## 2 Previous Work
Additionally, the power adaptiveness does not guarantee that avoidance of packet duplication and as well as the potential forwarder selection. In [20], authors proposed two routing schemes named as avoiding void node with adaptive hop-by-hop vector based forwarding (AVN-AHH-VBF) and cooperation-based AVN-AHH-VBF (CoAVN-AHH-VBF). The AVN-AHH-VBF selects the next hop forwarder based on the neighborhood information. The main selection criterion of the next hop forwarder is the node with maximum number of neighbors. It means that all the network nodes share their 1-hop proximity information as well using the neighbor request and neighbor acknowledgment packets. The holding time to suppress duplicate data broadcast in the pipeline is similar to AHH-VBF with only one additional parameter, that is the number of neighbors. On the other hand, the CoAVN-AHH-VBF works in a sender initiated manner, where it forwards the next hop forwarder(s) information in the data packet. The nodes that receive the data packet first search their ID in the packet and once they find their ID, they participate in the forwarding process. In addition to the next hop forwarder in the CoAVN-AHH-VBF checks whether the Bit Error Rate (BER) to the sender node is greater than the threshold or not. If BER is greater than the threshold then retransmission request packet is sent by the forwarder to the sender node to send data packet again. Both the schemes have large control overhead and incur more delay due to control packet exchange and practically they are not suitable for the underwater acoustic network scenario. Next, the authors have proposed sender initiated void avoidance routing protocols named two-hop adaptive hop by hop vector-based forwarding (2 hop-AHH-VBF) and quality forwarding adaptive hop by hop vector-based forwarding (QF-AHH-VBF) in [21; 22]. These protocols two hop neighborhood information to forward data packets towards the sink(s). The authors compute the composite function involving the energy and distance information of the potential forwarders in the virtual vector. In addition to that, the authors have also used the holding time to avoid duplicate data packet transmission in the virtual pipeline proximity. These schemes also face the same set of problems and face the same shortcomings as in [20]. For this purpose, minimization of void hole probability particularly in local sparse regions is focused on in this paper. The two-hop adaptive hop by hop vector-based forwarding (2 hop-AHH-VBF) protocol aims to avoid the void hole with the help of two-hop neighbor node information. The other protocol, quality forwarding adaptive hop by hop vector-based forwarding (QF-AHH-VBF), selects an optimal forwarder based on the composite priority function. QF-AHH-VBF improves network good-put because of optimal forwarder selection. QF-AHH-VBF aims to reduce void hole probability by optimally selecting next hop forwarders. Following is the discussion about the location based routing protocols for underwater networks that do not consider any holding time. The concept of directional power adaptiveness to overcome the packet flooding in underwater networks is also proposed in the Focused Bream Routing (FBR) [23]. Power and flooding angle are gradually increased according to the predefined gradients before forwarding data packets. A node requires to send many Request to Send (RTS) packets and waits for the Clear to Send (CTS) packets from neighbors in the beam direction. The nodes also share their location information in the RTS-CTS exchange. In order to confine the flooding direction, FBR selects potential node(s) in the beam direction that are eligible to forward the data packets.The power level is controlled by the FBR based upon the suitable number of neighbors located during the RTS-CTS exchange between the source and its neighboring nodes. The main drawback of the FBR is that it has a very large delay and energy consumption because of the initial RTS-CTS packet exchange.