# Opportunistic Multipath Routing in Long-Hop Wireless Sensor Networks
## 3 Energy-Efficient Reliable Multipath Routing Protocol Using Opportunistic Routing
### Overview  
In this section, we present the basic operation mechanism of the proposed routing protocol. The nodes are deployed over a field, and they could know a time \(T\) that has elapsed since the operation of a node began. Each node is aware of its own location by GPS or other techniques and knows the location of the destination. Moreover, each node keeps information about the location of its one-hop neighbor nodes by beaconing. The beacon interval \(T_{B}\) is predetermined before network deployment according to the requirement of the applications. As shown in Figure 1, a packet from a source node is transferred to the destination performing path branching and merging on the intermediate nodes. The source and each intermediate node predict the packet delivery ratio from itself to the destination with opportunistic routing, firstly. If the predicted packet delivery ratio is lower than the required packet delivery ratio, the node calculates the number of paths and performs path bifurcation to satisfy the required packet delivery ratio (see more details in Section 3.2). In Figure 1, the nodes with diagonals perform the path bifurcation. If the node could satisfy the required packet delivery ratio without path bifurcation, the node transmits the packet based on opportunistic routing only (a white node on the transport path in Figure 1). As the nodes receive packets through opportunistic routing, they should compete to determine the transmitting node. In this process, each node starts a timer according to the distance between itself to destination. The shorter the distance, the shorter the duration of the timer. The node where the timer is expired transmits a packet, and its competitors quit the packet transmission process if a sufficient number of paths have been constructed (see more details in Section 3.3). Unfortunately, as transfers are by opportunistic routing, the path merger problem could occur (a black node on the transport path in Figure 1). In this case, assigned packet delivery ratios on each path would be merged and it leads to degradation of the overall packet delivery ratio required by the source node. To address this issue, the node aggregates the assigned packet delivery ratio from each parent node and exploits the aggregated packet delivery ratio when it transmit the packet to its destination (see more details in Section 3.4).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Category**} & **Routing** & \multirow{2}{*}{**Key Features**} & \multirow{2}{*}{**Reliability**} & **Energy** & **Scalability** \\  & **Protocols** & & & **Consumption** & **(Long-Hop)** \\ \hline Opportunistic & \multirow{2}{*}{AOR} & Preventing energy hole & Medium & Low-Medium & Low \\ Routing & & Supporting real-time & & & \\ RTGOR & & transmission & & & \\ \hline \multirow{3}{*}{Learning Algorithm} & \multirow{3}{*}{FTRP} & Node logical clustering & \multirow{3}{*}{Medium-High} & \multirow{3}{*}{Low-Medium} & \multirow{3}{*}{Low} \\  & & with learning phase for reliability & & & \\ \cline{1-1}  & & Reliability based on & & & \\ \cline{1-1}  & & learning algorithm and & & & \\ \cline{1-1}  & & low energy consumption & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._