Deployment of Sensor Nodes  
The 3D UASN network model is adopted from the research conducted in [23], where the underwater environment is segmented into unequal virtual layers. The first layer which is closest to the SSN has the smallest width, whereas the widths of the remaining layers increased with an arithmetic progression. The uniform distribution of ASNs in each unequal layer can increase the chances of sparser deployment in the widest layer, which triggers the possibility of void area occurrence [23; 45; 46].  
For example, as per the deployment of ASNs considered in [47], the total sum of ASNs deployed in each layer is 100, and the volume (in cubic meters) of the first layer is 500 \(\times\) 500 \(\times\) 50 which is equivalent to 12,500,000, whereas the volume (in cubic meters) for the fifth layer is 500 \(\times\) 500 \(\times\) 150, which is equivalent to 37,500,000. There is a massive difference between the volumes of two distinctive layers. This massive difference affects the sparsity of the ASN deployment in each layer.  
Consequently, there is a greater likelihood of void regions in the network. The designed routing protocol may not be suitable to be deployed in underwater monitoring regions having higher depths. Since there is no mechanism provided for controlling the void issue, the ASNs are high priced; therefore, strategically placing them in the submerged region will be favorable. The mathematical expression given below can be used to obtain the number of ASNs to be placed in each layer based on the width while keeping the ASN density constant.  
\[\mathrm{N_{N}}=\mathrm{ASN_{d}}\times\varpi_{\mathrm{N}} \tag{14}\]  
where \(\mathrm{N_{N}}\) is the total number of ASNs to be deployed in layer \(\aleph\), \(\mathrm{ASN_{d}}\) is the desired ASN density (nodes per unit width) and \(\varpi_{\mathrm{N}}\) is the width of the layer \(\aleph\). Equation (14) ensures that the ASN density remains constant across all layers. The ASN density can be determined through Equation (15) by dividing the total sum of ASNs (\(\mathrm{ASN_{T}}\)) by the total width of the underwater networking region (\(\varpi_{\mathrm{T}}\)).  
\[\mathrm{ASN_{d}}=\frac{\mathrm{ASN_{T}}}{\varpi_{\mathrm{T}}} \tag{15}\]  
To maintain uniform node density across all the layers, the number of ASNs to be deployed can be strategically placed in proportion to the width of the layer. This ensures that the ASNs density remains constant across all layers at the time of initial deployment. For instance, if the \(\varpi_{1}=100\) m, \(\varpi_{2}=150\) m, \(\varpi_{3}=200\) m, \(\varpi_{4}=250\) m, and \(\varpi_{5}=300\) m, then the total number of ASNs to be deployed in each layer will be \(\mathrm{N_{1}}=100\) ASNs, \(\mathrm{N_{2}}=150\) ASNs, \(\mathrm{N_{3}}=200\) ASNs, \(\mathrm{N_{4}}=250\) ASNs, and \(\mathrm{N_{5}}=300\) ASNs. By doing so, each layer will have the same node density of one node per meter of width, ensuring uniform sparsity across all layers, which reduces the occurrence of the void region in the underwater networking region: for instance, as illustrated in Figure 3.