## 3 Related Work
The first scheme adaptively adjusted the transmission range of nodes for void avoidance. The second scheme used clusters of nodes to avoid the long paths for data transmission. The third protocol used backward transmission to establish data paths at the minimal expense of energy. The fourth scheme combined backward transmission in combination with the selection of forwarder nodes with minimum neighbors so that collision could be avoided. However, in all the schemes, packets were routed over a single path, which compromised reliability. In addition, enhancing the transmission range of nodes in depth based routing (DBR) further leads to unbalanced energy consumption. This is because low depth nodes already have high data traffic in DBR, and nodes forward data in a greedy fashion, which enhances the redundant packets transmission and, therefore, contributes to more energy expenditure. The authors in [15] proposed a stateless protocol that advanced packets to the sea surface in an opportunistic manner, i.e., a group of certain nodes, chosen under certain criterion, participated in information forwarding. When a void-hole existed, it was identified, and all the paths that led to such a hole were discarded for packets' advancements. This strategy saved energy consumption and improved the number of advanced packets to the end destination. However, identification of the paths with void hole inserted extra delay. In addition, for sparse network conditions, multiple paths may not be available, which in effect, challenges the theme of this protocol. Moreover, due to single link routing, the harshness of the sea channel may affect the reliable delivery of packets. An opportunistic routing that incorporated void avoidance was proposed in [16], in which a group of specific nodes participated in data advancement. At each node, all the routable paths were identified so that no void was left in the network. The reliability was addressed by the cooperation of relay nodes with source and destination nodes. However, the construction of routing graphs at every node constantly needed beacon signals, which added delay in packets' transfer to the sea surface. Geographic information of nodes was utilized to construct two routing mechanisms in [17]. The first utilized finding backward paths from a void-hole to transfer information to the sea surface. The second made use of forwarder nodes with the minimum count of neighbors so that collision did not occur in information transfer. However, the requirement of the geographic information of nodes increased the computational complexity of both schemes. In [11], the authors devised a variable transmission range of every node in a situation when a node is neighborless. In this case, it increases its transmission range, up to a limit, so that neighbors could be found. Moreover, the uniqueness of the time for which packets are held by nodes avoids transmission of multiple packets at the same instant of time so as to reduce interference. Furthermore, the condition of the geographical localization of nodes was relaxed, which made the network flexible in terms of scalability. However, reliability was compromised because information was advanced by every node over a single link. The proposed algorithms are unique in that the construction of the routing trajectories, cooperative routing, backward transmission mechanism, and two hop neighbors selection for void-hole avoidance did not involve the physical distance from the sea surface or the geographical coordinates of the nodes. This is because calculation of the hop length and mutual distances of nodes based on geographical coordinates or distance from the sea surface complicates the process of route determination as tides and current in the sea change nodes' positions. Moreover, the uniqueness of the time for which packets are held by nodes ensures that information packets intelligently reach the end target. That is, low power nodes hold the packets for a short time to avoid losing packets.