# A Two-Stage Routing Protocol for Partitioned Underwater Wireless Sensor Networks
## 1 Introduction  
Due to the rapid growth of technology in the last fifty years or so, science has ventured to explore and exploit previously unknown resources such as oceans and other water bodies that constitute more than 70% of the planet earth. Serious efforts have been made to tap the potentials of the world's waters in the fields including but not limited to mineral exploration, energy harvesting, defense and surveillance, communication, and underwater agriculture. Tools are being developed to support these efforts. Underwater wireless sensor networks (UWSNs) is one such tool which can provide basic technical support for gathering, organizing, and reporting mission-critical data [1]. UWSNs share some common characteristics with their terrestrial counterpart, which include limited energy, large-scale deployment, mobility, uncertain topology, etc. Nevertheless, UWSNs are faced with many issues that are alien to terrestrial sensor networks [2; 3; 4]. For instance, radio wave communication, which is prevalent in terrestrial sensor network, is not a suitable choice for underwater environments where acoustic waves are considered a better alternative [5; 6]. The acoustic channel has certain limitations, such as high error probability, multiple sources of noise, geometrical and cylindrical spreading, more pronounced Doppler's effect, etc. Moreover, nodes in underwater environments are prone to mobility with currents, resulting in an inconsistent network topology that may result in network partitioning and degradation in performance. Routing protocols for UWSNs have been extensively researched [7; 8; 9; 10; 11; 12]. Many protocols have been proposed for fixed networks and for networks with constrained mobility with water currents. One of the most important performance metrics addressed by these routing schemes is packet delivery ratio. The packet delivery ratio can be hampered by many factors such as, connectivity hole creation due to early death of nodes located near the sink nodes, adverse channel conditions and network partitioning. Many existing routing protocols propose solutions for energy management and connectivity hole avoidance. However, as per our knowledge, there has been no considerable effort to tackle network partitioning that may result due to free mobility of sensor nodes within the underwater streams. Network partitioning occurs when a certain number of nodes move out of the range of the nodes in the main body of the network and consequently cannot receive neighborhood information which is a critical building block for any routing protocol. Figure 1 shows the network architecture used by many of the existing routing protocols. The sink nodes initiate neighborhood information collection phase by broadcasting beacons. Based on the beacon, the receiving nodes record their distance from the sink nodes in terms of hop count. The receiving nodes update the beacon with information such as their hop count to the sink node and their residual energy. The updated beacon is retransmitted for the nodes further down the network which repeat the same process. The collected information is used by nodes in the data transmission phase to select a next hop node that will maximize the packet delivery ratio and improve energy efficiency. This architecture works effectively for networks with static or limitedly mobile nodes. However, in case of free mobility of nodes, the architecture will achieve very low packet delivery ratios as the sensor nodes move away from the static sink nodes and thus cannot receive beacons. These shortcomings dictate the need for network architecture and routing schemes which can efficiently tackle the above mentioned issues related to mobile UWSNs. In this work we propose a suitable network architecture and a routing scheme that can handle partitioning in mobile UWSNs and thereby improve network performance by delivering higher volumes of traffic to the sink node. Our scheme consists of two phases. In phase 1, neighborhood information is disseminated in the network using beacons.