## 4 Proposed Energy-Efficient Relay Mechanism
### SR-Based Packet Relaying  
After deployment and initial configuration of a particular wireless image sensor network, packets carrying visual data retrieved from the monitored field will be transmitted from source nodes to the sink through the multihop ad hoc structure created by the intermediate nodes. We assume that in the initial configuration each source node will compute a SR(s) and that each transmitted data packet will include the SR(s) of the transmitting source \(s\) in its header, as expressed before. In usual transmission, each packet may be acknowledged by a 1-hop ACK message during the multihop communication. When a packet is successfully received and acknowledged by an intermediate node, it is typically forwarded to the next hop in the path toward the sink. When employing the proposed relaying mechanisms, packets may be silently dropped in order to save energy in the relaying node and throughout the remaining path. Although monitoring quality may be somehow negatively impacted, dropping packets at intermediate nodes may turn the network active for a longer time. We propose a threshold-based dropping scheme where the current energy level of the relaying nodes (referred as _e_) indicates which packets can flow over the network. The SR(s) included in each data packet defines the packet's priority, which is checked by relaying nodes when deciding if packets will be forwarded or silently dropped. Adopting energy thresholds and a SR-based dropping scheme creates an adaptive behavior of the network, potentially enlarging the network lifetime. Three different energy thresholds are defined: _e1_, _e2_ and _e3_. Each energy threshold will be associated with one or more groups of relevance, indicating which packets must be forwarded to the next hop, as described in Table 3. When \(e\) is below one of the thresholds, only a subset of packets will be relayed, while the remaining packets are silently dropped. Considering that in the initial state of the \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Energy Level**} & \multicolumn{2}{c}{**Packets that MUST be relayed to the next hop**} \\ \cline{2-3}  & **Group of Relevance** & **SR** \\ \hline \(e\geq e_{i}\) & All packets & 1–15 \\ \(e_{2}\leq e<e_{1}\) & Medium, high and maximum relevance packets & 5–15 \\ \(e_{3}\leq e<e_{2}\) & High and maximum relevance packets & 11–15 \\ \(e<e_{3}\) & Maximum relevance packets & 15 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Energy thresholds for the SR-based relaying approach. network all intermediate nodes have a maximum energy level (\(e=1\)) and that intermediate nodes become inoperative when they run out of energy (\(e=0\)), we expect that \(0\leq e_{s}\leq e_{2}\leq e_{t}\leq 1\). All incoming packets will be forwarded to the next hop when \(e\geq e_{I}\). Low relevance packets are the first to be dropped, since they will not be forwarded when \(e<e_{I}\). Following this transmission scheme, packets from the most relevant source nodes are the last to be discarded, increasing theirs probability of successfully reaching the sink. As application monitoring quality is directly related to the sensing relevancies of the nodes, the proposed relaying mechanism is expected to have a reduced impact on the monitoring quality when compared with optimization algorithms based only on the relevancies of packets' payloads, as in [9]. Figure 2 shows two examples of packet relaying following the proposed approach. Consider that each square represents a data packet from a source node \(s\) with SR\({}_{\text{(s)}}\) as represented by the indicated number. The energy thresholds and the current energy level of a hypothetical intermediate node are graphically presented in a vertical bar graduated from 0 to 1. Based on the current energy level and the values of \(e_{I}\), \(e_{2}\) and \(e_{3}\), only a subset of data packets is allowed to be relayed to the next hop. It must be noticed that each intermediate node takes the decision of relaying or dropping based only on its residual energy, in an open-loop manner.