## 3 Underwater Routing Protocols
### Localization-Free Routing Protocols
#### 3.2.2 Protocols Addressing Energy Balancing  
EEDBR  
The energy-efficient depth-based routing (EEDBR) protocol considers the residual energy of nodes in addition to the depth to counteract the effect of the rapid death of low depth nodes in DBR [50]. When a sender node has to select a forwarder node, it selects a forwarder with the lowest depth and the highest residual energy among its one-hop neighbors. This leads to energy balancing and avoids energy holes in the network. However, the EEDBR does not guarantee the reliability of data at the final destination. This is because only a sender node decides the next forwarder in EEDBR, which may increase the probability of packet loss if the link quality is not good.  
ODBR  
The optimized depth-based routing (ODBR) protocol [51] assigns initial energy to nodes based on their vertical distances (depth) from the surface of the water. Nodes closer to the water surface are, therefore, assigned more energy than nodes farther from it. This results in balanced consumption of energy and prolongs the network lifetime. However, this protocol only works in shallow water zones. It does not perform well in networks that require deployment of sensor nodes at the bottom of the ocean because such nodes are assigned the least amount of energy.  
EBECRP  
The authors in [52] design an energy-efficient and balanced energy consumption cluster-based routing protocol (EBECRP) that divides the network into sectors. Each sector is assigned a cluster head to collect data from its neighbors and reduce multi-hoping. Two mobile sinks monitor the dense and sparse regions of the network based on the number of neighbor nodes. Nodes either send the data to the sinks or to the cluster heads. The sinks then collect the data from the cluster heads. The protocol prolongs the network lifetime by reducing energy consumption. However, the cluster heads are overloaded and die rapidly, which causes packet loss. Furthermore, the movement of cluster heads with water currents is a major issue that leads to packet loss.  
Hydrocast  
In the protocol proposed in [53], sensor nodes use water pressure in an opportunistic fashion to forward data packets to the surface sink. The selected set of forwarder nodes maximizes routing in a greedy manner and reduces energy balancing. A dead and recovery method is also proposed to ensure that packets reach the destination. The protocol achieves energy balancing, high packet reception at the sink and low end-to-end delay. However, in sparse conditions, the dead and recovery method does not perform well as nodes do not find enough neighbors. This compromises the performance of the proposed scheme. In addition, opportunistic routing prefers nodes close to the water surface as data forwarders. Such nodes die rapidly due to the high data load on them.  
OMR  
An optimal multimodal routing (OMR) protocol is proposed in [54] that combines acoustic and RF technologies during data forwarding. The selection of a technology is made to avoid bottlenecks and overburdening of relay nodes, as well as to avoid redundant data transmissions during packet forwarding. Every sensor node is assigned fair resources and technologies to participate fairly in data routing.  
As shown in Table 1, long range communications have a very narrow bandwidth. That means less data can be sent in long-range communications. Therefore, the use of acoustic waves in OMR for long-range communication results in reduced throughput. The use of radio waves in OMR cannot be used for long-range communications since these waves are severely attenuated by water.