# Self-Organized Proactive Routing Protocol for Non-Uniformly Deployed Underwater Networks
## Related Work
### Location-Based Protocols  
There are other routing protocols for UWSNs which are based on the depth of the sensor nodes, like LB-AGR, VBF, and DBR. The packet forwarding mechanism in LB-AGR is based on the level-difference between the neighbor nodes, available power, node density, and location. The traffic is routed according to its type as well. The traffic type is mainly divided into two categories: upstream and downstream. The traffic from the sensor nodes to the sink through is called the upstream traffic, and the opposite way is downstream traffic. The downstream traffic is further divided into three categories. The first is the downstream traffic for a designated area, the second is the downstream for a specific node, and the third is downstream for all the sensor nodes. Vector-based forwarding (VBF) is a location-based protocol based on the concept of virtual pipes. Nodes are selected from the source to the sink, which can possibly forward the packets. These selected nodes form a shape like a pipe, hence it is called the \"virtual pipe\". To avoid the issues of packet losses and node failures, multiple nodes within the virtual pipe forward the data packets. One major drawback is that the isolated nodes may create voids in the low-density network. The power consumption is also high due to three-way handshaking mechanism. Hop-by-hop VBF (HH-VBF) [9] proposes a virtual pipe node that is nodes-based, rather than from source to destination, to overcome the voids issue in VBF. Since each node forms a new virtual path before forwarding the packet, the voids are eliminated. HH-VBF uses more control packets to form a virtual pipe for each forwarding node than VBF, so the average energy consumption is poor due to broadcasting, but the packet delivery ratio is better than LB-AGR. Depth-based routing (DBR) is based on greedy routing. The path to the gateway is chosen depending on the depth of the next hop. Each forwarding node sends its depth information to the next hop in order for all the nodes to know about the depth of their neighbor nodes. The node which has the lowest pressure, or in other words, is closest to the gateway, will be selected as the next forwarding node. In the energy-efficient depth-based routing (EEDBR) [10] protocol, the nodes share their depth and residual energy information with their neighbors. Those with high residual energy are most likely to forward the packet, and those with low energy will try to avoid forwarding. There is a threshold value for residual energy to choose the node with higher residual energy: if the node has a residual energy higher than this threshold value, it will forward the packet. HydroCast [11] is similar to DBR but it avoids forwarding the packets to such a node which is not closer to the destination than the current node using the NADV link metric [12]. First, the nodes with higher priority will forward the packet. If they fail, then the nodes with lower priority will try. To avoid collision and redundant packets, the nodes with the lower priority will refrain from transmitting the packet and will listen to the packet forwarding by the highest priority nodes. This is possible because they are within the transmission range of each other. Like DBR and HydroCast, void-aware pressure routing (VAPR) [13] also uses the depth information, hop count, and sequence number to find the next hop and path to the sink. Sink nodes initiate the configuration process by transmitting beacon packets. A node decides its forwarding direction after receiving the beacon message from the deeper node or shallower node. The forwarding direction is set towards the sea surface if the beacon is received from a shallower depth node and towards the sea bottom if received from a deeper node. This way, the voids can be avoided by a change in packet forwarding direction. The energy-efficient routing protocol (EUROP) [14] is another depth-based protocol which uses pressure sensors.