# Two Hop Adaptive Vector Based Quality Forwarding for Void Hole Avoidance in Underwater WSNs
## 3 Proposed Schemes
### Two-Hop Adaptive Routing Scheme  
Energy is one of the most scarce resources in sensor-based networks. There are various reasons behind the inefficient depletion of nodes' energy, which degrade the network performance. One of the major concerns is the void hole. The occurrence of the void hole consumes unnecessary energy and stops the flow of data between the source and destination due to the breakage of routes and the unavailability of forwarder node(s). Instead of recovering from the void hole, we have proposed a mechanism through which the creation of the hole is avoided prior. For catering to the void hole problem, four parameters are taken into consideration. These parameters are: the residual energy of the forwarder node, the maximal distance between the sender and receiver within the communication range, a threshold value for the number of nodes to be in the pipeline for minimizing  
Figure 2: Problem identified regarding forwarder selection.  
retransmissions and the distance of the nominated forwarder node from the virtual vector. The reason behind the computation of the aforementioned parameters is to ensure that the forwarder node is not selected repeatedly, because it will dissipate the node energy quickly and the void hole will be created. Therefore, with effective forwarder node selection, an efficient battery dissipation is ensured. Energy computation at each hop is performed to accommodate any of the changes in the function for suitable node selection as forwarder for optimal results in terms of network performance.  
The transmission within the communication range means the signal to noise ratio is good; therefore, we have preferred a next neighboring node at optimal distance from the destination of sender node because the signal is broadcast within the transmission radius, which means the energy will be dissipated, so the node at the optimal distance is the priority. Similarly, the forwarder node having one neighbor is avoided due to node mobility. If more than one node is in the transmission range, the number of re-transmissions is reduced, and data packets reach the destination with low latency. Furthermore, the distance from the virtual vector accounts for if the data are forwarded before they reach the destination, and the neighbor node should be in the communication by keeping in view the mobility of the node due to water currents. All metrics are obtained at each up to two hops before nominating a forwarder node in order to minimize the void hole occurrence probability. The probability that the void hole is successfully detected and avoided can be computed as \((1-(1-p)^{2})\), where \(p\) is computed by \(p=\frac{\pi r^{2}}{R}\). Here, \(r\) denotes the sensing range of a sender node, while \(R\) represents the radius of the network. Due to the reception of duplicate data packets, much energy is wasted. Therefore, the holding time is computed based on the number of eligible forwarder nodes in the sensing range (Figure 3). The following equations are formulated to show that redundant transmissions are avoided for network lifetime prolongation.  
\[Thold_{2 hop}=\frac{\alpha\times T_{pre}}{2\times f_{Nn}}+\frac{R_{t}-|SF|}{V_{ sound}} \tag{1}\]  
where \(f_{Nn}\) is computed as given in Equation (2).  
\[f_{Nn}=\frac{1+sign(Nn-2)}{2}, \tag{2}\]  
\(sign(Nn-2)\) is calculated according to the following equation.  
\[sign(Nn-2)=\left\{\begin{array}{rl}-1,&Nn<2\\ 0,&Nn=0\\ 1,&Nn>2\end{array}\right. \tag{3}\]  
To ensure that each parameter has an equal contribution to selecting the potential forwarder node, a mathematical formulation is carried out in the following section.