# A Comprehensive Survey of Recent Routing Protocols for Underwater Acoustic Sensor Networks
## 3 Routing Protocols for UWSNs
### Ef-Iehm  
LF-IEHM [54] is a localization-free depth-based routing protocol. In LF-IEHM, forwarding decisions are made based on depth of nodes. The \"in range\" nodes nearest to the water surface are chosen as candidate relays. If two or more candidate relays have the same depth, the relay node with the smallest response time (which is mainly dependent on the distance from the source node) is chosen as relay. LF-IEHM improves packet delivery ratio and energy efficiency in the routing process using two strategies. Firstly, it alters its transmission power in order to avoid voids. If a node is stuck in a void, it increases its transmission power to reach out to a sink node, or to a node with smaller depth that can act as relay node (Figure 13). This technique is intended to achieve small end-to-end delay, by finding out the shortest path to sink nodes on the water surface. Secondly, in order to reduce interference among transmitting nodes, every transmitting node is assigned a unique holding time given by Equation (17):  
\[TH=\frac{N_{j}\big{(}PrL_{i}-PrL_{j}\big{)}}{C\big{(}\frac{E_{initial}}{E_{ Current}}\big{)}} \tag{17}\]  
where \(N_{j}\), \(PrL_{i}\), \(PrL_{j}\), \(E_{initial}\), \(E_{Current}\) and \(C\) denote the no. of neighbors, the pressure level of the sender, the pressure level forwarder, initial energy level of the forwarder, current energy level of the forwarder and the speed of sound in water, respectively.  
This strategy of assigning unique holding time to every transmitting node, minimizes interference, which results in increased successful delivery of packets, thus improving packet delivery ratio and energy efficiency. Moreover, this strategy also improves end-to-end delay by improving hop-to-hop successful delivery probability. The scheme improves packet delivery ratio in sparse networks using a \"variable transmission power\" strategy; however, this may increase power consumption as the network becomes sparser. Furthermore LF-IEHM considers only depth as the forwarder selection criterion. This (especially in the absence of considering residual energy) will result in early death of the nodes near the surface, thus creating connectivity voids.  
EDBF [55] is a depth-based forwarding mechanism. It selects next hop based on three parameters: (1) Depth of the forwarding node: a node whose depth is smaller than the sender is selected as a forwarder; (2) Residual energy: this prolongs network lifetime by achieving balanced load  
Figure 13: Adaptive neighbor determination. (**a**) No neighbor in the range of node A, (**b**) Node A increases power to be able to access neighbors.  
distribution, which results in preventing the creation of communication void; and (3) Forwarding quality; the forwarding quality of a node is determined based on link or node failures, the presence or absence of connectivity voids, etc. Inclusion of this metric improves the successful forwarding of data packets. EBDF reduces communication overhead; however, due to its simple design, especially in determining forwarding quality, the end-to-end delivery ratio does not improve significantly.