# Void Avoiding Opportunistic Routing Protocols for Underwater Wireless Sensor Networks: A Survey
## 2 Routing Protocols
### Void Area Problem in UWSNs  
Sensor nodes drift at different depths in the commonly deployed three-dimensional UWSN architecture to make it easier to identify or monitor a certain phenomenon. Multi-hop routing protocols depend on the relay nodes with positive advancement to transmit the data collected from the phenomenon from the source node to reach the target location on the water surface (sink(s)), as illustrated in Figure 1. In this figure, data can be transmitted through the path with dotted arrows to reach the middle sink or through the path with solid arrows to reach the sink on the right side. One of the very critical issues that face data transmission, particularly with UWSNs, is known as the void area problem as it appears in Figure 1. The data will be stuck in the relay node in the dotted path since the upper hemisphere area of the relay node is empty and there is no other node closer to the sink than this relay node. Numerous researchers have recently become more interested in this issue; however, much more research is required before it can be fully addressed. In this paper, we will follow the same UWSN routing protocol classification, location based and location-free based categories, as presented in previous works [15; 25; 29; 31; 37] in order to give a clear understanding of void area characterization. In location-based routing protocols, the detected data is transmitted from the source through the relay nodes with a shorter Euclidean distance to the destination on the sea surface. The upward region of the node's sphere is referred to as the void area if, during this procedure, any node that is holding sensed information could not locate a relay node in its communication range with a shorter distance to the destination to transmit the information to it. In this case, the node is known as a void node. In the location-free based routing protocols, the sensed data is forwarded by sensor nodes using their depths until it is delivered to the sea surface. The sensed data is transmitted from the source through low-depth relay nodes to the surface. In this category of routing protocols, a node holding information is referred to as a void node and the space above it as a void region if it is unable to connect with a node that is in its transmission range but has a lesser depth than itself. Therefore, a communication void or void area between underwater nodes exists when the area has an absence of nodes. The void area is one of the essential problems to investigate in the UWSN field. It can prevent communication between two or more network sensor nodes, which in turn can lead to a topological partition that results in decreased network connectivity and increased packet loss, which lowers the performance of the entire network. Through our research and review of the literature, we have come to conclude that any network architecture can experience the void area phenomenon due to one or more of the following causes [36]:  
1. Sparse topology deployment: Because underwater sensor nodes are expensive, it may not be possible to deploy enough of them to cover the area of interest. These sparse networks are prone to empty areas. 2. Underwater sensors failure: Owing to the harshness and peculiarities of the underwater environment, it is more likely that the sensors would malfunction due to corrosion and fouling, which could result in a void area issue. Figure 1: Void area in UWSN architecture. 3 Movement of the underwater sensor nodes: Water currents cause the underwater sensor nodes to move in both horizontal and vertical directions. Both node placements and the network topology will change because of this relocation, and a void area might be created. 4. Temporary obstacles: Many living organisms are found in the underwater environment. The movement of organisms could interfere with the underwater sensor nodes' ability to communicate. In addition, ships, boats, and other water-surface machinery might block the communication link between the network devices. As a result, void areas could be created. 5.