# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 3 Energy Efficiency and Consumption Metrics and Models
### Energy Efficiency and Consumption Metrics  
In this section, we describe the main metrics that are commonly used to study and evaluate energy efficiency and consumption in IoT devices and networks. Such metrics can be categorized as follows [66]:  
* _Energy per correctly received bit_, that specifies the amount of energy spent to transmit one bit of information from the source (e.g., an IoT device) to a destination (e.g., another device or the network);* _Energy per reported event_, that indicates the energy needed to report an event happening in the network. For example, if one device receives an information that is deemed more urgent, the transmission of this information takes precedence over information related to other devices; in this case, transmitting this information will require to consume more energy since its transmission is unexpected. In parallel, the information from other devices is halted until the urgent data is fully transmitted; hence, the normal flow of transmission is interrupted and starts again after the interruption, making the other devices to be in a waiting state for transmission to restart. This indicator is particularly used in specific IoT applications, for example, health care services, transportation systems (when it is require to transmit information of happened accidents), and monitoring and alerting systems;
* _Delay/energy trade-off_, indicating the time it takes to report an urgent event happening in the network/system. As explained for the above indicator, when there is an event happening, the network has to prioritize the data from this event in relation to other data; so there is a delay in the time it takes to transmit these data and the time it take the network to go back to function normally;
* _Bits-per-Joule system capacity_, that is commonly used to measure the throughput of the system for unit-energy consumption [24]. The bits-per-Joule capacity increases when the number of nodes in the network increases, as shown in [67]. The bits-per-Joule metric is studied in more detail in [68; 69; 70; 71], among others;
* _Battery lifetime_, that focuses on the device, and is inversely proportional to its energy consumption;
* _Network lifetime_, that provides instead a global network characterization. It can be defined as the operational time of the network during which it is able to perform the dedicated task(s), or as the time until the first device or group of devices in the network runs out of energy. A short network lifetime implies the need for changing (or charging, if possible)IoT devices more often, leading to higher network management costs;
* _Duty cycle_, as a way to measure energy consumption of the devices. Considering that the duty cycle consists of the period in which a node is active, it is an important metric in studying energy efficiency. Usually a network is duty-cycled to ensure long node and network lifetime, leading to the nodes being in sleep mode most of the time, with their radios turned off. When there is a transmission, the nodes have to wake up and then get ready to receive the transmission, consuming in this case energy for the time it takes to wake up, and then go back to sleep mode after the transmission has ended. When the duty cycle of a node increases, the energy consumption also increases, leading to the decrease of the nodes lifetime.