## 3 Underwater Routing Protocols
### Tc-Vbf
#### 3.1.2 Protocols Addressing Energy Balancing  
These protocols involve energy balancing and ensure that certain nodes within the network do not get overburdened due to frequent data forwarding. This is because the overburdened nodes deplete their battery power and die soon. The death of such nodes interrupts the reliable delivery of data to the surface of the water. Generally, nodes close to the surface of the water are overburdened as these nodes are heavily involved in data routing due to the closeness to the water surface. The description of these protocols follows below.  
HH-VBF  
In order to overcome the overburdening of the nodes in the routing pipe of VBF, a hop-by-hop VBF (HH-VBF) protocol is proposed [20] that defines the routing pipe for every forwarder node in contrast to the single major pipe in VBF. However, defining the routing pipe for every forwarder node increases the computational delay. In addition, nodes in the pipes are still preferred for selection as forwarders. This overburdens such nodes.  
REBAR  
A reliable and energy-balanced routing (REBAR) protocol establishes a varied radius cylindrical path from the source to the destination [21]. This path has a lower radius near the sink to involve fewer nodes. This avoids early death of such nodes and achieves energy balancing. The protocol assumes that every node has knowledge of its own position and the position of the final destination. A source node incorporates the information of the established path and its location in the packet and broadcasts it. Upon receiving the packet, a neighbor node calculates the difference between its own distance and the distance of the source from the water surface. If this difference is smaller than a certain limit, it forwards the packet and otherwise drops it. Redundant packets transmission is suppressed by using a history buffer. Furthermore, the protocol overcomes the void problem by making nodes near a void transmit the packets to all its neighbors that are independent of their positions and distances from the sink. Nodes' movement with water currents is used as a positive parameter to select different nodes to further achieve energy balancing. Routing in the restricted cylindrical path achieves energy efficiency, but compromises the number of successful packet receptions at the sink, especially in sparse conditions. Furthermore, significant node movement may cause an extra amount of energy consumption in position calculation.  
BEAR  
The balanced energy adaptive routing (BEAR) protocol considers a hemispherical network and divides it into sectors of equal radii [22]. Nodes that reside closer to the sink are assigned more power because of greater data load on them than the farther nodes. In addition, the density of nodes is kept higher near the sink than the rest of the network. Every node in a sector selects a forwarder node in the sector above it until data packets reach the sink. The protocol balances energy consumption and, therefore, prolongs the network lifetime. However, the protocol suffers from high interference near the sink as more nodes are deployed near the sink and their packets' transmission is not monitored by defining the packet holding time.