## 5 Simulation Results
(**a**) \(e_{1}=0.9\), \(e_{2}=0.7\) and \(e_{3}=0.3\). (**b**) \(e_{1}=0.95\), \(e_{2}=0.8\) and \(e_{3}=0.7\). Figure 10 relates energy consumption with the energy thresholds \(e_{I}\), \(e_{2}\) and \(e_{3}\) for node n2 in Scenario 2, for the source nodes configurations SR-based(3) and SR-based(5) and 12 days of transmissions. We define \(e_{I}=0.95\) and \(e_{3}=0.05\), varying the value of \(e_{2}\). There is a tradeoff between energy saving and monitoring quality, when employing the proposed energy-efficient relaying mechanism. In fact, energy consumption is inversely proportional to the values of the energy thresholds. Higher values for them incur in earlier dropping of low-relevant packets, reducing energy consumption. Thus, when we achieve higher energy saving, the monitoring quality is somehow prejudiced, since low-relevant packets are dropped in initial stages of the network lifetime. We also assessed the energy consumption in node n2, when the SR-DWT-based approach is employed in Scenario 2. We presented in Figure 11 the energy consumption after 12 days of image transmissions. The SR-DWT-based approach consumes more energy than the SR-based relaying approach, for the same configuration of sensing relevancies, since at least data packets carrying LL subbands from all source nodes will reach the sink (and all packets from source nodes with SR = 15). However, higher monitoring quality is expected for the SR-DWT-based approach. In general words, if applications require low-quality versions of the images transmitted by all source nodes, the SR-DWT-based approach should be employed. One should also note that, as in previous experiments, the Data-relevance approach may not be the best energy-efficient relaying approach, depending on the sensing relevancies of the source nodes. Figure 12 presents a visual scheme for the received images at the sink side, but now considering the SR-DWT-based relaying approach. Note that the optimal monitoring configuration is achieved when the proposed SR-DWT-based relaying is employed. Although the Data-relevance relaying approach achieves some energy savings, the quality of all received images decreases along the time, uniformly prejudicing the quality for all visual source nodes. The expected energy savings when employing the proposed relaying approaches are significant. If source nodes are powered by two AA batteries providing 20,000 J or more, the sensor network lifetime could be 3 days longer than the traditional SR-unaware relaying paradigm, depending on source nodes and network configurations. However, the network monitoring quality is somehow harmed when comparing with traditional relaying. In fact, the inner characteristics of visual sensor networks impose the need for a new understanding of QoS guarantees and monitoring quality. We can expect that the visual monitoring quality depends on the compliance of the performed monitoring with the application requirements, and such requirements are defined by users. Thus, the monitoring quality is related with the Quality of Experience [35] of the deployed wireless image sensor network. We proposed the concept of Quality of Viewing (QoV) as a metric to assess the loss of monitoring quality when exploiting the sensing relevance concept for network optimizations [38], but it is also a subjective concept. Figure 12: Average quality of the images that reach the sink for different transmission configurations and also considering the SR-DWT-based relaying approach. If source nodes are strictly assigned to groups of relevance according to the monitoring requirements, we can achieve high monitoring quality even with some data losses. However, how to compute a quality index to be used for comparison purposes is still a complex issue, turning prohibitive a direct relation between energy preservation and final monitoring quality [38]. Since it can not be said that a maximum-relevant source node is twice or three times more important than a high-relevant source, a different notion of quality must be designed.