# A Survey on Underwater Acoustic Sensor Network Routing Protocols
## 4 The Non-Cross Layer Design Method
### The Mobility  
In this section, the mobility of underwater sensor nodes caused by water currents is taken into consideration by the routing protocols. Due to the continuous movement of the sensor nodes, the routing is easily broken, which will need extra energy and processing time to recover or rebuild the routing, so the routing protocols which take the node mobility into consideration also have good energy consumption and propagation delay performance. Since the node mobility affects the link quality, some mobility aware routing algorithms take the link quality into account to improve the network performance. In [50], considering the unreliability of acoustic channels, the authors propose HydroCast. HydroCast is similar with DBR, the difference beingthat HydroCast take the wireless channel quality into consideration to improve the routing performance under continuous node movement conditions. The routing algorithm introduced in [51] also takes the link quality into account to increase the reliability of the communication links. By defining the flooding zone, the routing protocol can adapt to the dynamic changing of the network conditions and topology, see Figure 4. However, the disadvantages in these two algorithms are obvious: (1) in HydroCast, the authors do not consider the energy consumption of each node; (2) in [51], the nodes need to know their own location information and that of the destination node, which is not always available in underwater sensor networks. Figure 4: The principle of DFR. Considering the frequent routing maintenance and discovery in mobile UASNs, the authors in [52] propose a vector-based forwarding (VBF) routing protocol. In VBF, a virtual routing pipe is used to forward data packets to destination nodes. The nodes located in the pipe can take part in the data transmission. The routes from the source node to the destination node are redundant and separate, which can make the routing protocol robust to node movement. However, in VBF, the nodes near the pipe will be used again and again, which is not energy balanced. To overcome this problem, in [53], the authors propose a hop-by-hop vector-based forwarding protocol (HH-VBF which has better performance than VBF. In HH-VBF, each node can decide the pipe direction based on the current node location. However, due to the hop-by-hop transmission method, the overheard is heavier than in VBF; moreover, the performance of HH-VBF is sensitive to the pipe radius threshold. The interesting conclusion in [54] is that the authors regard the continuous node movement as having a good effect on energy balance. Based on this conclusion, the reliable and energy balanced routing (REBAR) algorithm is proposed. In REBAR, the nodes have different transmission ranges according to their distances to the sink node. The nodes near the sink node have small transmission ranges, and _vice versa_. The algorithm can reduce the energy consumption of nodes near the sink node. However, in fact, node movement can cause more problems than advantages. Void aware pressure routing (VAPR) [55] is also a robust routing protocol for dynamic topology networks, in which the node uses sequence number, hop count, and depth information to set up the next hop relay node. In this section, three different kinds of mobility-aware routing protocols are reviewed. The work in [50,51] considers the link quality in the routing protocol design. As for the node movement, the work in [51] can achieve better performance than HydroCast. However, in HydroCast, the authors do not consider the energy consumption of each node; in [51], the nodes need know their own location information and that of the destination nodes, which is not always available in underwater sensor networks. VBF and HH-VBF are routing protocols which can successfully reduce the routing rediscovery and maintenance. The HH-VBF has better performance on the energy consumption balance than VBF, but the overheard of HH-VBF is heavy.