# Reliable Link Level Routing Algorithm in Pipeline Monitoring Using Implicit Acknowledgements
###### Abstract  
End-to-end reliability for Wireless Sensor Network communications is usually provided by upper stack layers. Furthermore, most of the studies have been related to star, mesh, and tree topologies. However, they rarely consider the requirements of the multi-hop linear wireless sensor networks, with thousands of nodes, which are universally used for monitoring applications. Therefore, they are characterized by long delays and high energy consumption. In this paper, we propose an energy efficient link level routing algorithm that provides end-to-end reliability into multi-hop wireless sensor networks with a linear structure. The algorithm uses implicit acknowledgement to provide reliability and connectivity with energy efficiency, low latency, and fault tolerance in linear wireless sensor networks. The proposal is validated through tests with real hardware. The energy consumption and the delay are also mathematically modeled and analyzed. The test results show that our algorithm decreases the energy consumption and minimizes the delays when compared with other proposals that also apply the explicit knowledge technique and routing protocols with explicit confirmations, maintaining the same characteristics in terms of reliability and connectivity. accs: protocols; industrial communication; wireless sensor networks; linear networks 2021  
122018  
Different proposals have been developed to optimize the power consumption of the nodes and minimize delays. Many studies have focused on minimizing energy consumption during transmission and reception [7], while others have focused on minimizing the processing of information in the sensor node [8], a task which can also increase the delay. In LWSNs, nodes may be placed in a thin (one dimension, forming a line) or thick (two or three dimensions around a line) configuration, and can have identical (one-level) or differentiated (multiple-level, creating a hierarchical network) functionalities [9]. In the applications such as pipeline monitoring, a thin one-level network is cheaper and simpler to build and maintain, this being the configuration considered in this paper to propose solutions for the minimization of energy consumption and delays (while maintaining reliability and connectivity) in LWSNs. Nevertheless, a direct transposition of classical protocols on LWSN is not efficient, as linear topologies are the worst cases for hierarchical networks (spoilage) and for stochastic distribution (time for reparation) [10]. Hence, routing protocols designed for nonlinear WSNs should not be directly applied to LWSNs with hundreds or thousands of nodes due to their special topology and application requirements [11]. For example, IEEE 802.15.4 nodes can use ACKs to confirm the reception of a frame. This technique is known as explicit confirmation (eACK), but the transmitting node can also receive an implicit confirmation (or iACK) when it receives the same information after the receiving node retransmit it to the next hop. By using implicit acknowledgements (iACKs), we can address two challenges: reducing energy consumption and latency. This technique helps to decrease the packet exchange between nodes, and to reduce the communication stack by removing most of the functions usually implemented at network level layer (OSI layer 3). This is possible because in a wireless network when a node transmits a frame, all the active nodes in the coverage area receive it and in addition, in a linear topology, the routing is very simple, as packets can only go in one of the two directions of the linear infrastructure, and nodes just have to decide if they have to retransmit a packet or not. Thus, networking functions that are typically implemented in a separated layer, can now be integrated within layer 2 (link) functionalities. The proposed algorithm for LWSN was validated through real-world testbeds and contributes to the following aspects:  
* Reliability of the transmission, using iACK. * Reduction of the delay time caused by using acknowledgement frames.