# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 3 Network Structure
### Hierarchical
#### 3.2.1 Hierarchy Based
In case every node within a cluster has low energy, disabling it to operate as a cluster head, the network will perform a re-clustering. The use of EESSC achieves energy saving and solves the problem of energy holes. On the other hand, the distance between a node and a cluster head is not considered. _Distributed Energy Efficient Clustering_ (DEEC) [78] is a hierarchical routing protocol for heterogeneous WSNs following the operation principles of LEACH protocol. In DEEC during the cluster head election process, the nodes calculate an election probability, which derives from the ratio between their residual energy and the network's average energy. Furthermore, the number of rounds that the nodes will perform as a cluster head, defined as rotating epoch, is calculated from the initial and residual energy. Therefore, higher initial and residual energy nodes are more probable to be elected as cluster heads. By using DEEC, energy efficient performance is achieved in multi-levelheterogeneous networks with more than two energy levels. In addition, the global knowledge of the network energy is not required. However, DEEC is not suitable for time critical applications. _Hybrid Energy Efficient Routing_ (HEER) [79] is reactive routing protocol that combines the operations of TEEN and DEEC protocols. The cluster head election process is based on DEEC along with the threshold mechanism of TEEN. During the election phase of HEER protocol, nodes elect themselves as cluster heads by using the node's initial and residual energies as in the DEEC protocol with higher energy nodes becoming more often cluster heads. After the cluster head election process, the cluster head sends to its members the threshold values HT and ST, in a similar way as in TEEN protocol, resulting in a smaller number of data transmissions. In this way, the network lifetime is prolonged. Additionally, HEER can be applied in time critical applications, operating in both heterogeneous and homogeneous networks. Its disadvantage is that it lacks data aggregation at the sink node and causes data flooding. _Improved Inter-Cluster Data Aggregation HEER_ (IICDAHEER) [80] is an improved version of HEER, which adds inter-cluster data aggregation and additive and divisible functions to minimize packet number. Cluster head election and the values of HT and ST are the same as in the HEER protocol. Additive function combines different data packets generated from the nodes while the divisible function is used when the generated data packets, of the inter-cluster nodes, are the same. In this way, IICDAHEER prolongs even higher energy efficiency. However, the protocol considers the BS to be located at the center of the area. _Hierarchical Energy Efficient Reliable Transport Protocol_ (HEERTP) [81] is a hierarchical cluster based protocol, which reduces energy consumption during redundant data transport over a WSN and cluster head election. To address these problems, HEERTP protocol divides the network in clusters and elect cluster heads, which are responsible for collecting cluster's data and handle redundancy by aggregating redundant data. Furthermore, receiver nodes utilize a data table, which is updated in case they receive useful data and timeouts, which, when they occur, receiver nodes check for redundant data. In order to further improve energy savings, a cluster head is elected by its residual energy and position, without spending communication energy. HEERTP protocol can also elect a cluster head by electing a Root cluster head (RCH), which is a node close to the BS that analyzes data it receives from the other cluster heads. In HEERTP, both energy consumption during cluster head election and redundant data are reduced. However, useful data may not be sent to the BS. _Energy-Aware for cluster based sensor networks_[82] protocol suggests a different hierarchical approach, a three-tier architecture, in which clusters are formed before network operation. This algorithm deploys less energy constrained cluster heads that are assumed to know nodes location.