# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 3 Network Structure
### Hierarchical
#### 3.2.1 Hierarchy Based
On the other hand, cluster heads may have to transmit over long distances, thus increasing their energy consumption. _Threshold Sensitive Stable Election Protocol_ (TSEP) [93] is a protocol that combines TEEN and SEP protocols to provide an operation with three energy leveled nodes and adjusting between energy efficiency, accuracy, and low response time. When TSEP elects a cluster head, it uses three different probabilities assigned to each energy level of the nodes. In the start of every round, the cluster head broadcasts Hard and Soft Thresholds (HT, ST), as well as Report Time (RT) and Attributes (A). RT expresses the period of time, within which reports are sent from every node while A represents the physical parameters. TSEP protocol is suitable for applications that require continuous data transmission and provides both good energy efficiency and network stability. On the other hand, there is no awareness in the cases that thresholds are not reached. _Double-phase Cluster- head Election Scheme_ (DCE) [94] is a clustering protocol that implements an extra phase during the cluster head election:  
1. In the first phase, a tentative cluster head is chosen randomly with the use of a probability function that considers relative initial and residual energy levels. 2. In the new second phase, the non-elected nodes determine their minimum communication tentative cluster and compare their own remaining energy with that of the tentative cluster nodes, then the tentative cluster nodes are replaced accordingly in case the non-elected nodes have more residual energy. This set of phases ensures a distributed energy consumption, resulting in longer stability periods than other clustering protocols. During the procedure of cluster head random election, the network is enabled instantly, without delay, resulting in a more stable network. Additionally, the network is always active, and does not waste time for cluster head election. However, in every round, DCE may change up to two times cluster head selection, thus increasing the energy consumption of the network. Best Selection Double Cluster Head (BSDCH) [95] is a clustering protocol that divides the network field in equal regions and form chains with the nodes. Cluster heads are divided into main and secondary ones. A main cluster head sends data to the BS while a secondary cluster head receives data from the main and forwards them to the BS. BSDCH checks as cluster head the node having the minimum value of the factor \(F\) that is found by using (3):  
\[F_{i}=a\cdot\frac{E_{total}}{E_{remind-i}}+\beta d_{TO-CH} \tag{3}\]  
where \(a\) is the weighting coefficient of energy, \(E_{total}\) is the initial energy of the node, \(E_{remind-i}\) represents the current energy of node \(i\), \(d_{TO-CH}\) denotes the distance of each node to the cluster center, and \(\beta\) is the weighting coefficient of distance to the center, where \(\alpha+\beta\leq 1\). Similarly, the node with the most energy and the least distance to main cluster head and the best location to BS is selected as secondary cluster head. Secondary cluster head selection is based on the value of \(F_{i}\) factor:  
\[F_{i}=a\cdot\frac{E_{total}}{E_{remind-i}}+\beta\cdot d_{TO-CH}+\chi\cdot d_{ TO-BS} \tag{4}\]  
where \(d_{TO-BS}\) is the distance of each node to the BS, where \(\chi\) is the weighting coefficient of distance to the base station where \(\alpha+\beta+\chi\leq 1\). During initialization, the BS forms hierarchy clusters, configures the number of the levels, assigning each one with an ID, by taking into account distribution density, the BS location and number of nodes. Every node calculates its distance from the BS by the signal strength of the signal it receives from the BS. When clustering process is completed, the algorithm builds chains of nodes within each level; therefore, nodes of a chain have the same ID. Although BSDCH is based on PEGASIS, it can send data to the BS more efficiently since it uses fewer hops.