## 3 Two-Tier Camera-Based WMSNs Topology
### Proposed Lower-Tier Structure: Wireless Network of Sensor Nodes Equipped with Cameras  
Lower-tier is constructed of sensor nodes that are in contact with the physical environment. Therefore, in order to achieve practical multimedia applications in WMSNs, sensor nodes in this tier are equipped with cameras. Cyclops low-power camera sensor [17], CMUCam3 [19], Mesh-Eye [20], and Panoptes [21] are practical instances that can be utilized as camera nodes in this tier.  
Figure 1: a) Symbolic figure of proposed upper-tier structure at the beginning. b) Symbolic figure of proposed upper-tier structure after clustering and routing.  
The task of each camera node is to capture still images and transmit them to its cluster head in the upper-tier. The connection between lower-tier and upper-tier is single hop and there is no interactivity between camera nodes. In order to have maximum coverage of environment, the number of camera nodes in this tier is high; therefore, the fault tolerance increases because if one of camera sensors fails, each part of area will be covered by another camera. Also at first, all camera sensors are placed by network manager in a way that their density would be uniform and the field is covered completely. In addition, the adjacent cameras are placed in almost same directions; it means that images of camera sensors which are nearer to each other have more overlaps and can be stitched to each other. Figure 2 a,b shows the placement of indoor and outdoor applications. As it is depicted in these figures, in some applications sensor are placed in a flat area while they have a 3-D placement in some others.  
Each camera node covers a small section of environment, while the application purpose is to get high-resolution images that cover large areas. Hence, the nodes of this tier must be clustered based on their location to let the cluster head perform this task properly by stitching the images of each cluster. The acquired high-resolution image of each cluster head is independent from the other one and Sink requests it by sending a query to the corresponding cluster head. Supposing that nodes are placed with uniform density in this tier, if the number of node members in each cluster is equal, the areas which are covered by each cluster will be roughly the same size. As mentioned in subsection 3.1, the location of cluster heads in upper-tier are uniform; so, if every camera node which can connect to more than one cluster head selects the cluster head with fewer members, then the lower-tier clusters will automatically have almost equal member counts. To realize the mentioned goal, one possible algorithm used in this paper is Minimal Cardinality Variance Clustering (MSVC) [30]. In this algorithm, camera nodes are aware of their options for connecting to upper-tier cluster heads. Each camera node opts to connect to the cluster head which has the least number of child nodes. Obviously, if there is only one choice for connection, the camera node will connect to that cluster head. With regards to the structure of upper-tier and lower-tier, the overall view of two-tier heterogeneous sensor networks topology and its inter-tier or intra-tier communication is depicted in Figure 3.  
Figure 2: a) Outdoor placement of a cluster. b) Indoor placement of a cluster.