## 2 Related Work
Centralized and distributed depth adjustment with DA + CTC and DA + DTC are simulated with sonobuoys as taking as sink and provide robust communication without any occurrence of empty regions due to hybrid (centralized and distributed) approach and overall improved network lifetime. In [9], geographic and opportunistic routing work with depth adjustment is used for communication recovery (GEDAR) when the empty region comes into existence. These strategies are mostly used with the location-based application, where oil and minerals need to be explored. Basically, provide the exact location of the occurrence of the empty region by following the greedy-based approach. As the name implies for the opportunistic routing, it always finds the early realization of the area of the node where the gaps are near. It transfers the data packets by next forwarder selection of the set for GEDAR. The limitation of location-oriented problems of the destination nodes fix the depth adjustment for each hop. As the empty regions are located, it calculates the new depth for the sonobuoys for the delivery of data. The use of multi-purpose sonobuoys in each expect neighboring node is calculated with the next hope depth so that the chances of empty regions are minimum as expected. By calculating the depth at each node, the selection of neighboring sets has some trade-offs, because it does not ensure about the GEDAR finds the good and opportunistic set of neighboring nodes, the unexpected delays come with minimum throughput. The depth control routing protocol (DCR) [11] has a similar working nature, as discussed in GEDAR. The major difference between GEDAR and DCR is the moving style of the nodes. The vertical node movement is introduced in DCR to control the topology of the network. To know in advance the greedy approach, the DCR works better where the greedy mechanism of routing fails by using SEA-SWARM architecture. The determination of empty regions is not possible without checking the status of its neighboring nodes, so the main focus of depth control adjustment increases the lifetime and throughput ratio of the network. However, the long route by depth adjustment for neighbors may follow the network in large end-to-end delays. Void aware pressure routing (VAPR) [12; 13] used for monitoring purposes have 4G mechanisms, including space and time. By deploying the on-board pressure gauges at the surface of the ocean, it works directly to send the data packets to the master station. For this, VAPR uses three parameters like hop count, sequence number, and depth information for making the direct path to the closest sonobuoys. The central theme behind the pressure routing is worked in the presence of an empty region, and it is because the location and depth directly control the onboard surface station via an alternate route. Additionally, this scheme increases the network delays when the route is calculated on-board station. Layer-by-layer angle-based flooding (L2-ABF) is introduced in [6; 7] in which every node calculates its flooding angle towards the destination to forward the data packets in the layer hierarchy. The layered approach is used for the deployment of nodes so that there is no need for explicit location information about the nodes. Once the nodes have been deployed, the IDs are assigned that are static throughout the network lifetime. L2-ABF provides a 3D communication zone for its flooding so that both ends must complete the end-to-end delivery. Another approach AHH-VBF is adaptive hop-by-hop vector-based forwarding (AHH-VBF) [13], in which the radius information instead of angle is used. By using radius information, it quickly reduces the consumption of energy issues because the radius information is done by using acoustic wavelengths. Therefore, there is no need to take extra calculations between source and destination. This scheme enhances the rapid data packet forwarding ratio, reduces the delay, and makes an energy-efficient scheme.