 T3412 (timer) T3324 (timer) Up to 9.22s in NB-loT Paging occasions Sleep Sleep (monitoring) (device not reachable) Idle mode power consumption Up to 2.56s Figure 6.24 Illustration of PSM and eDRX operation [21,24]. agreed with the network, a reattach procedure is not required. However, in a similar manner to a radio module that has been powered off, a radio module in PSM cannot be contacted by the network while it is in sleep mode [7,8,21]. One problem with PSM mechanism is the ability to support device-terminated traffic, as the UE is unreachable when it is in PSM state. The device would become reachable by the net- work as the TAU timer expires (Fig. 6.24), which can introduce significant latency for device-terminated traffic. While periodic TAU can be configured to occur more frequently to match the UE's delay requirement, such configurations would result in additional signal- ing overhead from unnecessary periodic TAU procedures and increased device power con- sumption. To address this shortcoming of PSM, the extended DRX (eDRX) was introduced in 3GPP Rel-13. The eDRX is an extension of LTE DRX feature which can be used with NB-IoT devices to reduce power consumption. The eDRX can be used without PSM or in conjunction with it to obtain additional power savings. It allows the time interval during which a device is not listening to the network to be greatly extended. For an NB-IoT appli- cation, it might be acceptable not to be reachable for a few seconds or longer. Although it does not provide the same level of power saving as PSM, for some applications, the eDRX may provide a good compromise between device reachability and power consumption reduction. Fig. 6.24 illustrates the concept and the operation of PSM and eDRX mechan- isms. In eDRX, the DRX cycle is extended up to and beyond 10.24 seconds in idle mode, with a maximum value of 2621.44 seconds. For NB-IoT, the maximum value of the DRX cycle is 10,485.76 seconds [7,21]. 6.4.5 Paging and Mobility Paging is used to notify an idle-mode UE of a pending down