# An Energy Efficient Routing Approach for IoT Enabled Underwater WSNs in Smart Cities
## 2 Literature Review
### Cluster-2 Protocols Holding Heterogeneous Energy  
In [50], depth and noise-aware routing (DNAR) and cooperative DNAR (Co-DNAR) are proposed. DNAR fuses the degree of link clamor in grouping with the depth of a node to choose the subsequent data sending node. However, the data transmitted over a solitary link may get affected by the unpredictable behavior of the channel. Consequently, a cooperative scheme is added to the called DNAR, which uses source-relay-destination triplets. Co-DNAR diminishes the likelihood of data defilement, for the transmission over a solitary source-destination interface. The authors argue that network stability is increased in Co-DNAR; however, more time is consumed to choose the transfer relay nodes and, thus, Co-DNAR produces the maximum delay.  
The structure of two directing calculations EERD and CoEERD for U-WSNs is proposed in [51]. The EERD limits energy utilization, while the CoEERD improves the reliability of a network. The two plans use information sender nodes dependent on the weighting capacity factors (least Bit Error Rate, highest residual energy, and smallest distance). According to the author, depending upon the aforementioned and utilizing a single route, the EERD efficiently uses the energy of a network and it also extends the stability period of a network. However, the proposed EERD protocol is unreliable and inefficient under the unpredictable behavior of water by choosing a single way. The CoEERD protocol is proposed for achieving the reliability of the network. The concept of multi-way routing is introduced here. A similar standard is characterized for the information forwarder nodes choice and collaboration is held among the relay nodes and destination nodes. The authors argue that energy is efficiently utilized and the packets are reliably transferred to the sink node. However, likewise, Co-DNAR, more time is consumed to choose the transfer relay nodes and, thus, Co-EERD also produces the maximum delay.  
Authors proposed a protocol for heterogeneous networks, where all of the nodes in a network are supplied with two distinct levels of energy SEP [52]. Depending upon the energy, nodes are divided into two classes that are advanced and normal nodes. \(\alpha\) times more energy is provided to the advanced nodes concerning the normal nodes. Due to this varying energy distribution, advanced nodes have a higher possibility to evolve into CH. In any situation, the sensor node first forwards the packet to CH and then this respective CH is responsible for the next step, even if the sensor node is at a very nearest position to the BS than a CH. This causes extra consumption of energy and the energy diminution of nodes. Therefore, the lifetime of a network shrinks.  
A heterogeneous routing protocol DEEC [53], in which nodes have different initial energies. Node(s) having higher energy than the averagely computed energy of the network becomes CH(s), these CHs will transmit the aggregated packets to the sink. Because of varying energy distribution, high energy nodes are selected as CHs repeatedly. Hence, the network lifetime increases. CH selection criteria are the same as in LEACH; however, here the possibility of the node to evolve into CH is calculated with a distinct equation, which is explained in (ACH)\({}^{2}\)[54]. In (ACH)\({}^{2}\), the author highlighted the major issues, like unbalanced cluster size, backward forwarding, as in LEACH, TEEN, SEP, and DEEC. To thoroughly understand the existing protocols, a comparison chart of these existing schemes is shown in Table 1.  
The research works that have been presented here have been conducted on underwater nodes to collect data where optimum performance is obtained. Throughput maximization and improved lifetime of a network are the desired parameters in the proposed study.