# An Efficient Void Aware Framework for Enabling Internet of Underwater Things
## 2 Related Works  
In order to have an efficient opportunistic routing protocol, it is paramount to solve many challenges among which is Communication Void (CV) [16; 17]. This challenge is faced by the sending nodes when no neighbor node is within its transmission range, which impedes the node from forwarding the packet to the next-hop or destination [25]. Given the dynamic, sparse, and unreliable network topology inherited in Underwater Wireless Sensor Networks (UWSNs), these topologies suffer from high packet loss and low throughput especially if inefficient algorithms are used to handle the void communication problem [25; 26]. Needless to say that the adoption of an inefficient void handling algorithm may negatively affect the nodes' energy consumption and reduce the network lifetime. CV is also a common problem in Territorial Wireless Sensor Networks (TWSNs) [18; 27]. However, utilizing the same CV algorithms to UWSNs is not feasible due to the harsh characteristic of these networks. Furthermore, utilizing location-based algorithms for handling CV problems such as Vector-Based Void Avoidance (VBVA) [28], Directional Flooding Routing (DFR) [29], Focused Beam Routing (FBR) [30] are not suitable for UWSNs due to their reliance on acquiring the nodes' locations utilizing a GPS receiver, which is not feasible in UWSNs [14]. On the other hand, Multi-layer Routing (MRP) [31] protocol proposed a costly architecture that utilizes a super-node to address the CV problem. However, MRP focused on proposing a network architecture but not an efficient routing protocol thus making it not feasible to hand the CV problem [14]. Another algorithm to handle the CV is the Adaptive Power Controlled Routing protocol (APCR) [32]. APCR is an energy-efficient routing protocol that does not mandate any location information of the surrounding nodes. APCR addresses the CV problem by allowing the nodes to adjust their transmission energy in case of not finding a suitable neighbor node thus overcoming the CV problem and achieving a high packets delivery ratio. In contrast, several pressure-based algorithms have been proposed in the literature to handle the CV problem. For example, HydroCast which establishes a detour path between local maximum void nodes with the maximum Expected Packet Advance (EPA) [33]. However, all local maximum neighbor nodes should be aware of each other location information, which is a cost and inefficient process, to forward the packets from each non-neighbor local maximum node. Void-Aware Pressure Routing (VAPR) protocol proposed in [23] utilizes both the depth and hop-count information to assign UP-DOWN directions to the sink node using the sink node beacon messages. In this protocol, the void node is recognized if the beacon message is received from the deeper node. As such, these nodes are marked as DOWN to prevent them from being part of the packets forwarding process. However, VAPR is not efficient as it relies on the high-cost beacon messages for identifying voice nodes [34]. The Adaptive Mobility of Courier Nodes in Threshold-Optimized DBR Protocol (AMCTD) proposed mobile nodes that can gather packets from void nodes. However, AMCTD neither proposed an algorithm for identifying the void nodes nor demonstrated an efficient algorithm for controlling the movement of the mobile nodes. Finally, the Inherently Void Avoidance Routing (IVAR) [24] tackled the CV problem by allowing all nodes to acquire reachability information throughout the received periodic beacon signals sent by the sink node. In [35], the authors presented an Opportunistic Void Avoidance Routing (OVAR), a new routing algorithm that addresses the void issue without geo-location information. OVAR can circumvent all types of empty regions at low energy and delay while prioritizing the group of candidate sets. Each forwarding node can conduct a trade-off between energy consumption and packet advancement by altering the number of nodes in its forwarding set, depending on the density of its neighbors.