# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 4 Classification of Energy Efficiency Methods
### Energy Efficiency in LPWANs
#### 4.2.2 Cellular IoT (cIoT)
\begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**eDRX Timer** & **Description** \\ \hline Inactivity Timer & It is controlled by the eNB, and with the expiration of this timer, causes the transition from RRC Connected to RRC Idle state. \\ \hline Active Timer (T3324) & It determines the duration for which the device can be reached for DL through the RRC Idle mode. The timer is started when the device switches from RRC Connected to RRC Idle mode. The moment the timer expires, the device switches to PSM. \\ \hline Paging Time Window (PTW) & The total duration of a paging event, consisting of multiple DRX cycles. The paging event fits in the PTW. \\ \hline DRX Cycle & The duration of a DRX cycle and consists of the multiple Paging Occasions cycle; where the UE listens for Paging Occasions and sleeps for the following ones. \\ \hline eDRX Cycle & The time between two PTWs. \\ \hline On Duration & Time period during which the device monitors the DL control channel (DCCH). \\ \hline eDRX period & Time period during which the device stops monitoring the DCCH and also goes in deep sleep mode. \\ \hline \hline \end{tabular}
\end{table} 
Table 6: eDRX Timers. Figure 9: Power Saving Mode (PSM) and extended Discontinuous Reception (eDRX) mechanisms in NarrowBand IoT (NB-IoT). instructs the devices to switch to sleep mode using the RRC Connection Release procedure. The DRX cycles are then reconfigured to their original value, with an additional RRC Connection Reconfiguration message, after the multicast transmission has ended. Simulation results show that DA-SC increases the energy consumption, because the devices are required to switch back and forth between different cycles;
* DRX Respecting, Standard Incompliant (DR-SI): The mechanism modifies the paging protocol to notify the devices that a multicast transmission is about to start. Adopting DR-SI the devices can retain their DRX cycles, as in DR-SC. The device using the DR-SI have lower energy consumption, since it does not receive any paging in order to receive downlink data, so there is no need for this device to wake-up and connect to the network. This method is however not compliant with the NB-IoT standard. A contribution to the empirical study of energy and power consumption in NB-IoT using sleep/wake-up techniques is given in [154]. It includes measurements conducted during a period of several months, from October 2018 till October 2019, and gives an evaluation of the power saving methods supported by the available NB-IoT networks, taking into account different network configurations. The experimental setup consists in two NB-IoT modules, SARA-N211-O2B [155] and Quectel BC95-G [156], combined with the Otii Arc power measurement device to measure energy consumption. The measurements are achieved by connecting each kit to a laptop running different scenarios, for example, the sending of UDP packets of different sizes at different frequencies. The experiment also includes different locations, based on coverage conditions, classified as _good_ and _bad_ coverage. The collected parameters include RRC Connection and Release events, Signal to Noise Ratio (SNR), Transmission Power (TX Power), ECL (Extended Coverage Level), Physical Cell Identity (PCI), Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ). The experimental scenarios include three modes, that use the Release Assistance Information (RAI):  
* TX/RX (transmit/receive) Default Timers (RAI-000): In this mode the device sends an uplink packet to a server, which in turn echoes it back. Then, it stays in Connected mode, while monitoring the channel for any paging messages. Finally, it enters into PSM, after the RRC Release is achieved;
* TX/RX and Release (RAI-400): When the device receives a response from the server, it releases the RRC Connected mode;
* TX and Release (RAI-200): When the device transmits its packet, it releases the RRC Connected mode.