# 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
#### 5.1.2 Load-Balanced Tree Technique  
The load-balanced tree technique finds an energy-balanced and energy-efficient path(s) from the source node to the BS. A typical illustration of the load-balanced tree technique is shown in Figures 8 and 9. In Figure 9 for example, the source nodes (\(SN_{5}\) and \(SN_{17}\)) construct an energy-efficient and energy-balanced route to the BS. The route from the source node to the BS resembles a tree structure. Literally, a tree is made up of three parts: the root, the trunk, and the crown. In WSN terms, the root represents the source nodes, the trunk can be regarded as the intermediate nodes, and the crown technically represents the BS. Thus, as shown in Figure 9, the sensed information (which can be seen as the nutrient from the soil) is forwarded through the trunk (intermediate nodes) to the crown (BS) of the tree. In order to extend the network lifetime and functionality, researchers strive to balance the load from the root to the crown of the tree. Accordingly, in a load-balanced tree routing protocol, much attention is on the design of the trunk (relay node chain to the BS). The route construction can be in form of a multipath technique (Section Multipath Technique) or a single-path technique, as discussed in Section Single-Path Technique. These constructed route(s) often change in each data transmission round. The concept is that the load traffic carried by the source node is always evenly distributed as much as possible in each data transmission round [142]. In such a way, the bulk of the load traffic will not flow through the source node only, and the direction of flow to the BS changes all the time depending on the routing algorithm. The load-balanced tree approach has been extensively used to design a WSN routing protocol because it offers numerous advantages as discussed in the subsequent sections. In lieu of this, we study and classify some of the load-balanced tree routing protocols in the literature. Figure 8: Typical example of multipath load-balanced tree routing. Multipath routing is a load-balanced tree technique broadly used by researchers to develop routing protocols for WSNs in order to improve the performance of the network. The multipath routing approach has been used extensively in the past decade for different optimization problems in WSNs such as load balancing, fault-tolerance, bandwidth aggregation, security, data transmission reliability, and congestion control. Multipath routing is a routing approach that chooses multiple paths to convey data from the source node to the destination in the form of a tree as shown in Figure 8. As in the case of the multi-hop clustering technique, all multipath routing protocols fall under the multi-hop communication method. As shown in Figure 8, the source node uses different relay nodes to convey its sensed information to the sink. However, due to the dense placement of the SNs, several paths can be constructed from the source node to the destination. Multipath routing techniques are initially established to proffer solutions to the low data transmission rate associated with the single-path routing method. Nowadays, multipath routing techniques are used to proffer solutions to different WSN's optimization problems as mentioned. With heed to load balancing, multipath routing achieves load balancing by distributing the load traffic from the source node over more SNs towards the destination. For example, consider the expressions in Equations (4) and (6), where the load traffic is defined by \(q\). As shown in the equations, the load traffic \(q\) (message bit) is directly proportional to the energy consumption. That is, the more the load traffic \(q\), the higher the energy consumed by SNs in transmitting or receiving the message. Thus, with multipath routing, the traffic load is shared among the SNs from the source node towards the destination. In such a way, the network lifetime and functionality are prolonged for a reasonable period.