## 4 Next Forwarding Node Selection in UWSNs
### Taxonomy of Next Forwarding Node Selection in UWSNs
#### 4.2.1 Localization Routing Protocols
The major advantage of VBF is that the energy consumption and network traffic are significantly decreased because the number of candidate nodes is decreased. However, VBF is characterized by some disadvantages; VBF uses a location information metric in selecting the next forwarding nodes, which is too costly due to the inapplicability of GPS in an underwater environment. Moreover, the virtual pipeline is the key idea of VBF. Therefore, the performance in selecting the next forwarding nodes is directly affected by the radius of the virtual pipeline. Furthermore, VBF suffers from a lack of communication void algorithm that occurs during the selection of the next forwarding nodes. The Hop-by-Hop Vector-Based Forwarding (HH-VBF) Routing Protocol enhances VBF by designing an adaptive location-based routing protocol named hop-by-hop vector-based forwarding (HH-VBF) [31]. Different from VBF, this protocol employs a virtual pipeline from the sender node towards the destination/sink with dynamically changeable pipeline based on nodes' position, which increases the efficiency of selecting the next forwarding node. Figure 3 illustrates an example of how nodes A, B, and C create their virtual pipeline. As shown in this figure, each source node creates its pipeline towards the sink individually to forward the data packet. Similar to VBF, the energy consumption is significantly decreased, and the delivery ratio is increased. Moreover, the use of location information is the main disadvantage of HH-VBF due to a lack  
Figure 2: Single pipeline in VBF. of GPS in an underwater environment. Moreover, it did not come up with any solution to handle the communication void problem. Furthermore, this protocol did not utilize any energy-efficient or reliable metrics that help in balancing the energy consumption between nodes and further improve the network lifetime. Energy Efficiency  
Selecting the most energy-efficient next forwarding nodes is a major problem in routing protocols and has a direct impact on overall routing performance indicators such as energy consumption, packet delivery ratio, and network lifetime. A localization routing protocol provides virtual shapes using location information from the sender node to sink. This kind of solution is not energy-efficient due to the cost of using localization and the lack of GPS. Moreover, most of the existing localization routing protocols select the best forwarding node based on its distance to the sink. Specifically, VBF, HH-VBF, VBVA, and FBR employ only a distance parameter. SEANAR and PER use a distance parameter with a residual energy metric, whereas DFR utilizes a distance parameter along with link quality. The following paragraphs discuss the localization routing protocols belonging to this subcategory in detail. Power-Efficient Routing (PER) Protocol is another location-based routing protocol that has been proposed by [PERSON] et al. [43]; this protocol tackles the energy consumption problem in underwater environments. The architecture of UWSN is such that the sink node is located in the center, the ordinary nodes are scattered randomly in the water, and the source nodes are deployed at the bottom, as shown in Figure 4. PER is composed of two modules: a forwarder node selector and a forwarding tree-trimming mechanism. The fuzzy logic technique has been employed in the first module to select two next-hop nodes. Three parameters have been utilized: residual energy, distance, and the angle between two neighbors. Based on defuzzifier, two forwarding nodes have been selected. The second model has been applied if the number of duplicate packets is bigger than a predefined threshold. Otherwise, the data packet is sent to the two selected next-hop nodes. The main reason why PER has better performance than VBF is that it uses two hop nodes during the forwarding process, whereas VBF employs a flooding technique in its virtual shape.