# 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  
Organising SNs into clusters has been broadly used by researchers to achieve different single and multi-objective optimization problems relating to WSNs. The clustering technique is widely used in designing routing protocols for WSNs because it has many advantages such as scalability, efficient  
Figure 6: Classification of energy-efficient and energy-balanced routing protocols for WSNs. data aggregation, fault-tolerance, latency reduction, robustness, and reduced energy consumption. A clustered WSN mostly contains two sets of nodes: the coordinating nodes usually referred to as the CH, and the member nodes known as the nCH, as shown in Figure 7. The CHs in the network are used to process the information before sending it to the BS while the nCHs forward the sensed information to their respective CHs. The clustering routing solution can be seen as a two-layer hierarchy where the CH nodes operate in the upper layer while the nCH nodes operate in the lower layer. As such, clustering routing protocols are usually referred to in the literature as hierarchical routing protocols. In some cases, because the CH nodes perform more functions than the nCH nodes, the CH nodes are equipped with a better sensing subsystem, processing subsystem, radio subsystem, and power supply unit in comparison with the nCH nodes. If the components of the CH nodes are different from the nCH nodes, the clustering WSN is referred to as a heterogeneous clustering WSN. Otherwise, researchers refer to the clustering WSN as a homogeneous clustering WSN. A clustering WSN can be based on either the single-hop communication method (refer to Section 5.2) or multi-hop communication method. In this paper, we refer to a multi-hop clustering WSN as a network whereby the message from the cluster members to their respective CHs and/or the message from the CHs to the BS is transferred using one or more intermediate nodes depending on the travel distance \(d\). Therefore, we grouped the clustering energy-efficient and energy-balanced routing protocols for WSNs that require any form of intermediate nodes to convey the information either to the CHs or BS under the multi-hop clustering technique. Multi-hop clustering techniques have been extensively shown to prolong the network lifetime, which is a primary evaluation metric for WSNs. The approach or design decision variables used by different researchers might differ but the clustering concept is unchanged. Basically, in the multi-hop clustering technique, the WSN is clustered as shown in Figure 7. The number of clusters or the size of the cluster is specified in the routing algorithm. The nCH nodes send the sensed information to their respective CH directly or via one or more relay nodes within their cluster. This clustering mode of communication is regarded as intra-cluster communication. The CHs aggregate all the sensed information from their respective cluster members and forward the aggregated information to the BS, either directly or via one or more relay nodes. In the case of the CHs, the relay nodes can either be a nCH node or a CH node. This means that any of the SNs in the network can serve as a relay node to convey the aggregated message from the CHs to the BS as specified in the routing algorithm. The communication between a CH and other clusters in order to convey the message from its cluster members to the BS is referred to as inter-cluster communication. Moreover, most clustering algorithms divide the network operation into rounds and periodically re-cluster the network in order to rotate the CH duty. The CH nodes consume more energy because they perform more functions in comparison to the nCH nodes. Hence, in order to avoid untimely network partitioning, it is always desirable to rotate the CH duty and select nodes with higher residual energy to operate as the CH in each round. By so doing, the load in the network is uniformly distributed among the SNs.