# A Comprehensive Survey of Recent Routing Protocols for Underwater Acoustic Sensor Networks
## 1 Introduction
A routing protocol that uses locations to set up paths must deal with extra communication overhead incurred due to localization. _Mobility:_ Underwater sensor nodes move with water currents. This movement may disrupt end-to-end paths and create connectivity voids if a node drifts out of the range of its adjacent sensor node(s) along the path from source to sink. Therefore while designing a routing scheme, free movement of nodes should also be considered. _3D Deployment_: 3D deployment imposes further challenges, due to the addition of the third dimension, i.e., depth. Sensor nodes should be capable of relaying data to the sink nodes, forwarding  
Figure 1: Underwater Sensor Network Architecture [10]. it over multiple-hop paths. The nodes should adjust their depths in order to find a relay if there is no accessible relay using the current transmission range. However depth adjustment should ideally not create connectivity holes for other nodes. In the following text we briefly explain all the parameters based on which we evaluated the routing protocols in this survey. Tables 1 and 2 list all of these parameters, and their values for each of the considered protocols:  
_Mobility_: This parameter includes both limited and free mobility consideration. _Location Requirement_: Indicates the requirement of knowing the geographical locations of sensor nodes for successful operation of the scheme. _Main consideration_: Indicates the main issue addressed by the proposed scheme. _Next Hop Selection Criteria_: Indicates the criteria for selecting forwarding nodes. The next hop selection criteria determines the capability of the proposed scheme for addressing different issues related to routing. For example if \"Residual energy\" is considered, the aim is balanced energy dissipation of the node. Considering \"Link condition or SNR\" aims at improving reliability, etc. _Clustering_: Indicates whether the proposed scheme is a cluster-based scheme or not. Clustering offers advantages, such as low communication overhead, small contention domain, and transmission with low transmission power, etc. _Connectivity Void Handling_: Connectivity void handling includes two issues: (1) Prevention of void creation, e.g., by balancing energy dissipation; (2) Ability to work around voids to successfully deliver packets, for example, by using detouring techniques. _Sink_: Indicates whether the scheme uses a single-sink or multi-sink architecture. Both architectures have their own advantages and disadvantages. For example in the case of multi-sink architecture, the receiving plane is widespread. This helps in load distribution, and provides flexibility in path selection. Whereas in the case of single-sink architecture, all the traffic is directed towards the same destination, thus burdening certain nodes near the sink and shrinking path options. However, single-sink architecture offers less complexity, compared to multi-sink architecture. _Deployment_: Indicates whether authors have considered 2D or 3D deployment. The rest of this paper has been organized as follows: Section 2 presents a detailed survey of the routing protocols considered. It is further divided into two sub-sections dividing the routing algorithms into location-based routing algorithms, and location-free routing algorithms. Section 3 presents the conclusion, while Section 4 discusses open issues and challenges.