## 6 Visual Data Transmission in WVSNs
Their basic idea is to cluster the sensor nodes whose FoVs are overlapped and then the routing decision is based on cluster information and distance to the sink nodes. Since the FoV of the sensor nodes in the same cluster is overlapped, then relaying the visual data via the same cluster member with shorter distance to the sink nodes could reduce the visual information injected into the network. Ant-based WVSN routing protocols are proposed in [51, 52]. The basic idea of the ant-based routing protocols is to mimic the behavior of the ants searching for the food through pheromone deposition in the sense that future ants could locate food using the pheromones left behind by previous ants. By adopting this idea, ant-based routing protocols use reinforcement learning algorithms to locate the QoS aware [51] and energy efficient path [52] to the sink nodes. The reinforcement algorithm is to encourage the route that is close to the sink and discourage the routes that stray from the sink. Ant-based routing is a promising routing protocol in _ad hoc_ networks that it incorporates the good features of fully distributive, simple operations in the node, fault tolerant, adapt to changes in network topology and traffic demands [53]. We believe that ant-based routing protocols will also play an important role in resource limited WVSNs. However, ant-based routing protocols in WVSNs are still in the early stages and more research efforts are needed to tackle the problem of large bandwidth applications in limited resource WVSNs. Network coding is another promising technique to save the bandwidth transmission for routing in networks so that network coding could be a powerful routing scheme in bandwidth limited and energy limited WVSNs. The basic idea of network coding is to allow the mixing of the data at the intermediate nodes and then the original data could be extracted from these mixed data at the receivers. In [54], they propose a famous butterfly example to demonstrate that network coding could outperform routing. In Figure 3, we indicate this butterfly example in a WVSN. Two captured FoV datasets (i.e., FoV\({}_{1}\) and FoV\({}_{2}\)) from different camera sensor nodes are transmitted on the network to the sink nodes. If the capacity on each link can carry one FoV dataset, then traditional routing schemes would fail to deliver these two FoV datasets to the sink nodes because of the capacity limitations in the central link. Even if there is no capacity constraint, the central link still needs to transmit two visual datasets (i.e., FoV\({}_{1}\) and FoV\({}_{2}\)) so that the sink nodes could reconstruct the visual field. However, by using the network coding scheme, by transmitting the combined FoV data (i.e., FoV\({}_{1}\oplus\)FoV\({}_{2}\)) at the central link, the sink node is able to deduce the original two FoV datasets via simple XOR operations. For example, FoV\({}_{2}\) = FoV\({}_{1}\oplus\)FoV\({}_{1}\oplus\)FoV\({}_{2}\), then the left sink node can deduce the FoV\({}_{2}\) data from a simple XOR operation. Similar XOR operations could be performed at the right sink node to deduce the FoV\({}_{1}\) data. Since the XOR operation is a simple bitwise operation, the power consumption on XOR operation is much smaller than transmitting two visual data on the central link. Then the sink nodes could reconstruct the visual field with more bandwidth and in an energy efficient way. Future research works are encouraged to adopt the idea of network coding to transmit the visual data in WVSNs. In [55,56], the idea of network coding is adopted in their proposed routing schemes. In [56], a joint coding and routing scheme for WVSNs is proposed to optimize the tradeoff between network lifetime and video quality (distortion). First of all, they adopt the WZ-based multi-view video coding scheme [57] to encode the captured multi-view video streams into K or WZ frames. Then video quality could be measured by the video distortion of the received K and WZ frames.