## 4 Proposed Energy-Efficient Relay Mechanism
### SR and DWT-Based Packet Relaying  
Besides exploitation of sensing relevancies for energy-efficient packet relaying, visual data payloads may be also considered when defining the packets' priorities. We consider that camera-enabled  
Figure 2: Threshold-based packet relaying regarding the sensing relevancies of source nodes. source nodes are transmitting still images retrieved from the monitored field and that each image is processed by a 2D DWT before transmission. Wavelet transforms provide data decomposition in multiple levels of resolution, where DWT is achieved discretely sampling the wavelets. DWT decomposes a signal by passing it through two filters: a lowpass filter L and a highpass filter H. Digital images are two-dimensional signals, usually represented as a matrix of pixels. A 1-level 2D DWT processes such signal considering rows and columns, generating four subbands: LL, LH, HL and HH. The LL subband represents the lowest resolution and a half-sized version of the original image. In fact, it is the most significant information for the decoding process, while the remaining subbands contains vertical, horizontal and diagonal details for the decoding process. Such processing produces two groups of relevance, but LL subband can be transformed again to generate more levels of resolution. Figure 3 presents an original image that is processed by a DWT applied once and twice, resulting in two and three levels of resolution, respectively from left to right. We propose a SR and DWT-based packet relaying approach, where data relevance is considered as a second level of packet's priority. Images captured from the monitored field are encoded by a 1-level 2D DWT and transmitted (fragmented in many packets) to the sink of the network. When incoming data packets have to be forwarded to the next hop by a particular intermediate node that implements the proposed approach, the residual energy level of this node is compared with the three pre-defined energy thresholds to select packets that will be relayed according to the value of SR\({}_{(s)}\), in the same way as presented in Section 4.1. However, if packets within a range of SR are selected to be silently dropped, packets carrying the LL subband are always preserved and forwarded to the next hop. Doing so, it is ensured that low-quality playable images are still received by the sink even from the least  
Figure 3: Discrete Wavelet Transform (DWT) processing generating two and three levels of resolution. sensing-relevant source nodes. We can roughly expect that 25% of data packets generated for a single image after a 1-level 2D DWT are transmitting the LL subband. Intermediate nodes need to know the type of packets' payloads, in order to perform the appropriate action. A simple way to provide such information is to include a numerical index in the packets' headers. A 4-bit Data Relevance (DR) numerical value could be inserted just after the SR, as defined in Table 4. A 4-bit DR allows 16 different configurations, which could be extended to represent up to a 4-level 2D DWT (15 subbands). The value of DR represents a local QoS parameter, since it is generated exclusively by the transmitting source node and its significance is restricted to the transmission flows it belongs to. In such way, putting together SR and DR creates global and local scopes of relevance, which may be exploited for network optimizations with high accordance to applications monitoring requirements and to the encoding relevance of parts of transmitted images [18,19]. Figure 4 shows two examples of packet relaying following the proposed approach, where DWT coding is also considered to define the packets' priorities. Similarly to the scenario represented in Figure 2, sensing relevancies are indicated by numerical values inside the squares (packets). Additionally, some packets carry visual data of LL subbands from transmitted images, and those packets will always be relayed to the next hop.