# Investigating Master-Slave Architecture for Underwater Wireless Sensor Network
## 2 Related Work
### Localization-Free Routing Protocols  
We mainly focus on the localization-free routing protocols, also called depth-based routing protocols, as they are more closely related to our proposed work. The routing schemes in such protocols use the depth information to evaluate a communication route between the source and destination. This information is being processed iteratively on each hop until it finds the destination. Following are the well-known localization-free (depth-based) protocols in the literature. * DBR: Depth-based Routing (DBR) [14] is the base protocol that played a vital role in the transformation from localization-based to depth-based routing in UWSNs. DBR is an approach based on the greedy algorithm focusing on sending packets from source to sink nodes. It is a single-hop scheme that uses the next node for the data packet transmission. The nodes are randomly deployed and have the characteristic of mobility due to the ocean nature, therefore, resulting in the void hole problem. This protocol uses the depth information to select the next transmission forwarder, and the source node transmits the data in its premises, while every other node within the range of the network receives the packet. Based on the depth difference information, the surrounding nodes are divided into PFNs and suppressed nodes. Next, the nodes occurring in the lower hemisphere drop the packet while the nodes in the upper hemisphere (i.e., PFNs) determine the holding time using the local depth information, and set the timer to hold the data packet. During this holding period, if a node does not obtain a copied version of the captured data packet, it transmits the packet after the timer expiration. On the other hand, the packet is dropped. In this protocol, the sender makes decisions based on one-hop depth and remaining information, resulting in a lower rate of packet delivery as well as lower energy efficiency, and eventually leading to the occurrence of void and energy holes. * DSRP: [PERSON] et al. [15] presented the Distributed Delay-Sensitive Routing Protocol (DSRP) to address the mobility of nodes in a network. In this model, when a node reaches its destination, it stops there for a moment and is then shifted to the new destination by keeping account of the expected traffic, chosen paths, and localization of the nodes. The movement speed of the node is kept between maximum and minimum values. This high mobility results in high PDR with the cost of high energy consumption and end-to-end delay, as compared to low mobility and no mobility models. * ODBR: The Optimized Depth-Based Routing (ODBR) protocol [16] targets the energy balancing approach where it assigns the energy to nodes based on their depths. To prolong the network lifetime and balance the energy consumption, the nodes with smaller depths are assigned a high amount of energy while the nodes with higher depths get low energy. This approach is not applicable in a deep-water environment because the nodes at the highest depth get low energy and die quickly. However, it works well in shallow water because each node in the network gets the required amount of energy to transmit data due to its less depth. * EBECRP: To reduce multi-hopping, the authors in [17] presented an Energy-efficient and Balanced Energy Consumption Cluster-based Routing Protocol (EBECRP) which divide the network into sectors where each sector consists of a cluster head capable of collecting data from its neighbor nodes. The nodes first send data either directly to the sink or the cluster head and then the sink collects data from the cluster head. This results in low energy consumption due to optimized routing but high packet loss because of the early death of the cluster head due to energy reuse. * Hydrocast: The Hydrocast protocol [18] proposed a way to use water pressure as a driving force to forward a data packet to the sink which reduces energy balancing and economize the node energy.