# Towards Efficient Sink Mobility in Underwater Wireless Sensor Networks
## 2 Related Work
In [14], an autonomous underwater vehicle aided efficient data-gathering (AEDG) routing protocol is proposed. After the deployment of sensor nodes, AEGD selects gateway nodes. Afterwards, the shortest path tree (SPT) algorithm is run to link sensor nodes with gateway nodes. AUV gathers data from gateways nodes. In this way, the network lifetime in AEDG is maximized. The study in [17] discussed the impact of using multiple mobile sinks in increasing network lifetime. Mobile movement strategies like hexagonal tiling is also discussed in this paper. Two different techniques for sink movement: predefined path and non-predefined path are also discussed. Results show that a predefined path for the movement of mobile sinks has a better network lifetime than a non-predefined path. An EEDBR protocol for UWSNS is proposed in [18]. The performance of EEDBR is greatly improved than DBR. In DBR, energy consumption is high because of data forwarding nodes selection criteria. In EEDBR, selection criteria for data forwarding nodes are low depth and high residual energy of sensor nodes. In selection criteria of EEDBR, load on residual energy results in rapid decrease of network lifetime. Before forwarding the packet, sensor nodes in EEDBR holds the packet for definite time. Holding time of a node is calculated based on high residual energy. The node having high residual energy has less holding time than the one with less residual energy. This calculation of holding time allows high residual energy nodes to forward packets. In this way, EEDBR proves to be more energy efficient than DBR. In [22; 23; 24], the authors have used clustering schemes for routing in UWSNs. These protocols feature two phases. First is the set-up-phase in which CHs are selected for the formed clusters; second is the communication phase, in which member nodes of cluster gather data and send it to their respective CHs. In these protocols, the first phase is periodically repeated due to the ocean current. This periodic repetition of the first phase adds extra burden on networks. To avoid collision of packets from member nodes to CH, time-division multiple access (TDMA) is adopted in these protocols. Introducing clustering with TDMA results in the increase of end to end delay (E2E). The study in [25] discussed the energy hole problem in relation to ocean currents. In this paper, drift speed calculation is introduced. Drift speed at any time can be calculated if the current location of sensor nodes is known. Whenever ocean current is sensed by the mobile sink (MS), the MS changes its speed accordingly. The proposed scheme results in better throughput. Hop-by-hop dynamic addressing based (H2-DAB) was proposed in [26]. In this scheme, HopID is assigned to each sensor node on the basis of hop count from the sink. Foremost, the hello packet is broadcasted by the sink node. Each node that receives a hello packet is assigned by HopID. This process is incremented hop-by-hop thus node near to sink gets small HopID. The nodes with small HopID are the ones forwarding data. The selection criteria of data forwarding node in H2-DBR requires sending some inquiry packets to sensor nodes, which increases the utilization of residual energy and delay.