# Self-Organized Fast Routing Protocol for Radial Underwater Networks
## 1 Introduction  
An underwater wireless sensor networks (UWSNs) is an emerging technology for environmental monitoring and surveillance. Electromagnetic waves are not suitable for this transmission medium because of their high absorption rate. Acoustic waves provide better performance in this regard. However, the propagation speed of acoustic waves is only slightly larger than 1500 m/s, varying with temperature, depth and salinity, which significantly increases the propagation delay. Routing protocols designed for terrestrial wireless sensor networks (WSNs) are difficult to be adopted for UWSNs due to this physical constraint, as they assume virtually instantaneous propagation, thus allowing for more signaling overhead. The propagation delay is a constraint that must be handled at the medium access control layer, which will try to utilize the channel as much as possible. Additionally, acoustic transmission data rates are much lower than in aerial transmissions. Terrestrial protocols for ad-hoc networks often rely on location information of the nodes, which, for UWSNs, is at best a challenging task, due to the unavailability of underwater positioning systems. Routing protocols for underwater sensor networks, which are based in the location information of the nodes, usually require the existence of a special node, a sink node that is equipped with a global positioning system (GPS) receiver to ascertain its location. The location of the rest of the nodes is estimated with reference to the sink node. Range-based schemes use time of arrival (ToA) [1; 2] or angle of arrival (AoA) [3] techniques to estimate the distance between the nodes and, eventually, the location of the nodes. These techniques require additional devices to measure the distance, which makesthem bulkier and more energy demanding. They also require time synchronization, which is another challenging task for underwater networks [4; 5]. Autonomous underwater vehicles (AUVs) based protocols [3; 4; 5; 6; 7] typically use an AUV to visit the sensor nodes and collect the data to be transmitted to the gateway. The disadvantage of this scheme is obvious, as it requires additional resources to operate the AUV. A radial topology can be deployed in 2D or 3D, as shown in Figure 1, and can be used to monitor sea volumes such as delta plumes, navigable straits, and archaeological sites as well as to secure facilities such as harbors, trade routes, mining sites, etc. It can also be of high value to monitor the surroundings of geological activity, such as the underwater volcano Tagoro, which erupted recently near island El Hierro [8; 9]. The possibility of real-time video transmissions over these topologies have been analyzed in Reference [10]. In terms of interference, the worst case is when adjacent strings are parallel to each other, and this will be the case considered. Routing is not an easy to solve problem in networks similar to the example of Figure 1. In this paper, we describe a location free routing protocol for 2D/3D underwater radial networks. The algorithm is designed to recreate the physical topology in the logical routing tables. The paths are established in such a way that only the nodes located in the same radius (called strings, see Section 3) will forward the data packets coming from other nodes which are farther from the sink. This way, all the packets are transmitted via the shortest paths. The algorithm uses the knowledge of the topology to initialize the paths. It is designed to avoid collisions and minimizes the overhead due to signaling. All together, these characteristics help save as much energy as possible. This structure also helps to balance traffic among nodes that are at the same distance from the gateway such that energy depletion is as homogeneous as possible. Other proposals do not consider the physical topology when building the routing trees, and thus they do not balance traffic among routes, causing faster energy depletion of some nodes than others.