## 3 Data Routing Approaches in WSNs
### Energy-Efficient Approaches
#### 3.1.2 Energy Consumption Reduction  
Clustering is known as an energy-efficient structure for data routing in WSNs. Hierarchal multi-hop routing algorithms successfully utilize data aggregation to decrease the volume of data flowing in the network. There are several data routing approaches in this area which proposed effective solutions for cluster formation and cluster head selection. In [60], the proposed solution consists of two data aggregation mechanisms based on clustering: combined data aggregation and adaptive data aggregation. Combined data aggregation is able to use both static and dynamic clustering methods concurrently in the defined network according to the environmental variables of the network; therefore, it can make use of the advantages of both techniques simultaneously. In this scheme, the area of each clustering approach is not fixed and it may be changed according to the number of nodes in the network. In the initialization phase, first, a tree topology and a static cluster are constructed. Then, the nodes should select the data aggregation method they want to use. This selection is done based on two values of \(\alpha\) and \(\beta\). Value \(\alpha\) illustrates the start of the static clustering based data aggregation and value \(\beta\) states the start of the dynamic clustering based data aggregation. Hence, the nodes whose values are between \(\alpha\) and \(\beta\) send their data to the related cluster head using static clustering method. The values beyond the \(\beta\) will be aggregated using dynamic clustering method and the values below \(\alpha\) will be directly sent to the sink without any aggregation. Therefore, by using this scheme, dynamic clustering is provided for the nodes far from the sink and the nodes near the sink which need low transmission power can send their data directly to the sink (Figure 3). Figure 3: Combined Clustering-based data aggregation. On the other hand, the adaptive data aggregation scheme can select a suitable clustering technique according to the state of network and application. In this scheme, each sensor node can adaptively change its data aggregation technique regarding the state of the network which can be affected by the number of targets. In this mechanism, the static cluster based aggregation is selected based on the network traffic. When the traffic is high, it chooses the static cluster based data aggregation and when it is low, the dynamic cluster based aggregation is selected. The decision for switching between the data aggregation techniques is made based on a threshold which is adjusted and decided at the BS. The approach in [61] uses a three-tier architecture to propose a cluster-based routing algorithm. In this hierarchical scheme, the cluster formation is done before networking process. Cluster heads which are called gateways have more energy than other sensor nodes and it is assumed that they know the location of all other nodes. Thus, gateways nodes establish multi hop routes for data collection based on the maintained states of the nodes. In this approach, a TDMA like MAC is considered for data transmission between the sensor nodes and the gateway. A whole epoch is divided to some time slots and each node transmits it data in its own time slot. Also, all the nodes are notified by the gateway about the slot in which they should listen or transmit. The sensor nodes in this approach can be in four states: sensing only, relaying only, sensing-relaying, and inactive. In the sensing only state, the nodes sense the environment and generate environmental data. In the relaying only state, the nodes go to listening phase and wait for receiving data of other nodes. In the sensing-relaying state, the nodes perform data generation while they are in listening phase, too. Finally, when a node carries out none of these functions, it is considered inactive. Generated data of the sensor nodes are transmitted toward the gateway through the bidirectional links between the nodes.