# 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
The mobile sink approach is still a very recent technique used in solving the energy-hole problem but has been shown to protract the network lifetime and functionality [139; 140; 141]. However, a major setback to this approach is that the BS has to continuously broadcast its current position to the SNs in the network. This results in high network delay due to the complexity in sending the present position of the BS. Although the cluster formation phases are very important in order to balance the load traffic and subsequently maintain the WSN functionality for a reasonable period, the main focus of this paper is to study how the network load is balanced during the data transmission phase. Having discussed how the aforementioned multi-hop clustering routing protocols carry out the clustering formation steps, we study the decision variables used by these clustering routing protocols in setting up the multi-hop route from the source node to the BS. As explained earlier in Section 4.2, the proposed approaches used in selecting the route from the source node to the BS play a vital role in balancing the traffic load in the network. This, in turn, conserves the energy of the SNs in the network and the total energy in the network. We divided our classification of these multi-hop clustering routing protocols into two phases. We grouped the clustering routing protocols based on the decision variables used during the intra-cluster communications and the inter-cluster communications as shown in Table 3. From the table, some literature defines the intra-cluster communication routes or relay node's chain towards the CH based on the distance usually computed using Equation (3). Researchers strive to reduce the transmission distance by selecting the nearest node to the CH node. This approach tends to conserve the energy dissipated by the forwarding nodes as a result of the small transmitting distance. However, this approach does not necessarily balance the load in the network. Using the illustration of Figure 5a, the shortest path or distance route to the CH is not always the most energy-balanced route. Some of these relay nodes along that route might be low on battery energy. Subsequently, such nodes should be protected as much as possible from performing relaying duties, or else, the nodes will deplete their battery energy completely. Moreover, for example, given three nodes X, Y, and Z--assuming the nearest node to X towards the CH is Y, and that node Z is next to node Y--node X will send the sensed information to node Y; node Y will subsequently forward the message to node Z, and so on. Nonetheless, it is possible that node X will conserve the total energy dissipated in the network by transmitting the information directly to node Z. That is, the energy dissipated in transmitting the information from node X\(\rightarrow\)Y and Y\(\rightarrow\)Z is greater than the energy dissipated in transmitting the information directly (without using node Y) from X\(\rightarrow\)Z. As such, the shortest distance route might not always be the most energy-balanced route to the CH. Besides, as shown in Table 3, some papers define the intra-cluster communication routes based on the hop count or nearest hop to the CH. Similarly, this method conserves the energy in the network--most especially the energy dissipated by the source node--but this method has the same weaknesses as the shortest distance method. In the table, some authors select the most energy-efficient route to the CH. In the algorithms, the authors use the residual energy of the SNs as a key design decision variable in selecting the routes. The algorithms select the SNs with high residual energy to form the relay node's chain from the source node to the CH. As emphasised with the illustration in Figure 5b, if the travelling distance in sending the information between the participating nodes toward the CH is unreasonably high, the total energy dissipated in the network will equally increase.