## 4 Next Forwarding Node Selection in UWSNs
### Void-Aware  
Communication void is one of the crucial problems in routing protocols. This problem occurs if the sender node does not find any neighbor node in its transmission range. Since UWSNs suffer from dynamic and sparse network topology, these topologies impose a low packet delivery ratio and further minimize throughput due to a lack of algorithms that handle the communication void problem. Moreover, the use of weak void avoidance algorithms entails high energy consumption. In TWSNs, void-handling algorithms are not efficient to use in harsh environments due to the specific characteristics of UWSNs. The following paragraphs discuss the localization routing protocols belonging to this subcategory in detail. Vector-based void avoidance (VBVA) is another enhancement of VBF and is considered the first void avoidance protocol that provides 3D flooding mechanisms to deal with the communication void [25]. If there is no void area, VBVA works the same as VBF. The main difference between VBF and VBVA is that VBVA provides two 3D flooding mechanisms to deal with communication void, called vector-shift and back-pressure. In the vector-shift mechanism, the forwarding vector has been shifted, and the next forwarding node has been selected in the boundary of the void area. If the void is convex, the vector-shift mechanism can forward the packet around the void towards the destination. On the other hand, if the void is concave, the vector-shift method cannot work efficiently. Therefore, VBVA uses the back-pressure method to forward the data packet back to another node that can apply the vector-shift mechanism. One of the significant advantages of VBVA is that it can handle the void area efficiently. However, similar to VBF, VBVA uses a location information metric in selecting the next forwarding nodes, while finding this information is too costly due to the inapplicability of GPS in an underwater environment. Moreover, the virtual pipeline is the key idea behind VBF and VBVA. Therefore, the performance in selecting the next forwarding node is directly affected by the radius of the virtual pipeline. Based on the cross-layer approach, a location-based scalable routing technique, named Focused Beam Routing (FBR), is presented [40]. The use of a different power level is the main way this protocol reduces energy consumption. In the network architecture, the sink is placed at the corner of the network. In order to select the next forwarding node, the sender node employs a cone from itself towards the sink and sends a request-to-send (RTS) message with power level (P1). The receiver node replies with a clear-to-send (CTS) message including its ID and location information if it is placed in the cone. The sender node waits a certain amount of time to receive the CTS messages from its neighbors. The sender node then selects the closest one as the best forwarding node. Otherwise, it increases the power level and sends a new RST message, and so on. If there are no nodes inside the cone after increasing the number of nodes to the largest number of power level, it means that the node is placed in the void area. Therefore, FBR uses shifting cone to left or right to handle the communication void. The use of a cone can be considered the main advantage of FBR as it reduces the number of nodes that are involved in the forwarding process. However, the use of location information is the main disadvantage due to the lack of GPS in an underwater environment. Moreover, the method of handling the communication void requires increasing the power level to the maximum, which causes high energy consumption. An Efficient Directional Flooding-Based Routing (DFR) Protocol addressed the issues of VBF and HH-VBF despite their advantages regarding high packet loss [41]. An Efficient Directional Flooding-Based Routing Protocol (DFR) is a location-based protocol. In terms of architecture, the sink and source node are placed on the left end and right end of the area, respectively; also, the ordinary nodes are scattered at the bottom of the water.