# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 3 Network Structure
### Hierarchical
#### 3.2.1 Hierarchy Based  
Routing protocols of this kind create clusters with cluster heads and cluster nodes routing data through two levels of communication, a low level one for cluster nodes to cluster heads and a high level for cluster heads to BS. A core design attribute of hierarchical protocols is scalability. Within a single tier network when node number increases, the BS overloads resulting in communication latency and inadequate event tracking. The single BS design has scalability limitations for a greater set of nodes that cover a wider area of interest due to the fact that nodes are not capable of long distance transmission. Cluster's formation criteria usually are nodes' energy reservoir and proximity to cluster head. Actually, hierarchy based hierarchical protocols constitute the most popular subcategory of energy efficient routing protocols. In what follows in this section, 59 typical examples of this type are described. Low Energy Adaptive Clustering Hierarchy(LEACH) [46] is one of the first hierarchical clustering protocols. It creates clusters of nodes based on signal strength and use cluster heads to aggregate, compress, and transmit packets to the BS. The optimal cluster head number is estimated around 5% of total nodes, while all of the processes as data fusion and data aggregation are performed locally in the clusters. During LEACH operation, cluster heads change randomly to balance remaining energy and network lifetime. In the cluster formation process, nodes compare signal strength of their neighbor cluster heads and join the one with the strongest signal. When a node becomes a cluster head, it cannot \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Protocol** & **Advantages** & **Disadvantages** & **Scalability** & **Mobility** & **Routing** & **Robustness** \\ \hline ZRP [44] & It has low traffic via routing. & Great delays are caused. & Limited & Good & The best route is used. & Good \\  & It has increased & It lacks GPS & & & It uses single & \\  & network lifetime and & navigation and consumes energy & Good & None & hop from cluster nodes to cluster head. & Good \\  & reduces redundant & from periodic data & & & & head. \\  & & sending. & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Synopsis of Network Structure Protocols belonging to the Flat Hybrid subcategory. become once again for P rounds. In order for a node \(n\) to become a cluster head, this node picks a random number between 0 and 1 and calculates the threshold \(T\) by using (1):  
\[T(n)=\left\{ \begin{array}{cc}\frac{P}{1-P\left(r\cdot m\cdot d\cdot\left(\frac{ 1}{P}\right)\right)}&for\;n\in G,\\ 0&\end{array} \right. \tag{1}\]  
where \(P\) is the desired cluster head percentage, \(G\) stands for the set of nodes, and \(r\) represents the number of the round selected by the cluster head. Then, the specific node becomes cluster head for this round if the number is below this threshold. In LEACH, transmissions are reduced, resulting in reduced energy loss. In addition, global network knowledge is not required. However, as it uses single hop routing, it is not suggested for sensor networks deployed in large areas. In addition, dynamic clustering creates overhead that may shrink gain in energy consumption. _Low-Energy adaptive Clustering Hierarchy-Centralized_ (LEACH-C) [47] is a variation of LEACH that forms clusters using the BS. During clustering, nodes transmit to the BS energy level and location information. The BS divides the network into a fixed number of clusters and their cluster heads, based on the energy needed during data transmission from cluster nodes to their cluster heads. In LEACH-C, more energy efficient clusters for data transmission are produced. In addition, the optimal number of cluster heads is predetermined. On the other hand, overhead is produced to the BS.