# 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
### Single-Hop Communication Method  
In the single-hop communication method, the sensed information is conveyed to the BS from the source node directly. This method was largely used in the early discovery phase of WSNs. Yet, the single-hop communication method offers some advantages. For instance, the single-hop communication approach avoids delay in data transmission which is a useful attribute in real-time data transmission. The single-hop communication method also avoids the energy-hole phenomenon which is an important advantage for using this method in designing a routing protocol. Besides, the single-hop communication method ensures reliable data transmission while all the SNs are still alive. In the multi-hop communication method, part of the sensed information might be lost while conveying the message from one node to another towards the BS. On the other hand, in the single-hop communication method, the message is sent directly to the BS. Conveying the message directly to the BS technically avoids the situation whereby part of the sensed information is lost in the process of sending the information from one node to another. Nevertheless, the single-hop communication method is not suitable in a large-scale WSN. In such a network, the distant SNs will always send their sensed information to the BS over a large distance. Thus, they will deplete their battery energy quickly in comparison to the nearby SNs. Accordingly, the network lifetime and functionality will be hampered. So, in an attempt to extend the network lifetime and functionality for a reasonable period, researchers proposed different single-hop clustering routing protocols, as shown in Table 6. These single-hop clustering routing protocols strive to optimize several single and multi-objective problems associated with WSNs. Figure 10 depicts a typical example of a single-hop clustering WSN. As shown in the figure, the nCH nodes send their sensed information to the CH directly without the help of any intermediate node. Similarly, the CH nodes forward the aggregated data from their cluster members directly to the BS, irrespective of their positions from the BS. This single-hop clustering approach is somewhat useful because all the SNs do not need to send their sensed information to the BS directly. The nCH nodes send the message to their respective CH nodes which, in most cases, are at a shorter distance from them in comparison to the BS. The single-hop clustering approach does not necessarily solve the distance problem associated with the single-hop communication method. For example, in a large-scale WSN, some nCH nodes might be at a large hop-distance from their respective CH nodes. More realistically, some of the CH nodes in such a network will be at a large distance from the BS. If these CH nodes continuously send the aggregated data to the BS directly, they can easily deplete their battery energy completely. This will result in early network partitioning as explained in Figure 2. However, we studied some single-hop clustering routing protocols based on the cluster formation which includes the CH selection phase. \begin{table}
\begin{tabular}{l c c c c c c c c c c c c c} \hline \hline  & & & & & \multicolumn{3}{c}{Decision} & \multicolumn{3}{c}{Decision} & \multicolumn{3}{c}{Cluster} \\
**Year** & **Clustering Routing** & **HM** & **HT** & **CH** & & & & Variables & & LA & Variables & & Size & Optimization \\  & & & & & & & & & (CH  
Note that both the intra-cluster and inter-cluster communications in single-hop clustering routing protocols require no form of intermediate nodes, as in the case of the multi-hop clustering routing protocols. Therefore, the cluster formation stages should be carefully designed in order to evenly distribute the network load, so that the network lifetime and functionality are extended for a reasonable period. As a result, our classification of the decision variables used in conveying the sensed information to the BS in single-hop clustering routing protocols is limited to the cluster formation stages.