# Void Avoidance Opportunistic Routing Protocol for Underwater Wireless Sensor Networks
## 3 EEDOR Void Avoidance with Hop Count Discovery Mechanism
### Description of the Proposed EEDOR-VA Protocol  
In this paper, we extend our previous EEDOR protocol and enhance the network performance by addressing the void area problem.  
Our technique to address the void area problem consists of bypassing the void and trapped nodes. EEDOR-VA makes a routing decision according to the nodes' reachability to the surface sink(s). Our novel hop count discovery process was inspired by the source route discovery process used in DSR. In EEDOR-VA, Hop Count Request (HCREQ) and Hop Count Reply (HCREP) messages are used to update the node's hop count to the reachable closest sink. The void and trapped nodes in the transmission range of a source and/or relay nodes do not reply to the HCREQ message and are excluded from being one of the forwarding candidates, so the data packet is not trapped in these nodes. Therefore, each \(\text{P}_{\text{holder}}\) can form its forwarding set easily. Since the presence of the void area can prevent communication between some of the network nodes, which reduces the routing protocols performance, our goal is to increase the routing performance through developing a routing protocol that gains a high packet delivery ratio with less energy consumption by selecting the shortest routing path. To complete a successful transmission, each packet must reach the next forwarder successfully at each hop and one of the sinks at the end of the forwarding procedure. The information received from the hop count discovery algorithm is used to assist the sensor nodes to update their hop count from the sink(s) and exclude the  
Figure 3: Impact of noise level on (**a**) \(SNR\), (**b**) probability of packet delivery.  
void and trapped nodes in the Pholder nodes' transmission range. The hop count discovery mechanism in EEDOR-VA eliminates periodic beaconing and its associated costs.  
The main idea of EEDOR-VA is to determine multiple loop-free paths between a source node and a single sink or multiple sinks on the sea surface. This is achieved through a hop count discovery procedure inspired by the DSR algorithm to update the nodes' hop-count. These multiple routes make it easy for the protocol to modify the chosen route from one path to another through electing the next relay nodes from a different path if this relay node is the best choice in the next hop forwarding set. Thus, this procedure avoids starting a new hop count discovery process. Initiating a hop count discovery happens only when all routes to all sinks fail. Our proposal uses the route information to update relay node information and guarantees that nodes responding to the Pholder have a path to one of the sinks to bypass the void nodes.  
The operation of the EEDOR-VA protocol is illustrated in Figure 2. When a source node has a packet to send, it first sends HCREQ and all its neighbors n1, n2, n3 and n4 receive it. Each of these neighbors rebroadcasts the request to its neighbors. When sinks s2 and s3 receive the request message, each sink generates an HCREP and sends it downwards. Consider the reply from s2. s2's neighbors, n8 and n16, update their hop count and resend the HCREP to their neighbors. Each neighbor will update the HCREP with its depth and hop count and resend the reply hop by hop until the source node gets the reply. More discussion about hop count discovery is included in the next subsection. It is important to mention that the size and content of both HCREQ and HCREP in EEDOR-VA are reduced from those used in DSR to save energy and to allow multiple routes to the sink(s).  
The EEDOR-VA protocol is divided into rounds; each round consists of three phases as given: a hop count discovery phase, a forwarding set formation phase and a data packet forwarding phase.