# Pido: Predictive Delay Optimization for Intertidal Wireless Sensor Networks
## 2 Related Works and Contributions  
The related work can be classified into three categories: link estimation metrics, routing protocols in the WSN and delay optimization solutions. Choosing the path with good \"quality\" for packets delivering is fundamental for routing protocols in WSNs. Different requirements in applications have variant definitions about good quality. During the past decades, significant research has been conducted to estimate quality on different dimensions. Hardware based estimation methods select RSSI (received signal strength indicator) [13], SNR (signal noise ratio) [14] and LQI (link quality indicator) [15] as important indicators of link quality. Utilizing hardware based indicators is overhead free; however, the measurement of RSSI cannot reflect the condition of link quality in real time and may fail to select the best path in real systems. Software based link quality methods like CTP (collection tree protocol) [16] and 4-bit [17] search the path with least retransmission times in real time. CTP [16] exploits link quality by calculating packet delivery ratio bi-directionally, and 4-bit [17] provides a comprehensive method by combining quality of link layer, network layer and physical layer. These two methods are user friendly on implementation and wildly used in real WSNs. Besides CTP, other link quality metrics like ETOP (Expected number of Transmissions On a Path) [18] and L-NT in [19] are proposed to select route wisely. However, these link metrics emphasize link connection individually and do not consider the situation of nodes, which may not fit in the requirement of IT-WSNs. Besides link quality, energy-awareness becomes another important requirement in most WSNs. Sensors in WSNs are battery-powered, which makes energy become a valuable resource, and routing protocols such as EAP (Energy-Aware Routing) [20] and REAR (Reliable Energy Aware Routing) [21] are proposed to provide energy-efficient routing services. EAP provides a new clustering parameter for cluster head election and can balance the energy load among all nodes. REAR provides a reliable transmission environment for data delivery in an energy efficient way by introducing local node selection, path reservation and path request broadcasting delay. These routing protocols are focusing on energy consumption optimization. In IT-WSN, nodes consume less energy during the under-water phase. Users care more about the real-time performance, which cannot be solved with EAP and REAR. Delay optimization solutions are proposed to decrease system latency. For example, EDAL (Energy-efficient Delay-Aware Lifetime balancing data collection protocol) [8] generates routes with minimal total path cost among all source nodes, under the constraints of delay requirements. The global optimization mechanism in EDAL is not suited for intertidal environments, where nodes in the IT-WSN face variant routing problems. BCMN/A (Broadcasting Combined with Multi-NACK/ACK) [9] proposes a mechanism to reduce the energy consumption and delays during the broadcast procedure. In the IT-WSN, we focus on reducing the end-to-end delay of data transmission, and BCMN/A is inapplicable. [PERSON] et al. [10] proposed ORW (Opportunistic Routing in WSNs), which introduces EDC (Expected Duty Cycled wakeups) as a metric for routing selection. However, the delay caused by the node condition is not considered in ORW, which may not suit for the intertidal WSN. There are also several delay-constrained routing protocols specially designed for intermittent WSNs. DutyCon (Dynamic duty cycle Control scheme) [7] introduces a self-adaptive duty cycle control approach and decomposes end-to-end delay into single-hop parts based on well-established feedback control theory. BCP (Backpressure Collection protocol) [22] presents a dynamic backpressure routing in wireless networks and uses floating LIFO (last-in-first-out) queues to decrease the end-to-end delay.