## 2 Literature Review
### Depth-Based Routing
This is because DBR only considers the current forwarder node that is located on one hop local node's distribution through its depth differences, which may create void holes on the expected next hop forwarding. However, WDFAD-DBR considers the second-hop forwarding mechanism, which effectively increases the reliability of the network. Consider a scenario in which a source node forwards the packet to the current forwarder node, being present at an optimal position (that is, its depth difference is minimum among all of the other nodes so that it is a best forwarder node for the next forwarding), so that in case of DBR, it will directly send the packet to that node after calculating the depth differences of the current and the forwarded nodes. As we consider this node located at the optimal position and for DBR, it will be a best forwarder node. However, as DBR does not consider the second hop for this forwarder node, if this optimal forwarder node has a void hole in its transmission region, then it will not be possible for this optimal node to forward its packet to the next forwarder, as there are no other forwarder nodes in its range. However, for WDFAD-DBR, as it considers not only the current forwarded node but also the expected next hop-forwarded node for the transmission of a packet, WDFAD-DBR will not only consider the optimal node that was considered by DBR, but it will select those ones that have a next forwarder node at the second hop, which may not be on an optimal position. Additionally, WDFAD-DBR works on anticipating the next hop forwarder nodes and avoids void hole creation, which effectively reduces energy consumption, avoids the packet losses, and increases the packet delivery ratio of the network. There is some shortcoming in WDFAD-DBR, such as if there are void holes at the second hop also, there is the question of what this node will consider for itself to forward the packet. Thus, another novel protocol [19], DOW-PR for UWSN, is proposed to mitigate some of the pertinent challenges of WDFAD-DBR. DOW-PR has defined some important parameters, such as suppressed nodes and potential forwarding nodes (PFNs). In DOW-PR, the authors not only consider two-hop communication as in DBR, but they also consider the numbers of PFNs and the numbers of suppressed nodes while performing the transmission. This being considered, we have two paths to transmit the packet, and both paths have potential forwarding nodes, so WDFAD-DBR will consider the path having nodes on the second hop and their nodes present at the optimal position, but DOW-PR not only considers the node position, but also the numbers of PFNs present at that forwarded node. If the numbers of PFNs at one path is large, then the chances of creating more duplicate packets are increased, as a large amount of PFNs means more packet transmission, which results in more duplicate packets. Thus, DOW-PR considers those paths for two-hop communication that have a low number of PFNs at the first and second hops, such that the reliability of the network is enhanced and the duplicate packet creation is minimized, while WDFAD-DBR only considers the next forwarded node and not the numbers of PFNs. Additionally, as we have discussed earlier, if a void hole occurs at the second hop, then DOW-PR will consider the nodes from those suppressed nodes having suitable PFNs other than the source node. In this way, the packets will not be lost in DOW-PR as WDFAD-DBR does not consider this strategy. Additionally, the DOW-PR divides the transmission range into different energy levels. The source node receives the information of transmission energy level from the flooding of the request packet and from the reply of the acknowledgment packet. From this, the source node sets its transmission power level according to the optimal transmission energy level so that there is no packet loss during transmission while WDFAD-DBR floods the packets at random power transmission.