## 2 Related Works  
While most real-time multimedia communications on Internet backbones are delay-intolerant and can generally afford some packet loss, reliability is more critical to image transmissions over WSNs than delay. Usually, errors during packet transmission over hop-by-hop wireless links may happen when network face congestion (resulting in packet discarding in congested nodes), when intermediate nodes fail or run out of energy or even due to signal attenuation and interference. To cope with such errors, many works have proposed MAC and transport protocols to recover lost packets/data in wireless sensor networks, where retransmission is the most usual approach. Retransmission can be performed in an end-to-end or hop-by-hop fashion. End-to-end retransmission is easier to implement but can rapidly deplete the energy resources of the used paths as the number of intermediate nodes increases. A hop-by-hop retransmission mechanism consumes less energy of the network, but demands that intermediate nodes cache the transmitted packets until the transmission is successful. Although this additional complexity may count against hop-by-hop retransmissions, it is argued to be the best option for source nodes that transmit a large amount of data, as in video-based wireless sensor networks [4, 11]. An end-to-end retransmission mechanism for WSN applications is proposed in [12]. The sink verifies the sequence number of received packets searching for some packet loss. If a gap in the sequence number is found, an explicit request is sent to the source node requesting retransmission of the lost packet. In a different way, in [13] a transport protocol is proposed that performs in-network caching of transmitted packets for hop-by-hop retransmission. In that work, packet loss is identified by proper timers enabled in intermediate nodes and notifications are performed by explicit NACK messages sent to the next hop on the reverse reinforced path toward the source. Moreover, the work in [14] employs multiple redundant paths as well as hop-by-hop retransmission for increased reliability, where congestion is also mitigated dynamically choosing appropriate transmission paths. Although very advantageous for VWSNs, all packets have the same reliability level in those proposed hop-by-hop retransmission mechanisms. Alternative solutions to provide reliability guarantees have also been proposed in recent years. In [15], authors propose image transmission over sensor networks where intermediate nodes perform error recovery based on redundancy and correction codecs. As retransmissions may result in additional end-to-end delay, the authors propose correction mechanisms based on the transmission of redundant packets through multiple paths and the use of Forward Error Correction (FEC) codes, which are computed by each intermediate node instead of only in the source node and in the sink. However, the reduced end-to-end delay when compared with retransmission-based approaches also result in additional energy consumption, since more paths (and intermediate nodes) are necessary to transmit redundant packets, besides the energy consumption and required processing for computing the FEC code in intermediate nodes. In fact, we argue that energy is a more critical issue than delay for many WVSN applications. To the best of our knowledge, a mechanism for selective retransmission based on DWT coded images has not been proposed by the academic community [10]. However, some works propose other interesting cross-layer optimization solutions for DWT-based image transmission in wireless sensor networks, directly influencing our investigation. In [16] full reliable transmission is only required for high-priority data, since the remaining DWT subbands are transmitted in a semi reliable mode, where intermediate nodes silently discard low relevant packets if their residual energy is below a threshold. The expected outcome is the prolonging of the network lifetime, keeping an acceptable quality level for the received images.