## 2 Green Routing Algorithms in WSNs
### Optimizing Data Flow-Based Routing Algorithms
#### 2.3.2 Multi-Path-Based Schemes  
Multi-path-based schemes have good reliability in data transmission because there are multiple routing paths from the source nodes to the destination nodes. Even though some nodes in one path fail or run out of energy, packets can be forwarded through other paths.  
To construct multiple path segments according to the gradient information of nodes, the gradient-based multi-path routing protocol (GMRP) is proposed [61]. The node chooses the path segment that can forward packets to the sink fastest as its next path segment. The path segment selection process is repeated until the source node obtains the minimum delay path. In GMRP, each node should maintain the information of its own gradient and some path segments. The node finds each path segment, which starts from the node itself and ends when it encounters a node with a lower gradient. Then, the node chooses the minimum delay path segment as its next segment, and the first node of the selected pathsegment is the node's next hop. GMRP finds the minimum delay path to the source with less complexity.  
Randomized switching for maximizing lifetime (RSML) [62] is a tree-based routing protocol, and it achieves the balance of the energy load by changing the tree routing dynamically. In RSML, the energy load is computed as the ratio of energy consumption of the node and the node's energy. A parameter x is adopted that represents the magnitude of the network balance. Once the difference between the energy path load and minimum path load is more than x, the child node, which loads more, chooses a neighbor with a load as light as that of its parent. Repeat the process until the difference between energy path load and minimum path load is no more than x; then, the optimal balanced tree is created.  
A comparison of different optimizing data flow-based routing algorithms is shown in Table 3, which considers the optimized parameters involved in the single-path-based schemes and multi-path-based schemes.  
As shown in Table 3, the delay and residual energy are the most optimized parameters in different schemes. The delay metric is related to the number of hops from the sending node to the destination node. To some extent, the shorter the number of hops at both ends of the communication, the lower the delay and the lower the energy consumption. The residual energy of the node is related to the network lifetime of WSNs. If the residual energy of the node is too low to complete the communication task, the node will fail, which will decrease the lifetime of the WSNs. The difference between single-path-based schemes and multi-path-based schemes is the number of paths from source nodes to destination nodes. In general, multi-path routing is more reliable than single-path routing. From the perspective of the specific process of optimizing data flow, the single-path-based protocol mainly adopts the next-hop optimization method and completes the global optimization of the network through local optimization. The multi-path-based protocol, by contrast, utilizes the method of optimizing the next path segment or optimizing the parent node to achieve network optimization.