## 4 Underwater Sensor Networks
Combined with the observation plan in the South China Sea, authors provide numerical simulations of ocean models and data assimilation, and autonomous trajectory planning and optimization, which demonstrate the effectiveness of the system. [PERSON] _et al._ discuss a new architecture and routing protocol for underwater sensor networks called Energy-efficient aDaptive hiEarchical and robusT Architecture (EDETA) and EDETA-e protocol respectively in [25]. Authors have applied different delay-aware and non-delay-aware scheduling techniques, and different retransmission methods combined with them, to the architecture in order to study their performance in the underwater environment. EDETA-e is a power-aware routing protocol which organizes nodes in clusters and uses low-power modes at the times in which nodes have no need to be awake. In addition, the protocol adds fault tolerant mechanisms and has time-constrained properties. Paths can be dynamically adapted to topology changes and node failures, offering the maximum energy saving without exact location information. The authors analyzed the behavior of EDETA-e protocol in the subaquatic medium using Network Simulator 3 in terms of energy consumption, packet end-to-end delay, packet delivery ratio, number of duplicate packets, and packet loss rate. The performance evaluation of EDETA-e protocol shows a stable and efficient behavior of clusters and tree structures. Simulation results show that, taking advantage of the transmission delay when performing the scheduling can significantly reduce the energy consumption and delays, maintaining the same packet delivery ratio when packet errors are introduced. Thus, the protocol provides high reliability in terms of no data packet loss due to collisions and an optimal energy management during the normal operation phase, allowing the nodes to remain in a low-power state when they have no data to deliver to the sink. Moreover, they also proved that EDETA-e is more energy efficient than the Depth Based routing (DBR) protocol. Taking into account that in underwater sensor networks, the probability of node failure is high because sensor nodes are deployed in harsher environments than ground-based networks, [PERSON] at al. propose in [26] a checkpointing scheme for the head nodes of clustering routing protocols to quickly recover from a head node failure with the aim of improving the reliability of the network. In order to achieve this goal, backup nodes that periodically save the routing information and collected data sent by head nodes. If a head node experiences a transient fault, one of the backup nodes detects the head node failure and becomes the new head node. The head node can quickly recover from a transient fault by omitting the reelection of a head node and by preventing the loss of the collected information. Even if a head node experiences a permanent fault, a backup node could become the new head node immediately. Experimental results show that the proposed scheme enhances the reliability of the networks and makes them more efficient in terms of energy consumption and recovery latency when the head node fails than other schemes without checkpointing. In [27], [PERSON] _et al._ propose a new underwater routing scheme, namely Autonomous Underwater Vehicles aided Underwater Routing Protocol (AURP), which uses both heterogeneous acoustic communication channels and controlled mobility of multiple autonomous underwater vehicles (AUVs) to maximize the data delivery ratio and minimize the energy consumption of underwater sensor nodes. The network architecture consists of U-sensor nodes, gateway nodes (GWs), AUVs, and surface unit/mothership. AURP minimizes the total number of data transmissions using the following procedure. Underwater sensors send the sensed data, using a midrange underwater acoustic channel, to the GWs directly or in a multihop fashion.