# Energy Saving in Wireless Sensor Networks via LEACH-Based, Energy-Efficient Routing Protocols
## 3 LEACH-Based Energy-Efficient Routing Protocols
The advantage of this two-level hierarchy is that the majority of data transmissions is performed over short distances, and consequently, nodes consume less energy during transmissions and, therefore, achieve longer lifetime. The disadvantage of this hierarchy is that the primary cluster heads are associated with a heavier communication load and, consequently, become depleted faster than the secondary cluster heads. Improved LEACH (I-LEACH) [47] aims at achieving high energy efficiency in both cluster formation and cluster head selection. Specifically, the optimal number of clusters is found based on the locations of nodes and the base station. Additionally, during cluster head election, an appropriate threshold used considers for every node its residual energy,the number of its neighboring nodes, and its distance from the base station. According to the value of this threshold, a node is preferred to be elected as a cluster head when it has many neighbors, the residual energy is greater than the average energy of the network nodes, and it is located closer to the base station. I-LEACH achieves network stability and energy efficiency. On the other hand, it does not have a mechanism for data aggregation. The equalized cluster head election routing protocol (ECHERP) [48] performs cluster head selection based on the Gaussian elimination algorithm. Specifically, the specific algorithm is used in order to compute the energy outflow that every node is going to have if it is a cluster head during the next round for all possible node combinations. Eventually, ECHERP selects the arrangement of cluster heads that minimizes the overall consumption of energy. ECHERP performs well in terms of energy efficiency and scalability. Nevertheless, it does not take into consideration metrics related to QoS and time constraints. Enhanced heterogeneous LEACH (EHE-LEACH) [49] pursues energy efficiency based on the use of a distance threshold. Specifically, nodes whose distance from the base station is below the value of the specific distance threshold, i.e., nodes positioned nearby the base stations, do not form clusters. Instead, they communicate directly with the base station. All other nodes, which are located in more distant positions, are grouped in clusters. Within each one of these clusters, there are some advanced nodes, which have higher energy reservoirs than the rest cluster nodes. EHE-LEACH provides advanced nodes with higher probability of being elected as cluster heads and, subsequently, extends the network lifetime. However, non-clustered nodes do not adopt any sleep-awake scheduling and, consequently, have relatively high energy consumption. LEACH expected residual energy (LEACH-ERE) [50] makes use of the so-called expected residual energy (ERE) value, which represents for every single node its anticipated residual energy in case that node is elected as a cluster head. Specifically, the cluster heads are elected at the end of every round by making use of both a predefined threshold, which expresses the percentage of the tentative nodes, and a function that generates a random number between 0 and 1. The cluster nodes that are associated with greater generated numbers of the threshold become cluster heads and calculate the probability, utilizing a fuzzy inference system, with inputs the residual energy and the ERE value. A bigger probability results in a higher possibility of being elected as a cluster head. LEACH-ERE achieves stable performance. On the other hand, it provides no consideration of distance to the BS, resulting in the early death of faraway nodes. Orphan LEACH (O-LEACH) [51], which is a single-hop routing protocol, introduced the concept of orphan nodes that characterizes the nodes that have not been assigned to any cluster head. O-LEACH proposes two different schemes. In the first of them, orphan nodes transmit the data they sense to adjacent nodes, which, operating as gateways, aggregate and forward these data to the base station.