# A Novel Cross-Layer Routing Protocol Based on Network Coding for Underwater Sensor Networks
## 2 Related Works  
In this section, we review some of the typical routing protocols in UWSNs. To clarify the advantages and disadvantages of different routing protocols clearly, we classify current routing protocols into sender-side, receiver-side and hybrid protocols based on candidate set selection nodes. In sender-side candidate set selection procedures, the selection of next-hop nodes is decided by senders. Generally, destination nodes need to broadcast beacon signals periodically to ensure that every sending node in network can acquire the information of its neighbor nodes. Based on the neighbor information stored in sending nodes, a cluster of candidate nodes are chosen and sorted according to their priorities. The header of data packets should include the unique IDs of the chosen nodes. Once a node receives a data packet, it checks if it is in the chosen candidate set. Nodes in the chosen set then set a timer to forward the packet to the destination. Those nodes with higher priorities will send packets at an early time and therefore suppress the sending procedure of other nodes with lower priorities. Some typical underwater routing protocols in this category are hydraulic pressure based anycast routing (HydroCast) [17], void-aware pressure routing (VAPR) [18], geographic and opportunistic routing (GEDAR) [13] and pressure sensor based reliable routing (PSBR) [7]. In HydroCast, each node maintains a list of Expected Packet Advance (EPA) of its neighbors. When a node determines to send packets, it performs the opportunistic routing paradigm and the next-hop node priority is set according to EPA. A dead end recovery method is also proposed to search for new paths when there exists void areas. VAPR is similar to HydroCast but uses sequence number, hop count and depth information embedded in periodic beacons to set up next-hop direction and to build a directional trail to the closest sonobuoy. GEDAR improves VAPR by performing the depth adjustment of the void nodes. When the channel senses a void area, GEDAR can move void nodes to new depths to resume the geographic routing. PSBR adopts link quality estimator along with depth distance and residual energy to select a single reliable node for data forwarding. Protocols that fall into this category can choose a subset of candidate forwarding node in advance and therefore more complex and comprehensive fitness functions can be used to choose next-hop nodes. However, due to the node mobility in underwater environment, sending nodes need to keep updating neighborhood information frequently by using beacon messages, which results in high packet collisions and low channel utilization rate. For receiver-side candidate set selection procedures, the selection of next-hop forwarding nodes is processed by data packets receivers. Sending nodes do not store any information of their neighborhood nodes. At the beginning of sending procedure, sending nodes add some control information (e.g., its depth or position) into packet header. All neighborhood nodes determine locally whether they are responsible for forwarding the packets and set their priorities based on some principles (e.g., residual energy or distance to the sender) after removing the packet header. If a node determines to forward the packet to the destinations, it sets a timer before sending the packet according to its priority and the sending procedure will be cancelled if a duplicated packet is received in this period of time. Protocols that fall into this category are vector-based forwarding protocol (VBF) [11] and its improved version hop-by-hop vector-based forwarding (HHVBF) [12], depth-based routing (DBR) [10], weighting depth and forwarding area division DBR routing (WDFAD-DBR) [19] and so on. VBF uses the 3D geographic position of nodes to build a virtual pipe from the source node to the destination node. The nodes in the network have higher priorities to forward data packets if they are closer to this pipe and the destination node. HHVBF improves VBF by relaxing the restriction of the single pipe line.