# Energy-Aware Wireless Sensor Networks for Smart Buildings: A Review
## 6 Networking Layer
### Network Topologies for WSN  
Implementing a WSN can be in different topologies such as Star, Tree, Mesh, Cluster, and Grid topology. Here we discuss the impact of topologies and routing protocols used in WSN on energy consumption and the network lifetime (Tables 6 and 7).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Data Acquisition Technique** & **Topology** & **Star** & **Tree** & **Mesh** & **Cluster** \\ \hline Directed diffusion & YES & YES & YES & YES \\ \hline Chain construction & NO & YES & YES &
\begin{tabular}{c} YES \\ (Between cluster heads) \\ \end{tabular} \\ \hline Probabilistic model & YES & YES & YES & YES \\ \hline Data correlation & NO & NO & NO & YES \\ \hline Query model & NO & YES & YES & YES \\ \hline \hline \end{tabular}
\end{table}
Table 6: The usage of data acquisition techniques in different topologies.  
(1) Star Topology  
This is the easiest network topology to implement. In this topology, nodes are connected to the base station and cannot communicate directly. It has the disadvantage of single path communication; hence, there is no backup if the link to the base station is lost. Additionally, if a node wants to reach the base station far from it, it needs to generate a high-power signal. In such a topology, we cannot benefit from several data acquisition techniques such as data aggregation.  
(2) Tree Topology  
In this topology, WSN nodes form a tree where the gateway is the root of that tree. The path from any node to the gateway may be single-hop or multi-hop. Such topology allows for the usage of data acquisition techniques and energy-aware routing protocols. However, as there is always a single path from any node to the gateway, all the sub-nodes get disconnected from the network if one node in the path fails.  
(3) Mesh Topology  
The Mesh topology presents several advantages for WSN. On the one hand, the network is a full mesh where each node is connected to all other nodes. On the other hand, when each node is only connected to some other nodes, the network is called partial mesh. This topology has the advantages of self-healing, self-optimization, self-configuration, and self-protection, which are critical requirements in the design of WSN.  
(4) Cluster Topology  
In this topology, the WSN network is divided into clusters. At any moment, there should be at least one node working in each cluster, and this node is called the cluster head. The head is responsible for transmitting data packets and routing information from the subsequent nodes to the gateway.  
The topology choice directly affects the data acquisition technique that can be used.  
The authors in [91, 92, 93] concluded that the energy consumption depends on the WSN network topology. This energy consumption is due to network joining, route discovery, route update, and network self-healing. Therefore, the network designer should consider the number of sensor nodes deployed before considering a specific network topology. If the number of sensor nodes is low, the star topology will serve as the optimal topology since it offers better performance and reliability with low energy consumption. However, as the number of nodes increases, the designer should consider cluster tree topology as it divides the whole WSN network into zones where each cluster head manages its zone.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Routing Protocol–Topology** & **Star** & **Tree** & **Mesh** & **Cluster** \\ \hline LEACH & NO & NO & NO & YES \\ \hline TEEN & NO & NO & NO & YES \\ \hline APTEEN & NO & NO & NO & YES \\ \hline PEGASIS & NO & YES & YES & NO \\ \hline SPIN & NO & NO & YES & NO \\ \hline RR & NO & NO & YES & YES \\ \hline GEAR & NO & NO & YES & YES \\ \hline GAF & NO & NO & YES & YES \\ \hline EAP & NO & NO & NO & YES \\ \hline GBR & NO & NO & NO & YES \\ \hline AESC & NO & YES & YES & YES \\ \hline PERP & NO & YES & YES & YES \\ \hline ODYSSE & NO & YES & YES & YES \\ \hline \hline \end{tabular}
\end{table}
Table 7: The usage of energy-aware routing protocols in different topologies.