# A Survey on an Energy-Efficient and Energy-Balanced Routing Protocol for Wireless Sensor Networks
## 5 Taxonomy for Energy-Efficient and Energy-Balanced Routing Protocols
### Multi-Hop Communication Method
#### 5.1.1 Multi-Hop Clustering Technique
They select the nodes based on their proximity to the boundary of the other grids in the network. This approach tends to prolong the network lifetime because more than one node can be used as gateways. As a result, the gateway function is shared. A setback with this approach is that the nodes acting as gateways are not rotated and might deplete their energy completely at some point during data transmission. As such, the grid(s) will be segregated from the whole network. We classified the literature that assumes that the location of the SNs is known in order to select the CHs under location-awareness (LA). Furthermore, some of the works in the literature select the CHs based on some mathematical formulations derived using a combination of some of these decision variables, thereby finding a balance in the trade-offs by using any of these metrics. In view of the aforementioned, Table 2 classifies the studied clustering routing protocols based on the common decision variables used in selecting the CH nodes. In the table, RE stands for residual energy, DBS stands for distance to the BS, HCBS stands for hop count to the BS, TB stands for threshold based, and TPBS stands for transmission power to the BS. Note that the majority of these clustering routing protocols rotate the duty of the CH in order to balance the load in the network. Rotating the duty of the CH has been extensively shown to improve the network lifetime of clustering-based WSNs. Cluster head duty rotation strives to extend the network lifetime and functionality because the functions of the CH are shared among all the SNs in the network. Additionally, this approach avoids untimely network partitioning as shown in Figure 2. Table 2 showcases the reviewed routing protocol that offers this important load balancing feature in the multi-hop clustering technique. This CH rotation approach is more common with homogeneous clustering routing protocols as shown in Table 2. This is because all the SNs in the network assume the same battery energy at initial deployment. Rotating the CH duty in such a scenario will ensure that all the SNs share the heavy task of the CH, thereby evenly distributing the traffic load in the network. On the other hand, some of the heterogeneous clustering routing protocols assume large battery energy for the predefined CH nodes in comparison to the nCH nodes. Since these CH nodes have large battery energy, the CH duty is not rotated at any point. This approach tends to extend the network lifetime and functionality. However, the disadvantage is that, at some point, the CH nodes will completely deplete their battery energy because they perform more energy demanding functions. We classified the clustering routing protocols in which the CH is predefined and that are usually equipped with large battery power in comparison to the nCH nodes. As shown in Table 2, some authors avoid this disadvantage in heterogeneous clustering routing protocols by rotating the CH duty at some point during the data transmission phase. They define an energy level at which the CH duty is rotated among other SNs in the network. Moreover, some authors [95; 96; 97; 98] propose a multi-sink technique to solve this problem. In this case, two or more sinks or BSs perform the CH duty. The sinks forward the aggregated information from their cluster members to the main BS. These sinks are powered in the same way as the main BS; thus, they do not have \"energy consumption problems\". Although this approach solves the energy consumption issues with CH nodes, we assume it is not practical in reality. A major reason for our assumption is the practical cost implicated in deploying more than one BS to monitor an environment. This is not sustainable considering that WSNs are mostly designed to monitor a small geographical area. In this regard, we classified the studied energy-efficient and energy-balanced clustering routing protocols based on the assumed network infrastructure as a homogeneous (HM) and/or heterogeneous (HT) network.