## 2 Literature Review
### Depth-Based Routing  
Depth-based routing protocols are protocols that perform their routing according the depth of the sensors nodes from the sink nodes. A sensor node can measure its depth from the sink node through a depth sensor equipped inside every underwater sensor node. The protocol that follows depth base routing strategy is discussed as follows. As in every UWSN, the idea is to forward the data packet from the sensor nodes to the sink nodes without loss, and with a low energy consumption. The same phenomena are also discussed in all routing protocols, such as VBF, HH-VBF, AHH-VBF, etc. Here, also in Depth-Based Routing DBR [16], the node routes the packet to the sink nodes according to the depth difference with other nodes. Consider a transmission scenario (as sensor nodes transmit packets in all directions; that is, flooding the packet) where a sensor node receives a packet from another node. This receiver node compares its depth with the sender node depth, as the depth of every node is embedded in the forwarded packet through the depth sensor. If the forwarded packet (sanded by another node) contains greater depth than the current receiving node, it means that the current receiving node is closer to the water surface or the sink node than the other forwarded node. Thus, in this situation, the receiver node will take the packet and forward it to the next forwarder while embedding its depth in the packet. On the other hand, if the forwarded packet contains less depth than the current receiving node, it means that the current receiving node is far away from the surface or from the sink nodes as compared to the current forwarded node. Thus, in this situation, the receiver node will simply discard the packet. DBR uses a holding time calculation for scheduling the best next forwarder node to forward the packet. As also mention earlier in DBR, when a node receives a packet, it holds the packet for some time to take some suitable steps; that is, the node calculates its position from the received packet through depth differences, and also calculate its holding time; that is, how much time the node is required to hold the received packet. Due to the water currents, the sensor nodes inside the water are in constant motion, meaning that the nodes will be present in different positions. In this way, their holding times will also be different. Thus, the nodes that are closer to the surface nodes or the sink nodeshaving minimum depth also have a minimal holding time. In this way, the nodes present near to the surface or the sink nodes are the best forwarded compared to the others nodes. This also prevents the other nodes present at the same local regions from sending the same packet, which effectively reduces the energy consumption and improves packet transmission. In [17], the time-critical routing schemes DSDBR, DSEEDBR, and DSAMCTD are proposed, which are mostly focused on how to reduce the end-to-end delay. Due to the harsh underwater environment and the low acoustic communication, the propagation of an acoustic signal in a UWSN is five times lower than the radio signal on the terrestrial sensors network, which affects the performance of the whole network, especially for time bounded applications. In DBR, the selection of the next forwarder node is performed through a depth sensor equipped at each sensor node and through a holding time calculation that effectively increases the end-to-end delay of the network. Thus, the proposed schemes DSDBR, DSEEDBR, and DSAMCTD work on this through an active signal, delay sensitive holding time, and weight function, to minimize the end-to-end delay. Working on one factor that is end-to-end delay eventually affects the others. Here also, a quicker transmission of the packet affects the routing energy of the network. Another novel protocol, Weighting Depth and Forwarding Area Division Depth-Based Routing (WDFAD-DBR) [18] is proposed, which considers the void hole creation for next forwarder node, which affects the performance of the whole network.