# A Survey on an Energy-Efficient and Energy-Balanced Routing Protocol for Wireless Sensor Networks
## 4 Overview of Energy Consumption in WSNs
### SNs Energy Dissipation Issues
This route can be very unforeseeable, and might change with each data transmission round depending on the energy level of the SNs in the network and the distance travelled in conveying the data. Furthermore, an important observation in Figure 5a is that the energy-efficient route uses more relay nodes (hop count = 6) in conveying the message in comparison with the shortest path route (hop count = 5) in Figure 5a. As shown in Equation (6), this implies that the route in Figure 5b will dissipate energy more times (six times) to receive and convey the sensed message in comparison with the route in Figure 5a (five times). The hop count is a very important decision variable used by the developers of routing protocols for WSNs to optimize the energy consumption by the SNs in the network. Determining the optimal hop count when sending a message from the source node to the BS can also be a challenging problem with the multi-hop communication method. The smaller the hop count, the less energy consumed in receiving and relaying the message from the source node. As such, it is desirable to keep the hop count as small as possible between a source node and the BS, but this does not necessarily guarantee a balance in the energy consumption by SNs in the network. For instance, let us assume that the route in Figure 5a offers the smallest hop count from \(SN_{5}\) to the BS. As shown in this figure, \(SN_{21}\) forms part of the relay node's chain in transmitting the message from \(SN_{5}\) to the BS. However, the energy level of \(SN_{21}\) is very low in comparison with the other SNs in the network as shown in Figure 4. At this energy level, \(SN_{21}\) should not act as a relay node to \(SN_{5}\) or any other node in the network in order to maintain the full network functionality. In fact, \(SN_{21}\) should sleep all the time except when it is transmitting its own sensed information. Despite the fact that the route in Figure 5a offers the smallest hop count from \(SN_{5}\) to the BS, we can say that this lowest hop count route is not the most \"energy-balanced\" route to the BS from \(SN_{5}\). Thus, there must be a route with an optimal hop count from \(SN_{5}\) to the BS that balances the energy consumed by the SNs and the total energy dissipated in the network. Another obvious setback with the multi-hop communication method is the energy-hole problem [53]. In multi-hop communication, SNs closer to the BS consume more energy in receiving and forwarding the \(q\) bit message towards the BS. Because these SNs are randomly placed closer to the BS, most load traffic towards the BS passes through these nodes. As such, these nodes closer to the BS tend to dissipate more energy in comparison to the distant nodes. For illustration, most of the load traffic toward the BS in Figure 4 will pass through any of these nodes: \(SN_{8}\), \(SN_{11}\), \(SN_{16}\), and \(SN_{24}\). This implies that \(SN_{8}\), \(SN_{11}\), \(SN_{16}\), and \(SN_{24}\) have a high chance of completely depleting their battery energy in comparison to the other nodes in the network. Now, looking at the WSN in Figure 4 as a whole, the closer the nodes are to the BS, the more load traffic they carry, and as a result, they will consume more energy. This means that there is an imbalance in the energy consumption by SNs in the network. This imbalanced energy consumption will result in the untimely death of some SNs. The multi-hop communication routing protocol should be designed in such a way to mitigate the energy-hole phenomenon. Therefore, an energy-balanced multi-hop route from the source node to the BS must, at all times, solve the energy-hole problem related to the multi-hop communication method. As mentioned, the design of a reliable energy-balanced routing protocol for WSNs involves a couple of decision variables (hop count, residual energy, distance metric, etc.) which must not be overlooked.