# A Comprehensive Survey of Recent Routing Protocols for Underwater Acoustic Sensor Networks
## 3 Routing Protocols for UWSNs
### Localization-Free Routing Schemes
#### Eva-Dbr  
Energy efficient void avoidance depth-based routing (EVA-DBR) [46] is a stateless routing technique. The protocol works in two phases. In the updating phase, nodes transmit information, such as depth of node, node type, etc. to their neighbors, which use the received information to update their neighborhood tables. Base on the neighborhood tables, forwarding decision are made during the routing phase. Void regions and trapped nodes (Figure 11) are proactively detected and avoided while making forwarding decisions. The connectivity holes and trapped nodes are identified. The process is carried out locally, and does not involve extensive messaging, thus resulting in reduced communication overhead, and better scalability. Moreover, EVA-DBR improves its reliability in sparse area by adjusting its forwarding area based on how dense is the area ahead. This also results in better energy efficiency by throttling duplicate packets in denser parts of the network. However, it does not take residual energy into account while making forwarding decisions, which may result in disbalanced energy dissipation. Figure 11: Void regions and trapped nodes problem. EECOR  
EECOR [47] is a cooperative opportunistic routing technique that aims to achieve improved energy consumption. When a node has a packet to send, it selects a set of relay nodes based on their depth. This is followed by applying a fuzzy logic-based relay (FLRS) selection scheme on the selected set to choose the best relay nodes for each hop from source to the sink node. The packet is forwarded to the best selected relay node. This process is carried out at every hop, until the packet is finally delivered to the sink node. Holding time (Equation (16)) is used in order to avoid collision between transmissions of neighboring nodes. The holding time is determined based on two factors: (1) a neighbor's fitness factor, which is calculated based on depth, and (2) the propagation time between the current sender and the candidate relay nodes in its neighborhood. EECOR achieves high packet delivery ratios and energy efficiency. However, delay may be added due to the forwarder selection process, which involves repeated inspection and selection of the forwarder set:  
\[TH_{rj}=\Big{(}1-\mu_{r}\Big{)}\big{(}\tau_{max}\big{)}+\frac{R_{\text{max}} \Big{|}\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\  
EAVARP  
EAVARP [49] is an energy-aware and void-avoidable routing protocol designed for 3D mobile networks, which works in three phases: Initialization phase, layering phase, and data collection phase. The initialization phase sets the IDs, energy levels, layer information, and initial routing table of the sink and ordinary sensor nodes. The layering phase involves forming a concentric structure around the sink node. The phase is initiated by the sink node, which broadcasts a layering packet that is received and rebroadcasted by node. This phase determines the distances of sensor nodes from the sink node in terms of hop counts. Neighborhood information of sensor nodes based on the residual energy levels of the neighboring nodes is also collected. In the data collection phase data is forwarded using an opportunistic data forwarding scheme (ODFS). Moreover, the transmission capacity of neighboring nodes (determined during the layering phase) is also taken into account while making forwarding decisions. By using residual energy as part of the forwarder selection criteria, EAVARP achieves load balancing, which results in preventing connectivity void creation and prolonging network lifetime. However, it achieves low packet delivery ratio for a smaller number of nodes. Co-EEORS  
Co-EEORS [50] uses cooperative routing to find energy efficient routes from sensor nodes to sink node. Forwarding nodes or relays are selected based on two parameters: depth, and location value. Depth is measured using pressure sensors, whereas the location value (which does not refer to the geographic location of a sensor node) is measured in terms of a node's distance from the surface sink node.