. The UE MAC will reattempt the process at a higher coverage level RACH preamble set, if it does not receive the RAR message after the anticipated number of attempts at a certain level. If the contention resolution is not successful, the UE will continue to use the same coverage level RACH preamble set [2,21]. 6.4.4 Power-Saving Modes Power-save mode (PSM) is a power conserving mechanism in NB-IoT that allows the devices to skip the periodic paging channel monitoring cycles between active data transmis- sions, letting the device enter a sleep state. However, the device becomes unreachable when PSM is active; therefore, it is best utilized by device-originated or scheduled applications, where the device initiates communication with the network. Assuming there is no device- terminated data, an NB-IoT device can remain in PSM state for a long time, with the upper limit determined by the maximum value of the tracking area update (TAU) timer. During the PSM active state, the access stratum at the device is turned off, and the device would not monitor paging messages or perform any radio resource management measurements. In addi- tion, the PSM enables more efficient low-power mode entry/exit, as the device remains regis- tered with the network and its NAS state is maintained during the PSM without the need to spend additional cycles to setup registration/connection after each PSM exit event. Examples of applications that can take advantage of PSM include smart meters, sensors, and any IoT devices that periodically send data to the network. When a device initiates PSM with the net- work, it provides two preferred internal timers (T3324 and T3412), where the PSM time is the difference between these timers. The network may accept these values or set different ones. The network then retains state information and the device remains registered with the network. If a device wakes up and sends data before the expiration of the time interval it Internet of Things Rel-12 power save mode (PSM) eDRX Idle mode sleep-cycle extension