# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 3 Network Structure
### Hierarchical
#### 3.2.1 Hierarchy Based
DHAC and then uses the sequence of nodes merging into clusters following the schedule while every cluster node gets an assigned role and starts to communicate in turns with cluster head. By using DHAC, network lifetime is prolonged. However, the performance worsens when network traffic gets high. Hybrid Energy Efficient Distributed (HEED) protocol [85] is a clustering protocol in which cluster head election is based mostly on residual energy and other parameters as distance from neighbors or number of neighbors. In HEED, cluster formation function is triggered in given intervals for cluster head election and uncovered nodes, those without a cluster head, which can elect themselves as one. Furthermore, HEED parameters such as minimum selection probability, which is a probability for a node to be elected as a cluster head and network operation interval, which informs the user about how frequent cluster head election process happens can be tuned easily for better optimization in case of the requirements of an application such as network's density. The low communication cost and the good scalability are the main advantages of HEED. On the other hand, different energy levels are not considered. Stable Election Protocol (SEP) [86] is a hierarchical routing protocol for heterogeneous WSNs with two energy-leveled nodes, normal and advanced, which increases the stable period during the clustering hierarchy process. SEP is a dynamic protocol in terms that there is a random deployment of the two energy leveled nodes. In addition, during the cluster head election, the nodes elect themselves as cluster heads, depending on their initial energy in respect to the energy of other nodes without any requirement of global knowledge of residual energy during each cluster head election round. SEP achieves network lifetime prolongation. In addition, global knowledge of residual energy during the round of cluster head election is not required. Its disadvantage is that the cluster heads are elected based only on their initial energy level. Enhanced-Stable Election Protocol (E-SEP) [87] is an improved version of the SEP algorithm for heterogeneous WSNs. To maintain a more uniform energy consumption, among the nodes, it uses a clustering algorithmic approach with a three-tier node configuration. E-SEP uses Constant Bit Rate (CBR), a common traffic pattern used in clustering for heterogeneous networks. During the clustering process, the three-tier heterogeneous nodes elect themselves as cluster heads depending on their residual energy, resulting in uniform energy distribution between the nodes. E-SEP attains both increased network lifetime and improved resource sharing. Its disadvantage is the uncontrolled number of associated cluster nodes within each cluster results in load imbalances. Energy Aware Distributed Clustering (EADC) [88] is a cluster based routing protocol for non-uniform node distribution WSNs, which combines an energy-aware clustering algorithm and a cluster based routing algorithm and can operate with heterogeneous nodes. During the setup phase, the nodes are divided into equal clusters. EADC balances the energy consumption of the cluster heads by modifying the intra-cluster and inter-cluster consumption with the use of a cluster based inter-cluster routing algorithm. Each cluster head picks another cluster head as its next hop, based on its residual energy and the number of its cluster nodes. The non-uniform node distribution causes imbalanced energy consumption, which EADC implements an increased forwarding task of cluster heads in sporadic areas to solve it. EADC elongates network lifetime. The Relay node is elected based on residual energy and number of cluster nodes only. Improved Energy Aware Distributed Clustering (I-EADC) [89] is an improved version of the EADC protocol that solves the energy imbalance in non-uniform node distribution. During clustering, the network area is divided into equal clusters and cluster heads are elected according to their residual energy.