# RECRP: An Underwater Reliable Energy-Efficient Cross-Layer Routing Protocol +
## 2 Related Work  
The first important issue of an underwater sensor network is to improve its energy efficiency to prolong the network lifetime. Energy-efficient underwater routing protocols typically use some special node information to reduce the energy consumption. They can generally be classified into the following three categories: location-based routing, depth-based routing, and path consumption-based routing.  
**Location-based** routing protocols assume that each node knows its location and each packet contains the position of the transmitter, which may not be practical in floating oceans. [PERSON] et al. present VBF [PERSON] et al. (2018), a vector-based protocol where the source to sink vector acts as a virtual pipeline with a pre-defined radius to reduce flooding packets for energy efficiency. Because it is difficult to define the pipe radius, VBF is difficult to deploy in UWSNs. HH-VBF [PERSON] et al. (2018) is an enhanced version of VBF where the source-to-sink vector is replaced by the per-hop vector. However, the small pipeline radius has lower performance in sparse networks. AHH-VBF [PERSON] et al. (2018) is an advanced version of HH-VBF that adaptively changes the radius and sends duplicate packets to improve reliability. Because of duplicate packets, there is a higher probability of collisions and energy consumption in AHH-VBF. GEDAR [PERSON] et al. (2018) and DCR [PERSON] et al. (2018) are two typical distributed topology control algorithms to avoid communication voids. They avoid communication voids by adjusting nodes' locations. However, due to the large energy consumption of position adjustment, it is not energy-efficient. In a location-based routing protocol, the assumption that each node knows its location information in UWSNs is not necessarily true. Therefore, there are significant restrictions on the application scenario of location-based underwater routing protocols.  
**Depth-based** routing protocols use depth information to route packets instead of full location coordinates. To acquire depth information, extra sensors are equipped which increases the cost of sensor nodes. DBR [PERSON] et al. (2018) was presented to forward packets to upper nodes. Since it uses a flooding method to route packets, DBR is not energy-efficient. DBMR [PERSON] et al. (2018) leverages a multi-hop transmission model to deliver packets to reduce the energy consumption. While protocols like DBR and DBMR [PERSON] et al. (2018) cannot deal with communication voids, D-DBR [PERSON] et al. (2018) is an enhanced version of DBR that improves the time and energy costs. Since many nodes broadcast at the same time, D-DBR leads to a high probability of packet collisions.  
**Path-consumption-based** routing protocols select the next hop by their considered route selection metric, including distance, hop count, and residual energy, to find the most energy-efficient path. ERP2R [PERSON] et al. (2018) forwards packets according to the physical distance from sink nodes and the residual energy. EEF [PERSON] et al. (2018) uses residual energy, depth, and distance from sink nodes to evaluate the cost. However, both EEF and ERP2R do not consider the effects of channel frequency and dynamic transmission power on the energy cost.