# Energy-Aware Wireless Sensor Networks for Smart Buildings: A Review
## 6 Networking Layer
### The Network Layer Protocols  
One of the approaches to minimize data transmission is in-node and in-network data processing. It is more efficient to process data than transmit it, but data fusion complicates secure routing and introduces delay and unreliability. Another way is the use of hierarchical routing that helps to achieve energy efficiency in WSN. However, it may suffer from various issues such as optimum cluster size, cluster head selection, communication, cluster topology, and scalability. These issues become more challenging in the case of node failure where network topology changes, impacting network connectivity. Eventually, the majority of network energy will be consumed in cluster formation rather than data transmission. The multichannel routing approach allows sensors to use multiple orthogonal channels for sending data. Yes, it ensures parallel communications, decreases delay, and increases throughput in WSNs. However, it requires additional resources for channel scanning to decide upon available ones, channel decision to select a channel, and channel switching to switch from a channel to another one. For sure, this will require more energy to provide the needed Quality of Service (QoS) in WSN. In this section, we present the energy-aware routing protocols for WSN. Low-Energy Adaptive Clustering Hierarchy (LEACH)  
It is a protocol with self-organizing and adaptive clustering. It randomly distributes the energy load among the sensors in the network. LEACH assumes that all WSN nodes have enough power to transmit data to reach the gateway. In this protocol, WSN nodeshave enough computational power to support different MAC protocols. Nodes that are located close to each other have correlated data. A node called cluster-head (CH) in each cluster acts as the local base station. LEACH protocol randomly rotates the cluster head role between all WSN nodes in order to make activities equally shared among the WSN nodes; hence, they consume power equally. LEACH also performs data fusion when data is sent from the cluster heads to the base station, thus reducing energy consumption and extending network lifetime [50]. Threshold-Sensitive Energy-Efficient Sensor Network Protocol (TEEN)  
TEEN is a cluster-based hierarchical routing protocol for WSN that is based on LEACH protocol. TEEN is dedicated to time-critical applications, and it is based on two assumptions: first, the assumption that sensors and the base station have the same initial energy, and second, the assumption that the base station can transmit data to all nodes in the WSN without intermediate nodes. This protocol consists of simple nodes, first-level cluster heads, and second-level cluster heads. TEEN follows LEACH's strategy to form the cluster. Hence, first-level cluster heads are based far from the base station, and second-level cluster heads are based near the base station. The cluster head sends two types of data: the hard threshold and the soft threshold. The hard threshold represents data in the range of interest, reducing the number of packet transmissions. The soft threshold represents data out of the range of interest; hence, it is transmitted [59]. Adaptive Periodic Threshold-Sensitive Energy-Efficient Sensor Network Protocol (APTEEN)  
It is an improved version that has all the features of TEEN. It was developed for hybrid networks to capture periodic data collection and react to time-critical events [60]. APTEEN supports queries like historical analysis of past data values, snapshots of the current network view, and persistent monitoring of an event. After selecting the cluster head, the cluster head periodically broadcasts the following parameters: Attributes (interested physical parameters), Thresholds (hard threshold value and soft threshold value), Time schedule (time slot using TDMA), and the count time (the maximum period between two successive reports sent by a node). If a node does not send data during the count time, it must sense the environment and send the sampled data.