# A Survey on Underwater Acoustic Sensor Network Routing Protocols
## 6 Open Issues and Challenges  
Even through terrestrial wireless sensor networks have been studied for a long time and used in a wide range of applications, considering the differences between the terrestrial environment and the underwater environment, terrestrial wireless sensor network solutions cannot be directly used in underwater environments. The main challenges for the underwater acoustic sensor network routing protocol design are as follows. * The attenuation and absorption of acoustic wave in underwater environment is more serious than those of RF waves in terrestrial environments, which means that more transmission energy will be needed in an underwater environment, especially considering that the underwater sensor nodes are energy limited. * The propagation delay, the bandwidth, the link quality, and the bit error rate in underwater acoustic channels are worse than those of terrestrial wireless channels. * Due to the node movement and failure, the topology of underwater acoustic sensor networks changes frequently. Moreover, the architecture of underwater acoustic sensor networks is 3-dimensional, which is different from terrestrial wireless sensor networks. * Since underwater GPS devices and underwater location algorithms are expensive and complex, the location information of the underwater sensor nodes is hard to get. * The devices used in underwater acoustic sensor networks are much more complex and expensive than in terrestrial WSNs, because the devices used in underwater environments need to consider the waterproofness and the corrosion resistance under water. Since there are so many research challenges in routing protocol design for underwater acoustic sensor networks, there are also a large number of open issues that need to be investigated:  
1. The propagation delay model. Due to the fact the propagation delay in underwater acoustic sensor networks is serious, how to calculate the propagation delay and build propagation models is still an unresolved issue. 2. The energy consumption model. The energy in underwater sensor nodes is strictly limited, therefore how to reduce the energy consumption of underwater acoustic sensor networks is always the most important research area. However, there is currently no accurate and reliable energy consumption model for underwater sensor nodes and networks. 3. The movement model. Due to the water currents, underwater sensor nodes move continuously. Even though there are already a lot of mobility models for terrestrial sensor nodes, the special network structure and hydromechanics make the movement of underwater sensor nodes totally different from that of terrestrial sensor nodes. An appropriate motion model is needed for underwater sensor networks. 4. High efficiency and reliable communication. In underwater acoustic channels, the bandwidth, the link quality, and the bit error rate are all worse than those of terrestrial RF channels, so how to improve the efficiency and the reliability of underwater communication channels also deserves to be investigated. 5. The utilization of intelligent algorithms in the underwater environment. This is a new research area for underwater acoustic sensor work. Since there are only a few intelligent algorithms that have been used in UASN routing protocols, therefore, how to use the intelligent algorithms to solve the issues that exist in underwater acoustic sensor networks has been a hot open issue in recent years. 6. The location information acquisition. In underwater acoustic sensor networks, the location information is useful in routing discovery, however, the necessary GPS devices and location algorithms are expensive and complex. Therefore, how to get the location information of underwater sensor nodes easily and effectively is an important open issue.