The primary objective of the eDRX (extended Discontinuous Reception) mode in NB-IoT is to support longer paging listen intervals, thereby achieving low power consumption. The existing 2.56-second paging interval is due to high power consumption at the UE (User Equipment). However, when downlink data transmission frequency is relatively low, through negotiation and coordination between the core network and the user terminal, the UE can adjust most of the paging monitoring intervals to achieve power savings.

Cycle Differences Between eDRX and PSM

eDRX (Extended Discontinuous Reception) is designed to extend the time interval of traditional DRX (Discontinuous Reception), reducing the frequency and number of DRX cycles to save power. The minimum DRX interval was once 2.56 seconds, which is quite common for IoT devices that transmit data over long periods.

eDRX operates in both idle and connected modes:

  • In connected mode, eDRX extends the reception interval to 10.24 seconds.

  • In idle mode, eDRX can extend the paging monitor and TAU (Tracking Area Update) intervals to more than 40 minutes.

Differences Between Switching Between eDRX and PSM Modes

The key difference between PSM (Power Saving Mode) and eDRX lies in the time interval it takes for the terminal to transition from sleep mode to a receiving mode.

  • In PSM mode, the terminal must first transition from sleep mode to active mode, and then to idle mode.

  • In eDRX mode, the terminal is already in idle mode and can quickly transition to the receiving mode without requiring additional signaling.

Both PSM and eDRX significantly reduce terminal power consumption, but they trade off real-time performance for longer sleep intervals.

For applications that require remote and intermittent monitoring, such as remote positioning, calling, and configuration management, PSM is not suitable if real-time performance is needed. On the other hand, if some delay is acceptable, eDRX is a better option, and the paging cycle can be set as short as possible (with a minimum of 20 seconds, depending on the acceptable delay).

UEs (User Equipment) can request to enable PSM or eDRX during ATTACH and TAU, but whether PSM or eDRX is activated, or both, and the intervals, are determined by the network side.

Compared to PSM mode, eDRX mode is slightly less power-efficient, but it significantly improves downlink communication link accessibility

Additional Details:

  • Idle State: The UE primarily intercepts paging channels and broadcast channels. If data channel monitoring is required, the UE must switch from idle to connected mode. Paging DRX is controlled by the NAS (Non-Access Stratum), and when the coverage range is extended, the extension cycle supports paging channel reception.

  • Connected State: Coverage enhancement features can be used, and the repetition count is dynamically configured by the eNB (evolved NodeB) base station. Therefore, all eDRX timers use the PDCCH (Physical Downlink Control Channel) time intervals, which removes the short cycle DRX function. After data transmission timeout, the UE will initiate the eDRX timer.