# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 3 Network Structure
### Hierarchical
#### 3.2.1 Hierarchy Based
Relay nodes are cluster heads, elected based on their energy estimate, to transfer data from the other cluster heads to the BS. Cluster nodes transmit packets to their cluster heads using single-hop and cluster heads send their data to the BS via direct communication or via relay nodes. I-EADC extends network lifetime and is applicable in both uniform and non-uniform distributions. However, in this protocol, there is a lack of energy balancing in inter-cluster communication. Chain Based Cluster Cooperative Protocol (CBCCP) [90] is a clustering protocol suitable for real-time applications that divides the network into clusters, elects cluster heads as well as Cluster COrdinators (CCOs), and performs a multi-hop packet transmission from the clusters to the BS through the CCOs. During the clustering process, the number of clusters is derived from the SEP algorithm. Initially, cluster heads, used to aggregate cluster's data, are elected randomly as in LEACH protocol, but, in the next rounds, cluster head election is based on a predefined energy level threshold. When the residual energy of a cluster head drops below the threshold value, the cluster head election process begins again. During data transmission, CBCCP reduces energy consumption with the use of CCOs to transmit data packets to the next clusters until they reach the BS. Within every cluster, there are multiple CCOs depending on the number of the nearby clusters, each transmitting data to one of them. CCOs follow the same election process as the cluster heads. In CBCCP protocol, not only is the clustering process simple, but also load balancing is achieved and long delays are avoided. However, energy balancing problems for inter-cluster communication are not taken into consideration. Well Balanced TEEN (WB-TEEN) [91] is an improved version of TEEN that balances the size of the formed clusters. During the cluster formation process, every cluster head informs the other nodes with a packet including an identifier; then, every node sends a request packet with a strong signal to join the cluster. Cluster head picks those with the strongest one and, in case there are two nodes with the same signal strength, the cluster head picks one randomly. Furthermore, to limit the cluster size, every cluster head has a degree, to determine if the cluster head has reached its limit. In case the number of the cluster nodes is below the degree, then it can accept another node or, on the other hand, the node is rejected. The degree is calculated from (2):  
\[EGREE=\frac{NN-CH_{nbr}}{CH_{nbr}}+1, \tag{2}\]  
where \(NN\) is the total number of the network nodes and \(CH_{nbr}\) is the number of the cluster heads. By using WB-TEEN, the network lifetime is extended. However, far away nodes die too early. In order to overcome the early death of faraway nodes, the authors in [91] also introduce _Well Balanced TEEN with Multi-hop intra cluster communication_ (WBM-TEEN), which is WB-TEEN with multi hop routing within the clusters, overcoming path failures. Furthermore, WBM-TEEN is capable of data aggregation to further improve the performance. In WBM-TEEN, energy consumption is reduced. Additionally, increased reliability is attained. On the other hand, single-hop routing is used in inter-cluster transmission. _Heuristic Algorithm for Clustering Hierarchy_ (HACH) [92] is another clustering protocol that achieves energy efficient operation by electing evenly distributed cluster heads and utilizes sleep scheduling. HACH's operation focuses on two main ingredients: cluster head election and sleep scheduling. The SSIN (Stochastic Selection of Sleep Nodes) sleep scheduling that is used in HACH protocol deposits low energy nodes in sleep mode without affecting network's coverage. The distance from one another is calculated in order to evaluate network coverage ability and the nodes with more residual energy are chosen as cluster heads. In HACH protocol, the procedure of sleep scheduling prolongs network lifetime, even when different levels of heterogeneity are present.