## 4 Routing Layer
### Special Mechanisms  
In this sub-section, the routing protocols which use special physical mechanisms or devices in order to operate in an efficient manner are discussed. These special physical mechanisms can include, for example, mechanical modules that regulate sensor depth [122; 123], low-power wake-up systems [124], or autonomous underwater vehicles (AUVs) which have to harvest data from actual sensor nodes [124].  
The underwater wireless hybrid sensor network (UW-HSN) [122] protocol is a mix of acoustic and radio communications. UW-HSN uses radio communication for continuous traffic and acoustic communication for reduced data messages. In this manner, all nodes are equipped with radio and acoustic interfaces, using the acoustic interface for underwater communication and the radio interface if nodes are on the surface communicating with the sink. All nodes are equipped with a mechanical module in order to help the node to swim to the surface and dive back down to different underwater levels. Simulation results show that this protocol presents elevated throughput with reduced delays, but no results have been reported about the consumption and cost, which are key factors in this case because of the necessity of special hardware and devices.  
In temporary cluster-based routing (TCBR) [123], only special designated nodes need to include a mechanism to regulate their depth. This protocol is intended to solve the problem of multi-hop routing where nodes near the sink multiply their consumption. The idea is to deploy multiple sinks, and messages received at any sink are correctly received because they can communicate at an elevated data rate and have small propagation delays using RF communication. In TCBR, normal nodes are in charge of sensing and forwarding data to the nearer courier nodes. In order to promote energy consumption, a reduced set of courier nodes is used, and these can sense as well as receive the data from normal sensor nodes and forward them to a surface base station. The courier nodes dispose of a module that allows them to push the node down into the water at different depths and pull back the node to the surface. Courier nodes broadcast 'hello' packets at each defined depth in order to allow the nearest normal node to send the courier node the data. Sensor nodes must save their generated data in a buffer until a courier node appears in the vicinity; thus, it is impossible to provide a response in bounded time with this approach. Moreover, is possible that a node loses packets while awaiting the courier node's presence.  
Finally, the delay-tolerant data dolphin (DDD) [124] protocol follows another approach. It is intended to increase energy efficiency for delay-tolerant scenarios. This protocol uses dolphins, which are special mobile nodes that must visit the sensor nodes in order to retrieve their measured data. Using this technique, each sensor node must only send its locally stored data in one hop to the nearest dolphin node. Because it is unnecessary to use multi-hop routing, DDD saves energy in this manner in the sensor nodes. Dolphin nodes transmit a special signal, and in this manner, the acoustic modem must implement a low-power transceiver to detect the closeness of a dolphin node. Dolphin nodes will provide collected messages when approaching a sink on the water surface. When judging this protocol, the number of dolphin nodes is a key factor. It is necessary to have an equilibrated number of dolphin nodes in order to visit all sensor nodes frequently to prevent data loss, taking into account that an excessive number of dolphins will significantly increase the cost.