# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 4 Classification of Energy Efficiency Methods
### Mechanisms for RFID Active Tags
#### 4.1.2 Zigbee
The scenario is modeled as a network composed of \(N_{i}\) nodes, where each node \(N_{i}\) monitors its energy consumption, denoted as \(E_{i}\left(\Delta T_{j}\right)\), \(j\in\{0,1,2,\dots\}\) where:  
\[E_{i}(\Delta T_{j})=\alpha\cdot E_{i}(\Delta T_{j-1})+(1-\alpha)\cdot\bar{E}_ {i}(\Delta T_{j}), \tag{17}\]  
where \(0<\alpha<1\), \(E_{i}(\Delta T_{j-1})\) and \(\bar{E}_{i}(\Delta T_{j})\) are the previous and new estimated values, respectively. Equation (17) evaluates the energy consumption of a single node; then, the total energy of the network is obtained by considering the total energy consumption for any possible route. Denoted by \(L=\{L_{i}\mid i=1,\dots,m\}\) the set of all the possible routes, the average total energy for the entire network, including all paths, is then derived as follows:  
\[\overline{E_{total}}=\frac{1}{m}\sum_{i=1}^{m}E_{total}(L_{i}), \tag{18}\]  
where \(E_{total}\) is the total transmission energy for a path, \(L_{i}\) is a \(i\)th path, and \(m\) is the total number of paths. Simulation results showed that the residual energy of E-AODVjr is higher than the residual energy of AODV, extending in this way the network lifetime. Energy Aware Multi-Tree Routing (EAMTR) algorithm is a routing algorithm proposed in [118]. It is based on clustering and aims at building multiple routing trees in a sink-based network (a sink node represent a base station). The data collected by the other nodes that are part of a cluster will be transmitted to the CH, which in turn transmits these data to the sink node (base station). The EAMTR algorithm functions based on four phases:  
* _Initiation Phase_: During this phase, the sink node starts to accept nodes, thus creating the first tree. In this phase, the total number of nodes that are part of the tree is calculated, with the the sink node as a starting point, to continue with assigning the addresses to the nodes;
* _Tree Selection Phase_: It starts from the sink (root) node to the leaf nodes, and begins from the first-level nodes (the nodes that have 1-hop distance to the sink). The nodes in the tree form a relationship that can be also called a parent-child relationship, with some nodes acting as parent nodes, when they have nodes that are dependent on them, and the other nodes called child nodes;
* _Normal Phase_: During this phase, each node knows all the nodes it is connected to, and which of these nodes acts as parent to other different nodes. The sink node can be used as a router for the nodes that are the furthest from it, and helps redirecting the packets using the specified tree of each node;
* _Recovery phase_: In this phase, there is a route redundancy, considering that each node knows the route to use to send the packets at the sink. At the same time the network is more resilient in the case of node failure, and in the off chance of a node failure, a new node simply replaces the old node in the tree. For each node, EAMTR selects the least congested route to the node, thus balancing the energy consumption of the nodes and at the same time extending the network lifetime. The Improved Routing Protocol presented in [119] is another version of a tree clustering algorithm focusing on the residual energy of the nodes, and helps selecting the optimal nodes so to avoid in the nodes with low residual energy. In [120], the Energy-Efficient Shortcut Tree Routing (ESTR) was proposed and used to find the best next-hop node, based on load balancing on links and nodes, and on the shortest path selection. It provides the network with a low-delay route and a mechanism for energy-balancing. ESTR functions on the assumption that every node knows the neighbor's table, which contains the information about relationship between the nodes (which node is a parent and which node is a child) and link-state information about neighboring devices, when a node receives any frame from a neighbor, and so forth.