## 4 Next Forwarding Node Selection in UWSNs
### Energy Efficiency
The neighboring node will reply and forward the packet if its depth is less than the depth listed in the control message, as shown in Figure 11. This reply message consists of its weight (depth information and residual energy of the node) and ID using the equation \(F(ID)=Er/dr\), where \(dr\) is the depth of the current node and \(Er\) signifies the residual energy; otherwise, it will discard the message. After that, the largest weight (\(F(ID)\)) node will be selected as the next hop by each node when the holding time is over and stored in the routing table. In the sending packets phase, sending the packet to the sink requires gathering information about the next hop node from the routing table and choosing the best one. Then, the data packet is transmitted to the selected node directly to avoid communication overhead. Figure 10: Packet forwarding process in DBR. The advantages of DBMR are highlighted as follows. The employment of multi-sink helps reduce the energy consumption for nodes placed closer to the sink. Moreover, the network lifetime has been improved because of the use of single-hop, next forwarding nodes selection. Furthermore, the use of residual energy during weight calculation has a direct impact on balancing the energy consumption. However, the key drawbacks of DBMR are: first, due to the lack of an efficient retransmission mechanism in DBMR, selecting only one next hop entails high energy consumption and end-to-end delay. Second, calculating the weight in DBMR is not efficient because it uses only residual energy metrics without taking into account link quality metrics, which promote packet loss. Third, DBMR suffers from the lack of a communication void avoidance algorithm. Last, the discovery phase should be updated with short intervals, leading to an increase in the network overhead. Energy-Efficient Localization-Free Routing (EEDBR) Protocol is an enhancement of DBR aimed at improving the energy efficiency in UWSN [48]. Because of the replacement of the batteries, energy is one of the key issues in designing a routing protocol. For this purpose, an energy-efficient routing protocol has been proposed, called Energy-Efficient Localization-Free Routing Protocol (EEDBR). In terms of forwarding data packets, EEDBR utilizes the depth of the sensor nodes. Moreover, it takes into account the residual energy of the sensor nodes to improve network lifetime and reduce the energy consumption. EEDBR is divided into two phases, namely knowledge acquisition and data forwarding. During the knowledge acquisition phase the next forwarding node is selected. To this end, each node broadcasts a hello packet including its local depth information and residual energy to its neighbor nodes. Only the nodes that have less depth than the sender nodes store the data in their tables, sorted based on residual energy then depth information. In the data forwarding phase, the sender node broadcasts the packet, including the nodes' ID, to its neighbors. Each node that receives the packets extracts the ID. If the node matches its ID with the ID embedded in the packets, it will calculate the holding time based on residual energy using the following equation:  
\[T=\left(1-\left(\frac{current\ energy}{initial\ energy}\right)\right)\times \max\_holding\_time+p, \tag{1}\]  
where \(\max\_holding\_time\) refers to the maximum holding time and \(p\) is the priority value. Then, it forwards the data packets after a certain holding time. Otherwise, it discards the packet. If the node overhears the packets, it generates a random number and compares it with the delivery ratio; if this number is less than the delivery ratio, it will drop the packets; otherwise, it will forward the data packets after the holding time finishes. On the other hand, if the node does not overhear the packets, it will forward the data packets after its holding time finishes. Figure 11: Route discovery phase in DBMR. The main advantage of EEDBR is that it employs multi-sink, which has a direct impact on balancing the energy between the nodes that are closer to the sink.