# Sink-Type-Dependent Data-Gathering Frameworks in Wireless Sensor Networks: A Comparative Study
## 2 Data-Gathering Frameworks Depending on Sink Types
### Aerial Mobile Sink-Based Frameworks
#### 2.4.1 Node Density-Based Clustering and Mobile Collection (NDCMC)  
[PERSON] et al., proposed a mobile sink-based data-collection mechanism called NDCMC for WSNs [64]. NDMC employs a CH selection mechanism and a trajectory optimization algorithm for the mobile sink. This algorithm also aims to increase the lifetime of the WSN by reducing the transmission energy and by planning an optimum data-collectionpath for the mobile agent. NDCMC assumes the presence of GPS information for both the mobile agent and sensor nodes. In addition, this protocol assumes the presence of a static infrastructure, and that the sensors are also provided with information prior to deployment. To inform the presence of the mobile agent, a periodic beacon message is broadcast.  
Initially, all nodes broadcast \"hello\" messages by attaching their ID and positional information. In this way, all nodes obtain information about other deployed nodes. With the help of a geographic routing mechanism, all sensor nodes deliver their own information to the BS in a multi-hop fashion. Based on the received information, the BS elects the CHs from the densest zone and plans the tour of the mobile agent accordingly. NDCMC introduces another type of node, called a VH. Nodes that are inside the communication range of the mobile agent but are not CHs are called VHs. After electing the CHs and VHs, the mobile agent informs the entire WSN about a specific node's role. Upon receiving the information, the normal nodes locate their closest CH or VH. Figure 6 shows the working mechanism of the NDCMC protocol.  
Both CHs and VHs are responsible for gathering data. However, the trajectory of the UAV is only calculated based on the CHs, and this is the only difference between the VHs and CHs in NDCMC. In the NDCMC process, the CHs are mostly located at the center of a circular area. When the energy level of a CH falls below a threshold level, the CH becomes an ordinary node and broadcasts the message to the local nodes in its vicinity. Nevertheless, the mobile agent still enters the ROI, and the ordinary nodes transmit their data in a direct hop fashion. After the death of a CH, its one-hop member starts to play the role of a VH. The expected number of VHs can be expressed as follows:  
\[N_{v}=4 Rr\left(\frac{N_{s}}{A}\right)-1 \tag{5}\]  
where \(N_{s}\) is the number of sensor node, \(N_{v}\) is the expected number of VHs, the length of the mobile agent's path is \(2R\), the VH and mobile agents' distance is \(r\), and \(A\) is the area covered by the mobile agent.  
Advantages: The trajectory of a mobile sink is not static and depends on the density of the node. This mechanism helps the majority of the nodes reduce their transmission  
Figure 6: Working mechanism of the NDCMC protocol.  
energy cost. Isolated nodes are declared as CHs by the BS, and the mobile agent is given extra responsibility for visiting the node.  
Limitations: Initial \"hello\" messaging increases the number of exchanged control packets. As the assumption of NDCMC is that of the dense deployment of the WSN nodes, the initial \"hello\" message will also have an adverse impact on the buffer memory of sensor nodes.