# Void Avoiding Opportunistic Routing Protocols for Underwater Wireless Sensor Networks: A Survey
## 2 Routing Protocols
### Opportunistic Routing  
At the beginning of this section, we will give the definitions of some terminologies that are used in this paper to make it simple and clear. Neighbor nodes/neighboring nodes of node (i): are a set of nodes, which are in the transmission range of the node (i). The qualified set: is a subset of the node's neighbor, which meet the rules adopted by the author(s) to design an efficient routing protocol. Next-hop forwarder of node (i): the nodes within the transmission range of the node (i) and at the same time have depths less than the depth of node (i). Beacon messages: Status messages that contain the status information of the nodes, which are used to exchange this information between the neighbors. Pholder: is the node carrying the data packet in the current round. It could be the source node, which originally generated the data pocket, or a relay node. Relay nodes (i): any node located between source node and sink(s) that carries on forwarding the data packet starting from the source hop by hop until it reaches the sink(s). OR is a promising technique that was proposed for overcoming acoustic signal fading, high bit errors and losses due to shadow zones, limited bandwidth, high power consumption, and signal spreading [38]. The main concept of OR is to use the broadcast nature of wireless networks, which allows multiple nodes to overhear the transmissions made by any in-range sensor node. Therefore, various underwater OR protocols have been suggested in order to enhance communication in underwater networks. In OR protocols, a subset of a node's neighbors will be selected as next-hop forwarder set candidates. These nodes collaborate in a coordinated manner to continue forwarding the packet along toward the destination (sink) by using a prioritized technique according to the rules implemented by the protocol [1; 14; 39]. The forwarding candidate set selection and the coordination manner between these forwarding candidates to deliver the packet are the two main parts of OR construction. This OR approach is preferable to the traditional multi-hop routing approach, in which only a single node is selected to act as a next-hop forwarder, to increase the probability of delivering the packet [1; 25; 30]. This can be illustrated through the following example:  
Let us assume that the delivery probability of each link (which presented by the arrow in Figure 2 is \(p\) and (\(0\!\!<\!p\leq 1\)). In the traditional multi-hop routing approach, the delivery probability, \(D_{\textit{Prob}_{e}}\) from the source node to the sink using \(h\) hops can be presented mathematically as. \[D_{\textit{Prob}}=p^{h} \tag{1}\]  
In contrast, if all the relay nodes can transmit the packet by using the OR approach, the probability of delivering the packet to the sink is increased, as explained in [40]. For OR with m possible relay nodes in each hop, as shown in Figure 2, we can express the \(D_{\textit{Prob}}\) mathematically as  
\[D_{\textit{Prob}}=\left(1-(1-p)^{m}\right)^{h} \tag{2}\]  
where \(h\) is the number of hops between the node that originally generated the packet and the final sink, and \(m\) is the number of relay nodes in each hop. Consider the following numerical example: assume \(p=0.8\), \(m=3\), and \(h=4\). By using Equation (1), the delivery probability is 0.4096 for the traditional routing, while by using Equation (2), we get a delivery probability of 0.9684 for the OR routing. Figure 2 below illustrates both routing protocols. Hence, by taking into account the advantage of the broadcast nature of the wireless transmission medium [41] and using the OR forwarding technique, it has become possible to mitigate the effects of the underwater environment and its characteristics on the acoustic communication channel and improve the efficiency of the underwater acoustic physical links [1; 25; 42]. That is, the OR technique has been proposed to enhance network performance by reducing high bit errors and losses caused by limited bandwidth, high power consumption, and signal spreading [38].