## 6 Conclusions  
In this paper we have proposed a DWT-based energy-efficient selective retransmission mechanism for wireless image sensor networks, where packets are transmitted with different reliability guarantees according to the relevancies of the packets' payloads for the reconstruction of the original image at the receiver side. We designed comprehensive energy consumption models and performed extensive mathematical verifications, estimating the average energy consumption and the resulted image quality when some less relevant packets are lost.  
When applications can afford some loss in the image quality, it was showed that the proposed solution performs better than traditional fully-reliable transmission mechanisms, achieving considerable energy saving in intermediate nodes. Moreover, we noted that the visual quality of the received images may be highly acceptable in different error conditions, especially for the 1-level 2D DWT based selective retransmission approach. In fact, the conclusions are based on the average cases,  
Figure 15: Cont.  
which although not sufficient to attest the effectiveness of the proposed solution, present an average behavior that can be expected in most cases along the time. The performed mathematical verifications have brought numeric values that indicated better options in average, turning the proposed solution promising for wireless image sensor networks. Nevertheless, future works will be concerned with experimental verifications in a discrete event simulator featuring IEEE 802.15.4 MAC technology, in order to allow analyzes of punctual values of error rate, retransmission attempts and energy consumption in more realist communication scenarios. In fact, transmissions using duty-cycle contention-based protocols are controlled by messages as RTS and CTS and generally synchronization messages are transmitted among the nodes to optimize the sleeping time, adding new issues to the energy consumption estimation.  
Image transmissions over wireless sensor networks may be challenging when resource-constrained sensor nodes are deployed. The huge amount of information that has to cross the network considerably increases the energy consumption over the network, when compared with traditional scalar wireless sensor networks. A reasonable solution to address such particular issue is to compress data before transmission [29, 31, 32]. Future works will investigate how compression strategies can be combined with the proposed solution: if DWT is applied over original images, the resulted subbands could be compressed to reduce the number of packets to be transmitted, but still allowing differentiated transmission services according to the value of DR. We expect that the adoption of compression algorithms can reduce the overall energy consumption of the network, but the proportional gains of the proposed solution over fully-reliable transmission approaches should remain if packets can still be classified according to the payload relevance for the reconstruction of the original images.  
Moreover, we envisage that the proposed selective retransmission mechanism can be employed as part of a global solution, addressing issues as network congestion and node failure. Dynamic adaptation of parameters of the retransmission procedures can be performed according to the current network conditions. At last, although it is expected that many wireless image sensor network applications are delay-tolerant, we intend to investigate the potential benefits of the proposed selective retransmission mechanism for the end-to-end delay of the communication. As the use of ACK messages for acknowledgment and retransmission procedures demand additional time, the proposed solution may reduce the overall delay of the communication, potentially benefiting real-time WISN applications.