## 2 Literature Review
### Depth-Based Routing
Sink nodes are placed at the water surface; these take data from underwater sensor nodes and direct these data to a base station. However, in DOW-PR, there is one embedded sink inside the water and it is linked with surface sink through a high bandwidth connection. Thus, the nodes that are far away from the surface sink and close to the embedded sink can directly communicate with the embedded sink. In this way, the whole network energy will be in place while the packets reach the surface sink without any loss of energy of the other nodes. In comparison with WDFAD-DBR, the DOW-PR networks have a much lower energy consumption; more packets are delivered to the sink node, especially increasing the life time of the network. We know that, in UWSNs, a sensor node has a battery as an energy source for gathering information from its surroundings and transmitting it to the sink nodes located at the surface of the water. During the transmission, if the battery of a sensor node loses all of its energy, then this creates an energy hole that affects the whole transmission of the network. In some cases, this minimizes the life time of the whole network. In this regard [20], the AMCTD-DBR protocol is proposed in order to enhance the life time of the network. The transmission of a network is divided by the authors into different phases. Firstly, each node computes the local node information of the network, such as the node density and its movement. Secondly, they give priority to a node; that is, which node is an optimal node for the next forwarding, and take this information from the first phase. Lastly, taking into account the sparse local node distribution and the density of the network, and performing different variations in the depth threshold according to the sensor node movements, they decide which node is a suitable node for the next transmission. AMCTD-DBR thoroughly observed and worked on the above three phases so that a lesser amount of sensor nodes take part in the packet transmission. In this way, the network energy will remain optimized, which eventually leads to an enhanced network life time. To improve the performance of AMCTD, such as the holding time calculation, the path loss factor due to distant transmission, and the flooding of packet and energy limit, another novel protocol [21], iAMCTD-DBR, is proposed. The author for iAMCTD-DBR adopted the existing phases of AMCTD with the addition of some other phases, which are discussed here. Firstly, they implemented that the routing of data should be on demand; that is, sensors nodes will not only flood the existing data, but they will perform routing when it is in demand. This will facilitate the time-sensitive data application. Secondly, the routing of data varies according to the distribution of the sensor nodes of a network, to facilitate the routing in different node density networks, which leads to a minimization of the path loss factor. Additionally, in iAMCTD-DBR, they give authority to the sink node to identify the distribution of the sensors nodes that are sparsely and densely networked, for efficient routing on the basis of the control packet, to minimize the overhead of flooding on the network. Due to all the above factors--timing, sink-based, and on demanded routing, this leads to an efficient management of the energy consumption of the network. Additionally, in the next section, we discuss energy consumption and how to minimize it, in detail.