## 2 Literature Review
### Energy-Based Routing  
Energy is the soul of routing, whether it is a terrestrial or an underwater sensor network. However, here we discuss underwater routing, which requires special attention,because in the underwater sensors, the network battery is a source of energy that is placed inside the node, and we cannot easily recharge or replace it during the transmission of data. Additionally, some other factors, such as a low bandwidth of acoustic signal, more power consumption for transmission, and an increased delay rate are also a huge hurdle for UWSNs. The failure of the sensors node battery leads us to serious threats, such as the failure of the transmission of packets, or in some cases, the failure of the network. Thus, in this section, we discuss some of the pertinent protocols of UWSNs, including a focus on energy-based routing and its maintenance, to increase the life time of the networks and to avoid the creation of energy holes.By focusing on the energy of the network, some other parameters of the network, such as the packet delivery ratio and end-to-end delay are also affected; that is, decreased or increased, which will be discussed at each routing protocol, accordingly. The protocols that are in line with the energy-based routing are the following below. The authors in [22] proposed the DRP protocol, which they considered the probability of the collision of the packets between the sender and receiver nodes and its remaining energy. The collision probability depends upon the distance between the source and destination node. If the distance is increased, it means that the collision of packets is also increased, as more nodes will come in between the source and the destination node, and so, with this DRP route, data travel on the path where the distance between the source and destination nodes is a minimum, and less energy is required for transmission. DRP tries to find out a path for the transmission of packets that have a low collision probability and more residual energy, in order to maintain the network alive. Due to this, the energy distribution of each sensor node almost remains optimized, and the throughputs of the packets are increased. Similarly, an another routing protocol [23], SPR, also works the communication path; that is, it selects the shortest path for the transmission of the packets from the sensor node to the sink node, in such a way as to maintain the network alive and enhance its life time. Another novel protocol, EAVARP [24] is proposed, where the authors discuss two phases, the layering phase and the data collection phase. In the layering phase, the distances between the sink node and the source node are divided into layers, which comprise a circular shell around the sink node, with each layer separated by a fixed distance from another layer. The sensors nodes are randomly deployed around the sink nodes so the shell around the sink nodes covers many source nodes. In the second phase, the nodes, based on their location at the layer of the sink node, direct their data towards the sink node. EAVARP has taken into account the data transmission and the remaining energy of all of the nodes present in a shell, to avoid redundant transmission and a void energy hole. These phases are taken into account before the transmission of data, and their main purpose is to give direction to the routing towards the sink node. Thus, due to this shell approach, a limited number of nodes having more energy and that are nearer to the sink node will be involved in the communication process, optimizing the life time of the network. As we have discussed earlier, the unequal distribution of the energy of the source nodes create holes for the transmission of packets, which is the biggest challenge for UWSNs. This is because some of the nodes are involved in the communication process, while some are not. As with UWSNs, the sensor nodes with limited energy sources are deployed at different places, to monitor the environment and to route different data packets to the sink nodes.