# An Improved Forwarding of Diverse Events with Mobile Sinks in Underwater Wireless Sensor Networks
## 2 Background and Related Work
### Localization Free Routing Protocols  
This category consists of protocols which are either completely free of any localization information (based on hop count) or they may require partial localization information DBR (Depth Based Routing) requires only depth). In DBR [28], a receiving node from the set of potential forwarders utilizes its depth and depth threshold (a global parameter) to make the forwarding decision. Forwarding decision is done individually by each receiving node on the basis of receiving time of a certain packet and holding time of that node. Holding time is assigned to each node in such a way that node with lesser depth has less holding time and more probability to be forward. Other nodes are prohibited to forward the packet by assigning holding time values more than the optimal forwarder value. DBR performs well to enhance the stability period but when nodes start dying there is no mechanism to rescue and prolong the instability period (time period between the death of first and last node). Throughput of DBR in sparse networks is relatively low, which may require to implement multiple sinks. Additionally, if two nodes lies at same depth then there is no mechanism to suppress the transmission of redundant packets. Depth Based Multi-hop Routing (DBMR) [29] is a variant of DBR which strives to reduce flooding and redundant packet transmission issues. In DBMR, multi-hop communication is employed during the forwarding process. Weighting Depth and Forwarding Area Division DBR (WDFAD-DBR) is proposed in [30]. Void area occurrence is reduced by selecting next forwarder considering the depth of current node and expected next forwarding node. Weighted sum of depth difference of two hops have role to predict the holding time. There is a mechanism to change the forwarding region adaptively according to node density and channel condition. However neighbour node depth prediction error would lead to increase control overhead. Another depth based routing protocol is hydraulic pressure based anycast routing called: HydroCast [31]. In HydroCast, firstly an opportunistic routing mechanism is proposed which reduces co-channel interference and ensures maximum delivery. Secondly a simple depth based void area avoidance mechanism is proposed. In HydroCast, void nodes maintain the recovery route to a node who's depth is lower than its own depth. Then the void node packet is rerouted out of void using greedy forwarding approach. Opportunistic directional flooding is realized in Void Aware Pressure Routing (VAPR) [32]. VAPR operation consists of two parts: enhance beaconing and opportunistic directional forwarding. Additional information such as sender depth, hop count to sonobuoy, sequence number and forwarding direction, is added to each node's beacon in enhance beaconing phase. VAPR employs local greedy directional forwarding and utilizes a factor called surface reachability as forwarding metric. A Q-learning based cross layer approach is proposed in [33]. Instead of hop count or depth, Q-learning routing path selection process includes traffic load, latency and queue length. Q-learning in distributed nodes ensures balanced energy consumption. Path Unaware Layered Routing Protocol (PULRP) with uniform deployment of nodes and with non uniform deployment of nodes is proposed in [34]. It consists of two phases, the first phase called the layering phase, in which spherical layers of radius \(R_{r}\) with sink as layer 0 are formed and layering number is increased by each node. A node in layer \(l\) is to find forwarder from layer \(l-1\) on the basis of transmission range and layering radius. The second phase is called communication phase in which potential relay nodes from each layer are selected for packet forwarding. Energy optimized-PULRP [35] is an enhancement in which so called _on the fly_ routing is considered to select relay node. E-PULRP incorporate energy in routing decisions, which was not considered in previous versions. An improvement in DBR named as EEDBR was proposed by [PERSON] et al. in [36].