# Two Hop Adaptive Vector Based Quality Forwarding for Void Hole Avoidance in Underwater WSNs
## 2 Related Works
In the energy-efficient depth-based routing protocol (EE-DBR), energy conservation is obtained based on depth information. It incurs low end-to-end delay and less energy consumption compromising the network overhead [13]. The weighting depth and forwarding area division depth-based routing (WDFAD-DBR) protocol addresses the problem of void holes in UWSNs. In WDFAD-DBR, the extra energy expenditure and average end-to-end delay are reduced by suppressing duplicate packets in dense regions. Thus, enhanced reliability and reduced energy expenditure are achieved. In WDFAD-DBR, routing decisions are based on two metrics: firstly, the depth of both current nodes and the expected next hop forwarder are considered. Secondly, WDFAD-DBR adjusts the forwarding area according to the channel condition and node distribution \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Techniques** & **Features** & **Parameters Achieved** & **Limitations** \\ \hline VBF [16], location-based routing protocol for UWSNs & Confined forwarding area division to avoid packet duplication, directional forwarding along the virtual vector & High packet delivery ratio & Poor performance in sparse networks \\ \hline HH-VBF [17], location-based routing protocol for UWSNs & Change in direction of pipeline hop-by-hop, directional forwarding & High network throughput throughput & Inefficient void hole avoidance, poor performance in sparse networks \\ \hline AHH-VBF [18], location-based routing protocol for UWSNs & Change in direction of pipeline, adaptive transmission range hop by hop & High network throughput, energy conservation & Inefficient forwarder selection \\ \hline \hline \end{tabular}
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Table 1: _Cont._ \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline \hline
**TWSN** & **UWSN** \\ \hline Radio signal & Acoustic signal \\ High bandwidth & Low Bandwidth \\ Low propagation delay & High propagation delay \\ Low location error rate & High location error rate \\ High data rate & Low data rate \\ \hline \hline \end{tabular}
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Table 2: Difference of terrestrial WSN (TWSN) and underwater wireless sensor network (UWSN). density. WDFAD-DBR has achieved a lessened probability of void holes; however, considering the depth difference between two hops to avoid void holes does not eliminate them effectively, and eventually, packet drops affect the network lifetime [14]. Similarly, in [15], interference-aware routing protocols (Intar: interference-aware routing; Re-Intar: reliable and interference-aware routing) are presented. These protocols have adopted the sender-based approach to avoid void holes in UWSNs. The proposed protocols improve void hole avoidance along with collision probability at the receiver side; thus, the packet delivery ratio is improved. The Intar scheme computes the cost function (CF) based on multiple network parameters in the forwarding phase. The sender node selects the next forwarder on the basis of CF. The potential forwarding node with the least number of neighbors, the minimum number of hops from the sink and the maximum distance from the sender node computes the maximum value of the CF function. This approach reduces collision probability and avoids void holes by choosing the potential forwarding node with the maximum CF value. In Re-Intar, CF is computed using the depth difference between the source node and the potential forwarding node along with other parameters taken from the Intar protocol. For successful transmission, the source node transmits one hop backward in the case of the void region at the cost of increased accumulative propagation distance. In location-aware vector-based forwarding (VBF), the forwarding area for transmission is confined within a virtual pipeline that is directional towards the destination. The virtual pipeline direction and radius remain constant from a source to a destination. In sparse networks, the unavailability of forwarder nodes due to the constant virtual pipeline radius causes retransmissions.