## 2 Literature Review
### Energy-Based Routing
Thus, another routing protocol, DB-EBH [25], based on direct and multi-hop communication of the network, proposed a solution for this, in order to maintain the network alive. In this protocol, priority is giving to a node according to its location, in which the best nodes are selected for forwarding communication, and localization information are taken from the depth sensors equipped at each sensors' nodes. Additionally, a randomly deployed network is monitored in such a way as to balance the energy consumption and enhance the network life time. The target of the UWSN protocols is to perform reliable communications in such a way to overcome the challenges of the network; that is, to minimize the delay, improve the throughput, increase the network life, and to perform efficient data routing from the source node to the sink node, and eventually to the base station, without losses. As all these factors are interdependent on one another, enhancing one parameter will affect another one. For this, an another novel protocol [26], PA-EPS, is proposed, which takes notice of all these parameters and comprehensively checks each of the factors and their effects on the network, to collectively improve the network performance. In this protocol, a proactive routing approach is proposed for the dense sensors network and the sparse sensors network. For the dense sensors network, PA-EPS proposed a layering approach, in which the transmitting region is divided into small and equal distance-separated layers, so that each sensor node performs routing to the next shortest layer node in order to minimize the energy required for routing and avoid the creation of energy holes. For a sparse network, a cluster approach is proposed, in which each sensors node knows its own location and the location of the all others nodes present in the same cluster, as many nodes are present in a cluster, avoiding the creation of void holes. As different numbers of nodes are deployed in a network, the required information is gathered and the network is scaled accordingly. The number of nodes affects the scalability of the network, and to analyze its performance, in PA-EPS, the number of nodes is increased or decreased to between 100 and 500, to improve their scalability. The routing strategies are changed adaptively according to the local node distribution, which effectively decreases the dropping ratio of the packets and the delay for reaching the sink nodes. As we know, there are many sensors nodes that are deployed for collecting information and for routing the received information/packets to the forwarder nodes or sink node. Due to this, many duplicate packets are generated, which affect the energy of the nodes and of the networks. Thus, a holding time strategy is equipped for this, in order to suppress duplicate packet creation; only the nodes that are most desirable and at optimal positions can send the packets. However, the unfairness of the holding time creates problems for routing; that is, it is creates more delay, which affects the performance of the routing, especially for the time-critical application protocol, and as a result, this affects the energy of the network. For this, another protocol [27, 28, 29, 30], ESEVBF, is proposed, which works on this in order to enhance the effectiveness of the routing. ESEVBF operates in the following manner. Firstly, the information on the energy of all the sensors nodes distributed in the local region are taken, and the holding times of the forwarder nodes are scaled accordingly. After this, the next forwarding nodes are selected, which signify the routing according to its distance from the source node to the sink node, and preferably, on the basis of the holding time computation. Secondly, as nodes are located at different places in the local region of the sensors nodes, so the sensor nodes take the information of all the neighboring nodes and abbreviate the holding time, to minimize the end-to-end delay for the next forwarding.