# A Survey on an Energy-Efficient and Energy-Balanced Routing Protocol for Wireless Sensor Networks
## 6 Survey Findings and Possible Research Directions  
Different research works are ongoing in order to proffer a reliable solution to the energy consumption optimization problem relating to WSNs. The choice of the design input decision variable or the combinations of the design input decision variables is still an open area of research. We emphasised that using a single decision variable such as the residual energy cannot guarantee an energy-efficient and energy-balanced route in most cases. As discussed, all these decision variables have their advantages and disadvantages. Therefore, efficiently integrating these design decision variables can proffer a better solution to the energy consumption issue relating to WSNs in comparison to using a single decision variable. However, efficiently integrating these design decision variables in order to always ensure an optimal energy-efficient and energy-balanced route towards the BS from the source node can be an NP-hard problem. Besides the trade-offs in combining these decision variables, the proposed mathematical formulation might exhibit a high computational time and complexity delay. Moreover, the choice of the input decision variables can be application specific. Thus, the developers of WSN routing protocols should consider the network requirements before selecting the input decision variables. Furthermore, we observed that more than 50% of researchers use the residual energy, hop-distance, and hop count as the decision variables in deriving the routing protocol mathematical formulation. With the residual energy, they ensure that the participating SN has sufficient energy to receive and convey the sensed information for that particular data transmission round. The hop-distance which is a very important parameter in the WSN energy dissipation model is used to choose the next hop. The smaller the hop-distance, the less energy consumed in conveying the message. Hence, the importance of a small hop-distance. Lastly, the hop count is used to specify the number of relay node chains to the BS, especially in multi-hop communication methods. It is desirable to keep the hop count as small as possible in order to reduce the energy consumed in receiving and forwarding the message. Additionally, as shown in Tables 2-6, other parameters such as the AE and RSS/TP, can also be integrated in deriving the mathematical formulations while developing an energy-efficient and energy-balanced routing protocol. However, from the study of the WSN energy dissipation model in Section 4.1, we observed that the sensed information or the \(q\) bit message forms an important part of the model (refer to Equations (4) and (6)). In fact, the higher the value of the \(q\) bit message, the more the energy consumed in receiving and forwarding the sensed message. The weight of the \(q\) bit message has been largely ignored by most researchers in designing their routing protocol. The knowledge of the \(q\) bit message can also serve as an important decision variable that can be integrated with other decision variables in order to design an energy-efficient and energy-balanced routing protocol. Logically, if the weight of the \(q\) bit message to be transmitted is known, and the residual energy of all the SNs in the network is known, we can easily determine the SNs that can conveniently forward the message towards the BS or form the set of relay node chains toward the BS. More importantly, the weight of the \(q\) bit message will be useful in designing a load-balanced multipath routing protocol. If the weight of the sensed information is known, the load can easily be shared among the selected paths, where the paths with a high energy level carry more load in comparison to the paths with a moderate energy level. Thus, the weight of the \(q\) bit message can be considered in future works as a decision variable in designing an optimal load-balanced routing protocol. We also observed that the position of the BS plays an important role in the design of an energy-efficient and energy-balanced routing protocol.