IoT power saving

NB-IoT and LTE-M power saving: PSM and eDRX

Power Saving Mode and extended idle-mode DRX, and what they mean for reachability.

IoT4G

Overview

A UE using PSM remains registered with the network without re-attaching or re-establishing PDN connections, and is not immediately reachable while in PSM (TS 23.682). The NAS timers involved include T3324 (a GPRS timer 2 value in the Attach request, Attach accept and Tracking area update accept messages) and T3412, including its extended value IE (TS 24.301).

For eDRX, TS 23.682 states that for wideband E-UTRAN the extended idle-mode DRX values start at 5.12 s and go up to a maximum of 2621.44 s (almost 44 minutes). For NB-IoT, TS 36.304 allows eDRX cycles of 2 to 1024 hyper-frames (a hyper-frame is 10.24 s, so up to 10485.76 s). A Paging Time Window (PTW) is the period in extended DRX during which the UE monitors paging occasions following the DRX cycle (TS 36.304).

NB-IoT behaves differently from other UEs in connected mode: neighbouring cell measurements and measurement reporting do not apply to NB-IoT, and the NB-IoT reconfiguration message (RRCConnectionReconfiguration-NB) carries no mobilityControlInfo. TS 36.331 does define RRCConnectionReestablishment-NB and RRCConnectionResume-NB messages.

Call flow

1
UEtoMME
Attach Request with T3324 value
TS 24.301 defines a T3324 value IE in the Attach request.
2
MMEtoUE
Attach Accept or Tracking Area Update Accept
Both messages include a T3324 value IE.

Timers and parameters

Name	Where	Role
T3412 (and extended value)	UE	Periodic tracking area update timer. Default 54 minutes.
T3324	UE	PSM timer, coded as GPRS timer 2.
eDRX cycle and PTW	UE	Up to 2621.44 s for wideband E-UTRAN and 10485.76 s for NB-IoT (idle mode).

Full parameter and timer lookup →

Specifications

Spec	What it covers
TS 23.682	Architecture for machine-type communication, including PSM and eDRX.
TS 24.301	T3324 and T3412 (extended value) in attach and TAU messages.
TS 36.304	eDRX cycle and Paging Time Window in idle mode.
TS 36.331	NB-IoT RRC messages.

Failure behaviour in the specs

A UE in PSM is not immediately reachable, so network-originated traffic needs specific handling (TS 23.682).

Symptom-based troubleshooting →

Registration

5G registration and 4G tracking area update

NAS mobility management: registration in 5GS, tracking area update in EPS, and the reject causes.

4G5G

NTN

IoT-NTN: NB-IoT and LTE-M over satellite

Satellite assistance information in LTE system information for NB-IoT and eMTC, and how the UE uses it in idle mode.

IoTNTN4G

Idle mode

Cell selection and reselection

How an idle or inactive UE selects a cell by the S-criterion and keeps the best one by priority and ranking.

4G5GNTNIoT