## 2 Previous Works
In a tie situation, where two or more than two neighbors have the minimum lowest count, the decision is taken based on the path quality. If still that tie persists, then the next tie breaker metric is the depth of the node. The lowest depth node will get the chance to forward the data packet. The packets will be forwarded through the same set of nodes that have lowest hops, good path quality, and smaller depth. In result, it will penalize the low energy nodes and they will die out quickly. Moreover, the beaconing overhead to maintain the soft-state information is very high and that drastically consumes the network energy. Another _receiver initiated_ void avoidance routing protocol, called inherently void avoidance routing (IVAR), has been proposed in [18]. IVAR forwards packets towards Sinks using two routing metrics: depth and hop count of a node. In order to maintain the hop count at each node, all Sinks periodically broadcast the beacon message. This beacon message is forwarded as in [17]. Each data packet includes the hop count information. When a data packet is forwarded by the node, then all the receiving nodes that have the hop count smaller than the one in the data packet are eligible candidates. These forwarding nodes compute their holding time using their respective depth information. In IVAR, the nodes at the edge of a void region simply discard the transmission, because their hop count is either equal or greater than the one in the received data packet. IVAR avoids the void region and the communication is carried out through the nodes with the small hop count only and it fails to achieve and balance the energy efficiency. The authors in [19] improved the shortcomings of the IVAR and proposed a _sender initiated_ opportunistic void avoidance routing (OVAR). These shortcomings include; the duplicate packet transmission that is resulted by the small difference in the holding times of the candidate forwarder and the hidden terminal problem of IVAR. OVAR adapts the same periodic beaconing method to maintain information of the 1-hop neighbors. This 1-hop adjacency information at each forwarder helps it to select the next candidate forwarders. The candidate nodes are selected in a proximity to avoid the hidden node problem and reduce duplicate packet transmission. In the above-discussed forwarding protocols that use beacons to update proximity graph, e.g., LLSR, OVAR and IVAR, neighborhood information freshness depends on the beacon interval. The lower the beacon interval, the higher the neighborhood information accuracy and better the forwarding decision. However, the low beacon interval has severe impact on the battery power. Recently, a _receiver initiated_ depth based routing protocol, named weighting depth and forwarding area division DBR (WDPAD-DBR), has been proposed that avoids the void hole problem in UWSNs [20]. It achieves energy efficiency in the dense network scenario by suppressing the duplicate data packets and reducing end-to-end delay by considering the two-hop distance value in the holding time calculation. Routing in WDPAD-DBR is performed into two phases: First, the forwarding region is divided into one primary and two auxiliary forwarding areas to increase the chances of packet delivery. Second, it computes the weighted sum of the depth differences of two hops to minimize the packet duplication. This two hop information prevents the void hole encounter, however, merely two hop depth difference does not eliminate the chance of void hole encounter. The probability of packet drop and end-to-end delay increases with the increase in the network density because the holding time does not use any node density information. Figure 3 shows the void region scenario where node \(S\) sends the data packet and nodes 1-5 are within transmission range of node \(S\), \(t_{r}^{S}\). Nodes 2-4 are the forwarding candidates and the nodes 1 and 5 simply drop the packet because they have larger depth than \(S\).