## 2 Literature Review
### Data Fusion in Sensor Networks
#### 2.1.3 Typical Routing Protocols  
The network collects sensor information, but mobile wireless networks do not have a fixed topology. We review typical routing protocols to send requests to sensors for data collection and receive data back from the sensors. In query-based routing, the base station broadcasts the interest message to all sensing nodes to set up the path of data transmission and fusion. The interest message includes an inquiry request with list of attributes, duration of the request, and geographical area. Intermediate nodes on the path to the monitoring area cache the interest message and calculate the data gradient, including the data reporting rate and the next hop node. They use the path establishment mechanism to plan the optimized path to the base station, but also several sub-optimal paths to the base station. The base station may send a message of increased or decreased interest in the data sent by a path to change the data reporting rate. Data fusion of the directional diffusion route adopts the method of \"suppressing the replica\", that is, nodes buffer only new retrieved data, and do not use duplicate data. This approach is not only simple, but the routing technology can effectively reduce the amount of data in the network [17]. Having multiple paths ensures the robustness of the process, but it is not suitable for large-scale sensor networks due to the cost of building the data gradient. Figure 2 shows the stages of setting up the transmission path. Figure 1: Typical application of information fusion. Hierarchical routing is the method of network routing based on hierarchical addressing. Backbones connect routers, which in turn service a group. Both the Low Energy Adaptive Clustering Hierarchy (LEACH) [18] and Low Energy Adaptive Clustering Hierarchy-centralized (LEACH-C) use hierarchical routing. (Figure 3). Cluster heads handle data fusion tasks, and only send fusion results over the backbone to the base station to reduce data transmission. The cluster head performs the fusion operation periodically [19]. Nodes alternate in performing cluster head tasks to distribute energy consumption evenly. The choice of cluster head is based on the percentage of cluster nodes in the whole network and the number of nodes in the past operation. Assume that for a node n, n is a random choice of a number between 0 and 1, as a flag value. If the flag value of n is less than a threshold \(T(n)\), node n acts as the cluster head node of the current round. The threshold \(T(n)\) is as follows:  
\[T(n)=\left\{ \begin{array}{c}\frac{P}{1-P*\left(r\;mod\left(\frac{1}{p} \right)\right)},\;if\;n\in G\\ 0\;,otherwise\end{array} \right. \tag{1}\]  
where \(p\) is the percentage of the number of nodes in the network cluster node, \(r\) is the current number of rounds, and \(G\) is a set, and the nodes in the set are nodes of the cluster head nodes in the first \(1/p\) round. The symbol mod is a modulo operation symbol. Using this threshold, each node can act as a cluster head node in the \(1/p\) round. All nodes selected as cluster heads broadcast messages to the network informing the other nodes that they are cluster heads. All non-cluster head nodes receive the broadcast message and decide which cluster to join based on the received signal strength. LECAH-C is an improved version of LEACH. It uses the base station center control method, and applies an annealing algorithm to get K optimization of the cluster head and the corresponding cluster in the base station. The base station broadcasts the new cluster head IDentity (ID) information to each node to form a clustered network. After the selection stage of the cluster head, the remaining stages in LEACH-C and LEACH are the same, but its performance is better than the LEACH protocol. Figure 2: Directional diffusion process. Chain-Based Routing  
Power-Efficient GAthering in Sensor Information Systems (PEGASIS) [20] and its high-order algorithm have improved the fusion method of LEACH, as shown in Figure 4. It is based on two assumptions.