# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 3 Network Structure
### Hierarchical
#### 3.2.1 Hierarchy Based
Moreover, the cluster head closer to the BS is elected as a Vice Cluster Head (VCH), which acts as backup in the case that another cluster head fails. Otherwise, it performs as a simple cluster node. To further reduce energy consumption, UMBIC protocol makes use of MOIA when the energy reservoirs of the cluster heads are above an energy threshold. Likewise, during the clustering process, UMBIC protocol uses the UCM to create unequal clusters depending on their distance from the BS and their residual energy in order to configure the intra-cluster and inter-cluster energy consumption. To evaluate the results of UMBIC protocol, the Balanced Extent of Energy Dissipation (BEED) metric is used. Specifically, BEED measures the ability of the protocol to balance the energy dissipation. By using UMBIC protocol, network lifetime is prolonged. Additionally, extended stability is achieved and great scalability is accomplished. On the other hand, there is lack of fast and simple convergence. The main features of the aforementioned hierarchy based hierarchical energy efficient routing protocols are synoptically presented in Table 7. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Protocol** & **Advantages** & **Disadvantages** & **Scalability** & **Mobility** & **Routing** & **Robustness** \\ \hline LEACH [46] & There is reduction of energy loss. Global network knowledge is not required. It provides more energy efficient clusters for data transmission and predetermined optimal number of cluster heads. & As it uses single hop routing, no scalability is offered. Dynamic clustering generates overhead. & Low & Static BS & The shortest route is used. & Low \\ \multirow{4}{*}{\begin{tabular}{c} LEACH-C \\ [47] \\ \end{tabular} } & There is reduction of energy loss. Global network knowledge is not required. It provides more energy efficient clusters for data transmission and predetermined optimal number of cluster heads. & It produces overhead to the BS. & Low & Static BS & The best route is selected. & Low \\ \multirow{4}{*}{\begin{tabular}{c} Q-LEACH \\ [48] \\ \end{tabular} } & There is reduction of energy loss. Global network knowledge is not required. It provides more energy efficient clusters for data transmission and predetermined optimal number of cluster heads. & It may increase the energy holes of the network. & Low & None & The shortest route from each region to the BS is used. & Good \\ \multirow{4}{*}{\begin{tabular}{c} U-LEACH \\ [49] \\ \end{tabular} } & Energy consumption is reduced. Network lifetime is extended. & It does not consider distance between master cluster head and BS. & Low & Static BS & Data are sent from cluster nodes to the cluster head, and via the master cluster head reach the BS. & Good \\ \multirow{4}{*}{\
\begin{tabular}{c} H-LEACH \\ [50] \\ \end{tabular} } & It extends network lifetime & It has high number of unused nodes below the threshold. & Low & None & The shortest route is used. & Good \\ \multirow{4}{*}{
\begin{tabular}{c} TL-LEACH \\ [51] \\ \end{tabular} } & Few nodes communicate with the BS. Network lifetime is extended. & The three-layer scheme creates overhead. & Low & None & Cluster nodes send their data to the cluster head, which forwards them to the BS. & Good \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Synopsis of Hierarchical Protocols belonging to Hierarchy based subcategory. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Protocol** & **Advantages** & **Disadvantages** & **Scalability** & **Mobility** & **Routing** & **Robustness** \\ \hline  & & & & & & Nodes below the & \\  & & & & & & & distance \\  & & & & & & threshold use & \\ EHE-LEACH & It has great network & & & & & direct routing & \\ EHE-LEACH & It has great network & & & & & while those & \\
[52] & lifetime and improved & & mobility. & & & above it use & \\  & & & & & & their cluster & \\  & & & & & & head to & \\  & & & & & & communicate & \\  & & & & & & with the BS.