# Self-Organizing and Scalable Routing Protocol (SOSRP) for Underwater Acoustic Sensor Networks
## 1 Introduction  
Underwater Wireless Sensor Networks (UWSNs) are collections of many autonomous sensor nodes, networked together through wireless links, which perform collaborating tasks to monitor physical or environment parameters such as pressure, temperature, sounds, etc. These networks were initially developed using the concept of terrestrial WSN systems but the fundamental challenges of these two technologies are different. The early implementation of UWSN with radio frequency (RF) and optical links proved that new solutions and approaches are required for underwater environment, which confronts different challenges and limitations in terms of signal propagation, low efficiency of radio wave, transmission range of few meters and scattering in case of optical waves. Therefore, acoustic waves prove to be a promising communication technology, and because of this underwater sensor networks are also referred to as Underwater Acoustic Sensor Networks (UASNs). The use of acoustic communication imposes several constraints, therefore parameters such as carrier frequency, attenuation, noise, fading, propagation delay, and limited bandwidth are important to consider during protocols designing for UASNs. Furthermore, because of water currents and various underwater activities the underwater sensors remain mobile, which makes traditional routing inefficient since the network topology changes over time. Therefore, network topology is also a vital factor in protocol design. Reliability, capacity and energy consumption of network are affected and determined through topology control technique. The reliability of underwater network topology is highly importantbecause of high cost of sensor nodes. Moreover, the propagation environment also has substantial effect on energy consumption which results in node failure because of rapid energy depletion. For this reason, single-point topology should be avoided because a failure in a single node of the network could lead to overall network collapse. With the advancement in the field of wireless communication and sensor technology, researchers have proposed numerous routing techniques for UASNs [1]. In Reference [2], a new clustering algorithm is proposed using a Low-Energy Adaptive Clustering Hierarchy (LEACH) protocol to address the problem of large clusters and nodes at the edge consuming more energy. In the cluster head (CH) election phase, the position of CH is considered to be the cluster center from the points which were uniformly distributed in the network. For nodes to select the CH, weight factor is introduced which considers the energy consumption between node and CH, and every CH and a base station (BS): It balances the cluster size, reducing the total network energy consumption. Energy Aware and Void Avoiding Routing Protocol (EAVARP) operates in two phases: layering and data collection phase [3]. The sensor nodes are distributed in concentric shells built during the layering phase around the sink. The protocol uses opportunistic directional forward strategy (ODFS) for forwarding the data in a data collection phase to avoid the flooding, cyclic transmissions and voids. The protocol extends the network lifetime through balancing the energy in the network compared to other routing protocols. To address the problem of void nodes and energy-reliability trade off, a Stateless Opportunistic Routing Protocol (SORP) is proposed [4]. The protocol performs a depth-based stateless routing which can avoid the trapped and void areas. It selects the candidate forwarding node through calculating holding time for each node in the forwarding area, using the local information acquired in updating phase from the neighboring nodes. SORP decreases the energy consumption, packet loss and end-to-end delay in all scenarios, sparse or dense. In UWSNs, the efficient data delivering is still a challenge because of limitations of acoustic communication and underwater conditions. To address the packet delivery problem, a hop-based protocol is proposed in Reference [5], known as H\({}^{n}\) - PERP.