# A Comprehensive Survey of Recent Routing Protocols for Underwater Acoustic Sensor Networks
## 5 Future Work and Challenges  
Most of the routing protocols proposed for underwater acoustic sensor networks consider only static nodes and/or anchored nodes with limited mobility, while ignoring scenarios that feature free and unchecked mobility of freely floating untethered underwater sensor nodes. Unthered sensor nodes move freely with the underwater streams. Consideration of such scenarios gives rise to new challenges and demand for drastic changes in the prevalent architecture assumed in most of the routing schemes proposed for underwater acoustic sensor networks. For example with free mobility of sensor nodes, network partitioning becomes inevitable, as nodes drift away from each other over time. Network partitioning, which is different from connectivity voids, refers to the scenarios in which free mobility causes the network to split into two or more unconnected parts, thus creating scenarios where either sink nodes are not accessible at all, or the previously estimated routes are obsolete, due to the change in neighborhood information. Moreover, besides network partitioning, the magnitude of the connectivity void problem is also expected to increase. Besides mobility, connectivity voids may also be created due to environmental changes. This happens due to change in propagation characteristic of acoustic waves in different environmental conditions such as change in temperature and salinity of water. This may result in situations where, nodes that were previously accessible are not accessible anymore thus introducing connectivity voids in the network. Moreover, the changes in propagation speed may also affect accurate location estimation of a node, which in turn may affect the performance of the location-based routing schemes. Another issue related to the free mobility of nodes is the continuously changing topology of the network. For networks in which nodes move in the same stream, the topology may not change in the short run, due to negligible changes in the relative positions of node, because of movement in the same stream. However, in the medium and long run, the topology may change, resulting in inconsistent network density i.e., some parts of the network may have high density of nodes, whereas other parts may be sparse, with many potential voids. This gives rise to the need for routing protocols that can dynamically alter their routing strategy in different parts of the network based on sparsity. This further gives rise to the need to embed mechanisms in the routing protocols to determine network sparsity. Free mobility of nodes also dictates drastic changes in the network architecture assumed in most of the proposed schemes. This design seems infeasible, as sensor nodes (which move freely with water currents) quickly move far from the sink nodes, and are unable to communicate with the sink. This results in inability to acquire the sensed data. Propelled sink nodes could be used, which are capable of tracking the underwater network, and adjusting their positions accordingly, by moving with the network in such a way that they are always in communication with a certain number of underwater nodes. In most of the routing schemes proposed for underwater sensor networks, the nodes near the surface are more often selected as relays due to their close proximity with the sink nodes. This results in almost inevitable earlier energy drainage and consequently early death of such nodes. The connectivity holes and/or partitions thus created hamper reliability of the network. Deeper nodes may experiencehigher end to end delays, and low packet delivery ratios thus reducing overall utility of the network. This area still needs to be explored. More effective solution should be proposed to balance energy drainage of nodes in the network. In order to optimize performance of the network, cross layer design should be investigated.