Underwater Wireless Hybrid Sensor Networks Kashif Ali and Hossam Hassanein School of Computing, Queen's University Kingston, Ontario, Canada, K7L 3N6 {kashif, hossam} @cs.queensu.ca Abstract Underwater Sensor Networks (USNs) promise innovative and exciting applications, viz. oceanographic data collection, environment monitoring, exploration, and tactical surveillance. Underwater Wireless Sensor Networks (UWSNs) though pose significant research challenges due to the harsh underwater environment. In UWSNs, acoustic is thought to be the only vi- able means of communication. Underwater Wireless Acoustic Sensor Networks (UW-ASNs) present a wireless channel with key challenges, specifically in shallow oceans such as long propagation delays, signal attenuation, man-made and ambient noise, low bandwidth and high transmission energy. We pro- pose a new paradigm for UWSNs, namely Underwater Wire- less Hybrid Sensor Networks (UW-HSNs), which introduce the concept of hybrid communication. UW-HSNs combine the best of both worlds, i.e., the practicality of underwater acoustics and the high-peiformance of radio communication. The basic idea is to use radio communication for large and/or sustained traffic and traditional acoustic methods for small data volume. Furthermore, we introduce TurtleNet, an architecture based-on UW-HSNs concept, and we propose an asynchronous and dis- tributed routing protocol for TurtleNet. Based on the node's state, the protocol decides which communication channel to utilize. TurtleNet is simulated using the ns-2 simulator. Sim- ulation results reveal the promising peiformance for TurtleNet, and hence validate the UW-HSNs concept. 1. Introduction Current research [1,2,13,14] is focused on improving the UW- ASNs performance on all frontiers, from architecture to de- ployment strategies and in between e.g., routing, MAC, phys- icallayer, etc. The proposed solutions are capped by the lim- ited performance of acoustic communication due to its inher- ent physical limitation, Le., inadequate bandwidth (hard limit of 100KHz [9]), low physical propagation speed of acoustic waves, a shift of five magnitude slower (around 1.5 x 10 3 mls), compared to the radio (around 3 x 10 8 mis, high signal absorp- tion [10], noise level and immense error rate. The use of mo- bile data collectors for performance improvement is an active research area. Such an approach requires the availability of 978-1-4244-2703-1/08/$25.00 ©2008 IEEE specialized AUVs acting as data carriers [16]. This approach considerably increases network capacity, yield longer lifetime and cuts down the delays. However, it requires sophisticated AUVs and their scheduling, and has limited applicability sce- narios due to economic constraints. In this paper, we pioneer a new paradigm in underwater sensor networks, namely Underwater Wireless Hybrid Sensor Networks (UW-HSNs). UW-HSNs combine the best of both worlds, viz. the practicality of underwater acoustic with the high-performance of radio The intuition is to leverage the radio communication research from WSNs for the benefit of UWSNs. The basic UW-HSNs philosophy is to use radio for large and/or sustained traffic and acoustic for small data volumes. Because of limited radio propagation in water we envision the nodes of UW-HSNs having means to emerge to the ocean surface. This is achievable by employing sophis- ticated maneuvering techniques [6,11], or a simple buoyancy mechanism [4, 12].When a node is under-water it utilizes short- range multi-hop acoustic modem for small data transfer. Upon emerging to the surface the node uses using radio communica- tion. The philosophy is to incorporates mobility of underwater sensor nodes to increase the overall throughput of the network. We also propose TurtleNet, an architecture based on UW- HSNs concept. In TurtleNet, nodes submerge and emerge to the surface using a piston-based buoyancy mechanism [4]. Fur- thermore, we offer an asynchronous and distributed routing al- gorithm for TurtleNet. The algorithm, Turtle Distance Vector (TDV), is conceptually based on the distance vector approach, Le., calculating optimal routes with minimum information ex- change. Depending on the node's state, the algorithm decides on the communication channel with the goal of minimizing the events average delay. The event delay is the time duration be- tween its creation and successful reception at the base-station. TurtleNet, along with the TDV algorithm, is simulated using the ns-2 simulator. Simulation results substantiate the promised performance of TurtleNet and hence validate UW-HSNs con- cept. The remainder of the paper is organized as follows. Sec- tion 2 introduces the UW-HSNs concept. Section 3 describes TurtleNet, followed by the Turtle Distance Vector routing algo- rithm in Section 4. Section 5 provides simulation details and experimental observations. The related work is overviewed by 1166