# A Comprehensive Survey of Recent Routing Protocols for Underwater Acoustic Sensor Networks
## 3 Routing Protocols for UWSNs
### Localization-Free Routing Schemes
#### Eva-Dbr
The sink node transmits beacons periodically, in order to help sensor nodes update their location estimates. Based on the two values, the node closest to the sink node is selected as a relay node. Once a relay node is selected, data is forwarded to it, which relays it to the destination. The destination node sends acknowledgment to the source node in the case of a successful reception, or requests retransmission in the case of a failure. CoEEORS adds reliability by incorporating acknowledgements for received packet in the protocol design. Acknowledgement informs the sender of successful reception, or requests retransmission in the case of erroneous reception. However, acknowledgements add additional overhead. SORP  
SORP [51] is a depth-based stateless opportunistic routing protocol. This focuses on the issue of voids and trapped nodes faced by routing protocols in UWSNs. It uses information received from neighboring nodes to detect communication voids. It can also detect trapped nodes locally, and exclude them in the route establishment process, to ensure high end-to-end delivery ratio. The protocol works in two phases: updating, and routing phases. The updating phase is carried out periodically, in which information is shared between neighbors. The routing phase deals with routing data to the sink node while selecting the best forwarders, based on the information collected during the updating phase. The information is used to detect and exclude voids and trapped nodes from the route establishment process, using a passive participation approach. SORP proposes a technique to employ an adaptive forwarding area (Figure 12). The forwarding area is adaptive in the sense that it can be changed, and/or its size can be altered, based upon the density of nodes in the local area and position of the candidate relays, thereby improving the reliability and energy efficiency of the network. The impact of variable delays and shadow zones on the performance of the proposed technique has also been analyzed by the authors. SORP achieves a good packet delivery ratio (PDR) in dense networks; however, PDR can be very low in the case of sparse networks. The same is true in the case of end-to-end delay. RMCN  
Reference [52] proposes RMCN, which is a radius-based courier node routing protocol for long-term underwater sensing applications. RMCN divides the network into two types of areas: sink areas, which constitutes one-quarter of the total area, and node areas, which constitute the remaining three-quarters of the total area. Sensor nodes in the node area are equally divided into static and mobile courier nodes. Forwarding decisions are based on four metrics, which are: depth of nodes, energy levels of nodes, track ID, and a cost function. The packet delivery ratio is improved through efficient coordination between courier and static nodes. RMCN improves energy efficiency by avoiding data-flooding and the creation of multiple copies of the forwarded packet. However, the drawback of RMCN is that it assumes that nodes are deployed according to a set criterion. Therefore, random deployment of nodes, (which may be the case in many applications), may result in reduced performance of the protocol. RECRP  
RECRP [53] is a single copy, cross-layer routing protocol. The aim of this protocol is to enhance 2-hop delivery, while achieving better energy consumption. To this end, the protocol proposes a minmax technique that can adjust power and frequency dynamically according to the requirement. The minmax technique is used in conjunction with a mixed integer linear programming model to design energy efficient forwarding. Moreover to improve routing, the protocol is designed as a cross-layer framework. This enables information to be acquired, like Doppler shift from lower layers for estimating relative speed among underwater sensor nodes, and regulating transmission power and frequency to achieve better transmission efficiency. The protocol works in two main phases: Routing table update phase, and routing phase.