# Energy Saving in Wireless Sensor Networks via LEACH-Based, Energy-Efficient Routing Protocols
## 3 LEACH-Based Energy-Efficient Routing Protocols
In this way, the base station acquires global knowledge of the status of its network nodes and processes the relative information, in order to create clusters that consume less energy during transmission. The base station calculates the average residual energy of the nodes and extracts from the list those that are eligible to become cluster heads during the specificround the nodes whose remaining energy is less than the average energy. Upon the formation of clusters and the selection of cluster heads, the base station broadcasts the identities of the cluster heads that have been selected. LEACH-C performs better than LEACH in terms of cluster formation and energy efficiency. However, not only does it require the knowledge of the position of each node, but also, it is associated with the problem of the early death of cluster heads with low energy, since it does not guarantee that the nodes with the highest energy reserves are selected as cluster heads. LEACH-CE (LEACH centralized efficient) [43] is another single-hop routing protocol. Actually, it is an improved version of LEACH-C in the sense that it eradicates the main disadvantage of LEACH-C, i.e., the early deaths of the cluster heads. Practically, LEACH-CE follows the same process as in LEACH-C but only during the first round, until the clustering is completed. Next, the base station defines every cluster node with the highest residual energy among the cluster nodes as a cluster head. LEACH-D [44] is a multi-hop routing protocol. In this protocol, the selection of the cluster heads is performed by considering the density distribution of the nodes along with their residual energy. Specifically, the nodes that have high priority in being selected as cluster heads are those that have high energy residues and are located in high-density areas. During the clustering phase, each cluster head node decides what the radius of its own cluster will be according to its distance from the base station and the degree of connectivity. The nodes that have not been selected as cluster heads select the proper cluster to join by taking into account their distance to the neighboring cluster heads and the residual energy of these cluster heads. Finally, the cluster heads transmit their data to the base station via multi-hop communication. LEACH-D performs fine regarding energy efficiency and scalability. Yet, it is associated with high complexity and overhead. In LEACH-L [45], which is a multi-hop routing protocol, the cluster heads that are located close to the base station constitute the group of nodes that transmit their data directly to the base station. Every cluster head that does not belong to this group, because it is positioned far away from the base station, transmits its data to the base station via one relay node belonging to the aforementioned group. The selection of the most appropriate relay node is based on its residual energy and its distance from the base station. LEACH-L achieves remarkable energy saving, especially in large-scale WSNs, in which the base station is located at a distant position. On the other hand, the requirement of location knowledge is associated with higher communication load and complexity. In two levels LEACH (TL-LEACH) [46], which is a multi-hop routing protocol, a two-level hierarchy is adopted. Specifically, the cluster heads located close to the base station are characterized as primary, while cluster heads located away from the base station are characterized as secondary. The secondary cluster heads perform the aggregation and partial local processing of the data received by their cluster nodes and transmit the resultant data to the primary cluster heads. The primary cluster heads carry out the complete local computation of the data that they receive from both their cluster nodes and the secondary cluster heads and transmit the corresponding data directly to the base station.