# Retransmission Avoidance for Reliable Data Delivery in Underwater WSNs
## 1 Introduction  
Recent progress in underwater wireless sensor networks (UWSNs) has procured much attention and it encompasses an extensive range of applications like pollution monitoring, coastline surveillance, disaster prevention, oceanographic data collection, marine life monitoring, etc. [1]. UWSN is composed of sensor nodes deployed in the deep water which forward data to sink(s) situated off shore. Radio and optical waves used in terrestrial communication cannot be used in underwater environment because they get absorbed or scattered rapidly. Therefore, the acoustic signals emerge as a suitable choice due to low absorption rate. Acoustic signals face high end-to-end delay because of low propagation speed, whereas, multipath fading, doppler effect, path loss, noises like shipping, wind, thermal, etc. cause high bit error rate (_BER_) resulting in low reliability and high energy consumption [2]. Acoustic signals are stronger near the source and get impaired from noises as they propagate away. Noise intensity in shallow water is high as compared to deep water. Therefore, less number of different paths are required in deep water to reduce the energy consumption. Sensor nodes are costly and equipped with limited batteries which are difficult to replace. Therefore, energy efficiency, maximum data gathering, reliable transfer and low delay are the objectives of protocol designing. Some multipath routing protocols [3; 4] have been proposed in recent years to achieve reliability. Data packets are forwarded linearly through different paths towards the sink(s) situated at the surface of the water. If these copies, which are less in number, somehow survived energy holes, there are high noises in shallow water which make them erroneous resulting in data packet drop at the sink and high energy consumption. Data packets are also forwarded by following tree structure in which every sensor node multicasts the data packet upon reception in its transmission range and becomes the parent while, the receivers further multicast data packet and make subtrees. The reception of multiple copies at the sink increases the chances of uncorrupted data packet reception. However, a high number of copies increases collision and energy consumption which is unbearable with the availability of limited node battery. Authors of layered multipath power control (LMPC) [5] send multiple copies of a data packet by following binary tree structure packet. These copies are directed through different paths which faces low noise to achieve reliability. They are combined at the surface to generate original data packets. LMPC generates binary tree from every source node resulting in high energy utilization in dense network. Sparse regions created by random deployment or death of sensor nodes result in data packet drop because of void holes (unsensed zone) in LMPC. In this paper, we propose forward layered multipath power control-one (FLMPC-One) and FLMPC-Two routing protocols to forward the data packets towards the surface through multiple paths with the aim of eliminating retransmissions and avoiding void holes. Retransmission increases delay and energy consumption which effects the performance of time critical scenarios. In order to reduce the energy dissipation, the network field is divided into multiple heterogenous sized layers. From each layer, the source node unicasts the data packet which resides near the layer, whereas, node on the layer known as cross node generates binary tree upon reception of the data packet by using IP multicast technology. Current forwarder in FLMPC-One forwards data when it is confident about finding neighbors up till two hops. While, FLMPC-Two uses three hops neighbor information. The goal of FLMPC-One and FLMPC-Two is to achieve energy efficiency and reliability of the data packet. The main contributions are mentioned as follow:  
* Binary tree establishment from the sensor node lies on the layer nearest to the source node which results in less energy consumption.