## 4 Multimedia-Based Cross-Layer Optimization in VSNs
### Video-Based Cross-Layer Optimization
#### 4.2.2 Multiple Description Coding  
Multiple description encoding [54,55] may potentially benefit wireless multimedia sensor networks. In recent years, such techniques have been exploited for multipath routing, especially when sensor nodes collect visual and audio data from the monitored field. The context-aware multimedia-based cross-layer optimization scheme proposed in [12] exploits multipath routing along with the relevance of the encoded data for efficient path selection regarding the end-to-end communication delay. It is not properly specified a coding technique, but the proposed routing scheme is very suitable for multiple description coding. That work defines the MPMPS (Multi-Priority Multi-Path Selection) algorithm to find the paths with lower end-to-end delay for multimedia streaming in WMSNs, considering a set of available node-disjoint paths. Such paths are node-disjoint when they have no common intermediate nodes. The node-disjoint paths are discovered employing the Two-phase Geographic Greedy Forwarding (TPGF) algorithm [56], which finds the maximum number of optimal node-disjoint routing paths in terms of path length and the end-to-end transmission delay, potentially benefiting wireless multimedia sensor networks applications. The authors argue that traditional multipath routing protocols do not provide a powerful searching mechanism to find the multiple optimized routing paths and to bypassing holes [56,57]. Figure 2 shows a visual example of multiple node-disjoint paths from one source to the sink. Note that video data are split in two streams that flow over potentially lower-delay paths (four intermediate nodes). In that example, the unique audio stream flows over a path comprised of six intermediate nodes. The work presented in [12] proposes the splitting of the source stream into image and audio substreams, giving to each resulting substream a particular priority according to the current monitoring being performed. The paths with lower delay are assigned to the higher priority substreams, leaving the remaining paths to the lower priority substreams. The expected delay is measured by the number of intermediate nodes of the paths (the less is the number of intermediate nodes, lower end-to-end delay is achieved). The authors of [12] cite an interesting communication scenario for the proposed solution. In fire monitoring, visual information is more relevant for the application and should be delivered with minimum transmission delay. The audio stream could be transmitted over the remaining paths, complementing the received visual data. In practice, as some available paths may have higher transmission delays than the time constraint of the application, they are not considered by traditional single description coding applications (audio and image together). On the other hand, even available paths with high delays could be used by the application for transmission of the lower-priority substream in [12], maximizing the attainable communication throughput. The concept of context-aware multimedia-based cross-layer optimization presented in [12] is further investigated in [57]. In both works, the original data (72 kbps) is split into an image stream (48 kbps) and an audio stream (24 kbps). A generic treatment of multistream multipath transmission is given in [58]. [PERSON] _et al._[59] exploit multiple descriptions coding for multipath-selection. However, while TPGF was employed in [12, 57] to create multiple node-disjoint paths, the work in [59] extended the Direct Diffusion protocol [60] to discover multiple node-disjoint paths. This is in fact the main contribution of [59], since MDC is employed just to assess the performance of the proposed multipath routing protocol. The work presented in [46] also combines MDC with multipath transport. However, in a different way from [12], the proposed scheme continuously monitors the path to decide the number of substreams that have to be created.