# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 2 IoT Technologies
### Low-Power Wide-Area Networks (LPWANs)
#### 2.2.2 Cellular IoT  
Cellular IoT (cIoT) technologies are one of the two categories comprising the LPWAN technologies. Cellular LPWANs extend the existing cellular systems, allowing the reuse of the same architecture for long-range IoT scenarios and applications, and in particular mMTC. They operate in the licensed spectrum, ensuring reliability while using a consistent and standardized infrastructure. There are three types of cIoT technologies, that is, NB-IoT, LTE-M and the less used EC-GSM-IoT. All three technologies derive from cellular networks and share some common characteristics. The first one is the presence of extended coverage mechanisms, in compliance with 3 GPP TR 45.820 [64], defining coverage classes related to the quality of the network signal experienced by the IoT device. A second common characteristic is the presence of two common Radio Resource Control (RRC) modes, that is Idle (the UE is not connected and there is no radio link, but the UE is registered to the network and can be reached if needed) and Connected (the UE is connected and the network can transmit/receive data to/from it). Finally, cIoT technologies employ similar operations when a device requests to initiate a data transfer. This process happens in three phases [56]: (1) Idle mode operation; (2) system access procedure, with the device initiating a connection to the network; and (3) resource assignment by the network to the device, that concludes with the device entering in Connected mode. While in Idle mode, several procedures can take place, that must be taken into account in analyzing and modeling energy consumption:  
* _Cell Selection_: During this procedure, the device performs a full scan of the supported frequency bands, and collects information needed to identify the most suitable cell with respect to a set of criteria set by both device and network;
* _Cell Reselection:_ During cell selection, the device selects a suitable cell and then camps in that cell. Sometimes, a device may need to change the cell it is connected and select a new cell;
* _System Information Acquisition:_ After selecting and camping on a suitable, the device needs to acquire the full set of System Information (SI), which are grouped into messages called System Information Block (SIB) [56], where in LTE-M and NB-IoT, SIB1 and SIB2 are the SIBs that contain the most important SI;
* _Paging:_ It is a procedure that the network uses in order to determine the location of the subscriber, before the data exchange can be established. It is also used to alert the decide of an incoming data transmission. In case the device is in Idle mode and there is a need to re-establish a connection, the base station broadcasts a paging message reporting the device tracking area, which can include several cells. The paging message contains a set of different IDs, considering that the base station pages several users at a time. When the device decodes its own ID, it starts changing its mode form Idle to Connected;
* _Random Access Procedure:_ This procedure allows for a device to identify itself in the network and establish a connection. In essence, this procedure allows the device to transit from Idle mode to Connected mode;
* _Access Control:_ This mechanism is used in order to protect cellular networks from overuse. It is initiated in special situations, for example, during network power outages. Beyond Idle and Connected modes, devices can move to other modes that are particularly relevant from the point of view of energy efficiency. In particular, a device can enter the Power Saving Mode (PSM), during which it does not apply any of the Idle mode operations, e.g, it does not transmit or monitor paging messages and, in general, it uses the smallest possible amount of energy. PSM is discussed in detail in Section 4. Although the three cIoT technologies share the characteristics discussed above, the also possess different capabilities and provide specific features. Therefore, we briefly discuss them in the following dedicated paragraphs.