## 5 Research Directions  
In the last few years, many cross-layer optimization approaches exploiting characteristics of multimedia coding have been proposed. Such works present a wealth of contributions to energy preservation, transmission rate adjustment, congestion control, error recovery and multipath selection. However, the problem of multimedia streaming in resource-constrained visual sensor networks is not yet completely solved, and new challenges are still arising. We discuss some research directions for cross-layer optimization based on multimedia coding. A natural consideration to guide future research is the way multimedia coding techniques might evolve. When analyzing the last twenty years we note a clear evolution line of audio, image and video coding techniques. In visual sensor networks, new codecs have been considered all the time, with different results in computational complexity, data compression and error resilience. Old standards have been further enhanced, while new approaches as distributed video coding and in-networking processing have been considered for visual sensor networks. Regarding the evolution of multimedia coding, it is worth wondering about what information is actually relevant for a given application. For example, the work in [69] investigates video compression using address-event representation, which intends to achieve frame-difference coding with low computational cost. Frame-difference data can be used to indicate motion of a moving target in VSNs. The authors argue that for some applications, information about the moving behavior of a target is more relevant than the visual information itself. Doing so, the identity of the target may be preserved, but the observer can still be able to understand the actions of the target. We must also outline that too few bytes are necessary to present frame-difference information, when compared even with grayscale images. The evolution of the coding approach can be even deeper. [PERSON] and [PERSON] [70] conducted a cross-layer performance evaluation of Compressed Sensing video streaming in WMSNs. Compressed sensing is a new paradigm that claims to allow the faithful recovery of information requiring fewer measurements than traditional sensing. It is shown that CS-encoded images exhibit an inherent resiliency to link errors, unlike JPEG images, due to unstructured image representation. Based on the works surveyed in the previous sections, we can expect the evolution of the coding and sensing paradigms will influence the cross-layer optimization in VSNs. But there are other issues that should be properly considered. The work in [13] defines a tracking system for wireless visual sensor networks, where there are multiple mobile sinks. This potentially changes the way protocols perform path selection, congestion control and error recovery, as well as the coding strategies for multimedia streaming. [PERSON] _et al._[68] consider sensors endowed with an ultrawideband (UWB) transceiver. Although UWB may considerably increase the sensors costs, this physical layer technology allows transmission rates much higher than the ZigBee technology [2]. As UWB is claimed as the ideal physical layer technology for wireless multimedia sensor networks [5,6], cross-layer multimedia coding will have to consider the benefits and streaming challenges of _ad hoc_ sensor networks where intermediate nodes are interconnected by UWB-enabled links. The adopted MAC protocols may also influence the multimedia-based cross-layer design. For example, IEEE 802.11 technology is considered as the link-layer technology for [20,38,46,59] and IEEE 802.15.4 is used in [19]. We expect that those MAC protocols will be not employed in nearfuture real-world VSNS applications, mainly due to energy constraints of the sensor nodes. However, some energy-aware MAC protocols have been considered for wireless multimedia sensor networks, as T-MAC [71] in [30] and ET-MAC [72] in [43]. The nature of the wireless links may also impact the cross-layer architectures, but in a different way.