# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 3 Energy Efficiency and Consumption Metrics and Models
### Energy and Power Consumption Models
#### 3.2.1 LPSANs  
Zigbee  
A model for the energy consumption of a Zigbee node is presented in [74]. It assumes that the network operates with one-hop transmissions between two nodes. Hence, the node consumption while communicating information bits, for both the transmitting and receiving node is modeled as:  
\[E_{k}=\frac{(I_{t}+I_{r})V_{r}k}{R_{b}}, \tag{3}\]  
where \(I_{t}\) and \(I_{r}\) are the transmit and receive current, \(V_{r}\) is the transceiver supply voltage, \(R_{b}\) is the bit rate, and \(k\) is the number of transmitted bits. Based on the above formula, the total energy consumption for transmitting data packets through a wireless link, including a Header (H) and a Frame Check Sequence (FCS) is given by:  
\[E_{P}=\frac{(I_{t}+I_{r})V_{r}(k+H+FCS)}{R_{b}}. \tag{4}\]  
The energy efficiency can be then evaluated as:  
\[\eta_{k}=\Big{(}\frac{E_{k}}{E_{p}}\Big{)}r, \tag{5}\]  
where \(r\) represents the reliability of the wireless link in terms of packet acceptance rate.  
Bluetooth and BLE  
A model that evaluates the energy consumption of a Bluetooth network is given in [75], where a Markov chain is used for the analysis. A later work in [76] provides instead a model taking into account Bluetooth-specific hardware characteristics. This model has been further extended in [77], where the Bluetooth network composition in terms of scatternets is taken into account. In [77], the power model for Bluetooth assume that each node in a scatternet can be in active or low-power sniff modes (as mentioned in Section 2, see Bluetooth subsection). The authors validate the model over different scatternets and real devices. Hold and Park modes are however not included in the model.  
As regards BLE, an empirical model based on real measurements is proposed in [78]. In particular, a power monitor is used to measure the power consumption of BLE devices in two scenarios: (1) before a connection is formed between slave and master nodes, that is, taking into account the energy consumption of these two separately; and (2) once the master/slave connection is established. Measurements confirm that BLE has a rather low consumption and fulfills its low consumption goals. In the same work, the consumption of a BLE network is modeled in order to evaluate the trade-off between energy consumption, latency, and piconet size, taking into account the following parameters: (1) _comSlaveLatency_, that is, the number of consecutive connections during which a slave is not required to listen to the master and can keep its radio off; (2) _comInterval_, that is, the time between the beginning of two consecutive connections; and (3) _scannWindow_, that is, the time during which a device (master or slave) scans o data exchange channel. The model allows the authors in [78] to conclude that bit errors significantly affect the energy consumption. As a matter of fact, a slave receives a packet from the master at the beginning of a connection event. When this does not happen (due to bit errors), the slave is forced to keep on listening, until the packet is received correctly, which ultimately causes a consumption increase.  
Based on [78], a BLE energy consumption model is also defined in [79], focusing in particular on the impact of device discovery. The model considers (1) the energy consumption caused by the mechanism adopted for Advertisement events [80], during which a BLE device transmits the same packet on three advertising channels in a row in order to increase the probability of being discovered by a nearby device; (2) the total energy consumption in the Transmitter and Receiver devices during data exchange; and (3) the energy consumption required to switch across channels. Results show that the model can accurately predict energy consumption due to advertisement events, and provide a detailed analysis of the impact of settings adopted in advertisement procedures on energy consumption levels.