# Reliable Link Level Routing Algorithm in Pipeline Monitoring Using Implicit Acknowledgements
## 2 Related Work  
WSNs have been widely studied, and there are different protocols that have been designed to support a wide range of network topologies (star, mesh, etc. ), for example, 6 LowPAN and ZigBee, which can use IEEE 802.15.4 for the lower layers. Nevertheless, linear networks have very specific requirements which are not well addressed by generic protocols, as shown in [12], where the authors review and compare different aspects of routing protocols (addressing policies, discovery mechanisms, routing, etc.) designed specifically for LWSNs or for more traditional WSNs, by analyzing metrics such as end-to-end delay, reliability, or deployment requirements. Therefore, we can conclude that LWSNsrequire specifically designed communication stacks. In this sense, there are different proposals for new MAC layer protocols for LWSN. LC-MAC [13] is a long-chain MAC protocol where the forwarding nodes schedule the transmission of packets to reduce the delay of an end-to-end extended sensor network without sacrificing energy efficiency. CH-MAC, a delay-aware MAC protocol with implicit control frame Request to Send (RTS) employed in the medium access control, is proposed and compared with eACK in [14]. The authors conclude that it prevents the \"avalanche\" effect, and the use of iACK optimizes the use of energy. In [15] a token-based MAC protocol for linear sensor networks has been proposed to manage the access to the medium and improve the network performance. Also, many protocols have been presented to optimize the energy consumption of large WSNs. For example, in [16] MI-MAC, a new link level protocol, is proposed. Finally, reference [17] includes an analysis and comparison of different MAC protocols for linear LWSN such as Dual mode MAC, Disc-MAC, LC-MAC, MFT_MAC, CMAC-T, AC_MAC/DPM, and WiWi protocols. Different routing protocols have also been suggested to meet the specific LWSN needs. In [10] a robust discovery, addressing and routing protocol for dynamic LWSN is proposed. The application of LWSN to rail transport is considered in [18], and energy efficient routing protocols for linear wireless sensor networks are also studied. The authors recommend two energy efficient routing protocols, Minimum Energy Relay Routing (MERR) and Adaptive MERR (AMERR). A Chain-based Anonymous Routing (CAR) unicast-based broadcast to send data is studied in [19]. In this proposal, each node can only sense the adjacent nodes on the same chain, and the traceability of the system has been enhanced; thereby increasing network security. In [20] a new routing algorithm is proposed for wireless sensor networks for post-disaster road monitoring. For this approach, the nodes are arranged in a linear topology. It includes route establishment, packet forwarding, and route maintenance. This proposal assumes that the nodes are previously identified in sequence. Finally, the AODV Routing Algorithm in Linear Topological Wireless Sensor is studied in [21]. Transport and application protocols have been also developed for WSNs to guarantee reliability, flow control, and congestion control. However, large-scale networks require a complex implementation complicated by the limited processing and power resources of the nodes. For pipeline monitoring the main issue is reliability, since controlling the monitoring period helps to minimize other problems such as congestion or flow control. For example, a comparative analysis of the transport and application protocols CODA, ESTR, RMST, PSFQ, GARUDA, and ATP, proposed for large-scale WSN networks, is presented in [22]. The comparison includes the use of end-to-end ACK and NACK, but the studied protocols do not use eACK or iACK at the link level. The Rate-Controlled Reliable Transport (RCRT) protocol is presented in [23]. It uses a base station or receiver to discover missing packets and explicitly requests the packets from the sensor nodes for end-to-end recovery.