# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 3 Network Structure
### Hierarchical
#### 3.2.2 Zone Based  
In this subcategory, the network area is divided into zones. A zone, depending on the operation of each protocol, can operate either as a cluster with cluster head election or as a direct communication zone without cluster heads. Furthermore, each zone could be assigned a particular energy level of nodes depending of the routing algorithm or include relay nodes that transmit data from sub cluster heads to the base station. In what follows in this section, four typical examples of zone based energy efficient routing protocols are described. _Distributed Regional Energy Efficient Multi-hop-Maximum Energy_ (DREEM-ME) [103] is a routing protocol for homogeneous WSNs that uses a fixed number of cluster heads in each round and ensures that these cluster heads have the maximum energy. DREEM-ME separates the network into concentric circles, forming four equal sectors that divide the area in regions, excluding the first, which is in the center of the area containing the base station. Each region elects cluster heads and utilizes multi-hop communication between the other regions with the exception of region one, which uses direct communication to communicate with the base station. Furthermore, DREEM-ME reduces packet loss, during bad wireless link status, with the implementation of the Random Uniformed Distribution Model mechanism. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Protocol** & **Advantages** & **Disadvantages** & **Scalability** & **Mobility** & **Routing** & **Robustness** \\ \hline \hline \multirow{8}{*}{M-BDCP [96]} & \multirow{8}{*}{It has increased network lifetime.} & It needs balancing improvements and the BS is in the center of the network. & \multirow{8}{*}{Low} & \multirow{8}{*}{None} & Nodes send data to their cluster heads, which & \\  & & It has increased & improvements and & & & forward them to & Low \\  & & the BS is in the center & & & the BS using & & \\  & & of the network. & & & & multi-hop & \\  & & & & & & routing. & \\  & & & & & & Nodes send data & \\  & & & & & to their cluster & & \\  & & & & & & heads, which & \\  & & & & & & forward them to & Good \\  & & head selection. & & & & the BS using & \\  & & & & & & single-hop & \\  & & & & & & routing. & \\  & & & & & & & routing. & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\ EDGE [99] & It avoids flooding. & It causes traffic & & & & & children nodes & Low \\  & & BS and fixed sensor & & & & to their parent & \\  & & & & & & nodes, and then & \\  & & & & & & to the BS. & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & \\  & & & & & & forwarded from & \\ CTPA [100] & It has reduced energy consumption. & The clustering buffering mechanism may cause overhead. & Low & None & via the cluster heads using & \\  & & & & & & minimum & \\  & & & & & & spanning tree. & \\  & & It faces increased & & & & The children & \\  & & energy consumption & & & nodes route data & \\ GSTEB [101] & lifetime and avoids & due to direct routing & & & & to their parent & \\  & collisions. & & and energy overhead & & & nodes who & \\  & & due to large number & & & & forward them & \\  & & & & & reach the BS. & \\  & & & & & & Intra cluster & \\  & It extends both & & & & & single-hop and & \\  & network lifetime and & & simple convergence. & & & inter cluster & \\  & stability period. & & & & multi–hop & \\  & & & & & routing is used. & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: _Cont._Thus, by using DREEM-ME, not only is energy saving achieved, but also a lower number of packets are dropped. Additionally, extended stability period is attained. On the other hand, energy depletion is not balanced. In addition, scalability is limited. Regional Energy Efficient Cluster Heads based on Maximum Energy (REECH-ME) [104] is a clustering protocol that divides the network in nine regions with the first positioned in the center of the network, being the largest one containing the base station.