## 3 Background  
### Traditional Routing Protocols  
Traditional routing protocols can be classified into three categories: one-hop protocols, multi-hop protocols and cluster-based protocols [29].  
One-hop protocols, also known as direct communication protocols, mean that each sensor node in the network transmits the message to the base station. Figure 1a shows a one-hop protocol. They are the simplest protocol models in wireless networks [30]. However, they have several disadvantages. First, since the sensor nodes have limited transmission range, nodes far from the base station may not have enough power to reach it [31]. Second, even if the sensors are close to the base station, the network might be overly dense so the network efficiency would be seriously degraded by collisions [32]. Third, sensor nodes located in the same region may transmit similar results to the base station, which means WSNs exhibit inherent redundancy in their transmissions [33].  
Figure 1: Traditional wireless network routing protocols: (**a**) One-hop protocol; (**b**) Multi-hop protocol; and (**c**) Cluster-based protocol.  
Multi-hop protocol are a more practical approach for dealing with the above problems. As shown in Figure 1b, node \(A\) which is far from the base station does not transmit the information to the base station directly, but rather forwards its data to one of its neighbors (node \(B\)) which is closer to the base station. Then, node B will in turn send data to its neighbor \(C\) that is closer to the base station. In this way, the data will finally travel from the source node \(A\) to the base station by hopping from one to another. Since the path between two neighbor-nodes is much shorter, and consumes less energy, all the nodes in the network are reachable in the end [34; 35]. Meanwhile, data aggregation can be used on the travel path, which will reduce the inherent redundancy of the WSNs and further reduce the power consumption. The main drawback of multi-hop protocols is that data experiences serious delays. Four steps are required to transmit data from node \(A\) to the base station. By the time it reaches the base station, it may already be obsolete [36; 37; 38; 39; 40].  
Cluster-based protocols are a hierarchical approach that breaks the network into clustered layers. For example, there are two clusters in Figure 1c. In each cluster, there is a cluster head that has the responsibility of routing from the cluster to the base station. Compared with multi-hop protocols, this moves the data faster to the base station and reduces the transmission delays greatly [41]. Only two transmission steps at most are required for transmitting data from one node to the base station. Clustering provides inherent optimization capabilities such as the possibility of data aggregation at the cluster heads [42; 43; 44; 45]. Further, in cluster-based models only cluster-heads perform data aggregation while each intermediate node performs data aggregation in the multi-hop model. The wireless data transmission is completed by the cluster-based method, which overcomes the problem of low detection accuracy caused by the large span of pipeline space. At the same time, as they also have the ability to deal with local signals, sensor nodes using data fusion technology can complete on their own a lot of signal processing, effectively reducing the volume of data transmission, so the decisions haveve higher reliability and flexibility, improving to a great extent the monitoring system running speed. Therefore, cluster-based models are more suitable for time-critical applications than multi-hop models. They have low power consumption, and can accomplish the practical functions of real-time monitoring and alarming.