# Opportunistic Multipath Routing in Long-Hop Wireless Sensor Networks
## 2 Related Works
As it could only increase the link reliability, AOR may suffer from degradation of the packet delivery ratio under long-hop networks. Reliability and real-time requirements based on opportunistic routing (RTGOR) [13] have also been proposed. RTGOR exploits the expected delivery reliability and transmission delay as the criterion. Similar to AOR, the node has a higher expected delivery reliability compared with that of a certain criterion and a lower transmission delay of the packet could acquire the highest priority. Through the reliability metric, the intermediate node filters out neighbor nodes that cannot satisfy the criteria. The node then exploits the delay as the other criteria to sort the remaining neighbor nodes. RTGOR could satisfy the reliability and real-time requirements in a small-sized network. However, the packet delivery ratio of RTGOR also decreases when the network scale is increased. In the learning algorithm [19], a node constructs the best routing path through machine learning. In the fault tolerant reliable protocol (FTRP) [17], the nodes are in learning mode and broadcast a status of not being in a sensor domain in preparation to join one. If the node has not yet joined a domain and if it is not a member of the cluster, the node receives an answer from the sink node to be a cluster head. Otherwise, if the node receives an answer from some cluster head, the node will join the cluster. After constructing a cluster, the member node delegates the packet to its designated cluster head. The cluster head stores the packet until it receives an acknowledgement (ACK) message from a sink node. When it receives an ACK message for its previously sent packet, the packet is considered lost and retransmitted to ensure reliability. However, since it ensures reliability between the cluster head and the sink node by retransmission, it is hard to ensure the packet delivery ratio when the network scale is increased. This phenomenon would cause a buffer overflow on the cluster head because of the manystored packets retransmitted. The Energy-efficient algorithm for reliable routing (RRDLA) [18] considers the dynamics of links in finding a path from a source node to a destination by the QoS constraints such as end-to-end reliability. In the learning process, all of the paths from source to destination are evaluated. The source node finds the best path after finishing the process. The source node learns the best path and transmits the packet through the best path in the transmission process. The RRDLA could find the best path from the source node to the destination under QoS constraints through the learning algorithm. However, as the RRDLA exploits only one path to transmit the packet, it suffers from a degradation of the packet delivery ratio, even though the RRDLA selects the best path. That is, for a wireless sensor network in a multi-hop fashion, augmentation of end-to-end distance leads to an increased hop count. As a result, the augmentation of distance causes a degradation of the packet delivery ratio. Table 1 summarizes the key features with a comparison of the reliability, energy consumption, and scalability of the related studies. MMSPEED and RGMR could satisfy the packet delivery ratio by increasing the number of paths according to the requirement. However, other protocols have difficulties in achieving the required packet delivery ratio because they exploit a static number of paths. As the energy consumption is heavily influenced by the number of paths, MMSPEED and RGMR consume more energy than the other protocols. Finally, with respect to the scalability, most studies cannot achieve the required packet delivery ratio because the overhead is proportional to the number of hops. In Section 3, a proposed scheme is described that satisfies the packet delivery ratio with a lower energy under long-hop sensor networks.