# PSM and eDRX

https://www.datasim.com/technical/psm-edrx/

Quick answer. PSM lets a device switch its radio off for long periods while staying registered, so it is unreachable until it next wakes. eDRX lets it sleep between brief paging checks, up to about 44 minutes on LTE-M and 2.9 hours on NB-IoT. The device requests timer values and the network decides what it grants.

## Simple

Most IoT devices run on batteries and send a little data now and then. Keeping the radio listening all the time would drain the battery in days. PSM and eDRX are two standard ways to let a cellular device sleep for long periods and still stay part of the network.

### Power saving mode

With power saving mode (PSM), the device agrees two timers with the network. After it has sent its data, it stays reachable for a short active time. Then it turns its radio off and sleeps until the next periodic check-in, which can be hours or days away.

While it sleeps the device stays registered. It does not have to join the network again from scratch when it wakes, which saves time and energy. But nothing can reach it. Messages and data sent to it have to wait until it next wakes, either for its scheduled check-in or because it has something to send.

### Extended DRX

Extended discontinuous reception (eDRX) works differently. The device does not go fully silent. It wakes briefly at a fixed interval to check whether the network is trying to reach it, then sleeps again. The interval can be from a few seconds to about 44 minutes on LTE-M, and up to nearly three hours on NB-IoT.

So eDRX trades some battery life for being reachable within a known delay. PSM saves more power but leaves the device unreachable for longer.

### The network has the final say

The device asks for the timer values it wants. The network decides what it actually grants, and it can give different values or refuse. Support varies by operator, and a network the device is roaming on may handle these features differently from the home network, or not offer them at all.

### What it means for your devices

Design the application around the device's sleep. In most cases the device should contact your server when it wakes and collect any commands waiting for it, because the network holds only a limited amount of data for a sleeping device, and only where the operator has set that up. Text messages sent to the device also wait until it wakes. Test the timers your devices actually receive on each network they will use, and check them again before relying on a battery life estimate.

## Advanced

### Two ways to sleep

Power saving mode (PSM) was added in 3GPP Release 12 and extended idle-mode DRX (eDRX) in Release 13. Both are negotiated in NAS signalling during attach and tracking area update, and matter most on LTE-M and NB-IoT. TS 23.682 describes the features (clauses 4.5.4 and 4.5.13), TS 24.301 the EPS procedures and TS 24.008 the encodings.

### Power saving mode

TS 23.682 describes PSM as similar to power-off, except that the device stays registered and keeps its PDN connections, so it does not re-attach when it wakes. Two timers control it.

- **T3324, the active time.** After the device returns to idle, it listens for paging until T3324 expires. Then it enters PSM.
- **T3412, the periodic tracking area update timer.** When it expires the device wakes and performs a TAU. A device using PSM can request a long value with the T3412 extended value IE.

The device leaves PSM early whenever it has something to send. In PSM it does not listen for paging at all, so it cannot be reached until its next TAU or uplink.

The device asks for both values with the T3324 value IE and the T3412 extended value IE in ATTACH REQUEST or TRACKING AREA UPDATE REQUEST. The MME takes the request, subscription data and its own configuration into account and returns the values it allocates in the accept message. It can grant different values, or none, in which case PSM is not used.

### How long can a device sleep in PSM?

Both timers are one octet: a 3-bit unit and a 5-bit value from 0 to 31 (TS 24.008, clauses 10.5.7.4 and 10.5.7.4a).

| Timer | IE format | Units available | Longest value |
|---|---|---|---|
| T3324 active time | GPRS Timer 2 | 2 s, 1 min, 6 min (decihours) | 31 × 6 min = 186 min |
| T3412 extended | GPRS Timer 3 | 2 s, 30 s, 1 min, 10 min, 1 h, 10 h, 320 h | 31 × 320 h = 9,920 h, about 413 days |

Unit bits 111 mean the timer is deactivated. For T3412 extended that is treated as if the IE were absent. The 320-hour unit applies only to the extended periodic timers, and the device reads it as 1 hour if the message was not integrity protected.

### Extended DRX

In normal idle mode a device checks for paging once every DRX cycle, which lasts a few seconds at most. eDRX stretches the cycle. At the end of each eDRX cycle the device opens a paging time window (PTW) and checks for paging on its normal DRX cycle within that window, then sleeps again.

The device requests a cycle with the Extended DRX parameters IE (TS 24.008, clause 10.5.5.32). The network answers with the cycle and PTW it grants, which may differ from the request.

| | LTE-M (WB-S1) | NB-IoT (NB-S1) |
|---|---|---|
| eDRX cycle | 5.12 s to 2,621.44 s (about 44 minutes) | 20.48 s to 10,485.76 s (about 2.9 h) |
| Paging time window | 1.28 s to 20.48 s, in 1.28 s steps | 2.56 s to 40.96 s, in 2.56 s steps |

The 4-bit field codes 16 cycle lengths. A few short ones exist only for LTE-M, and the two longest (5,242.88 s and 10,485.76 s) only for NB-IoT.

eDRX keeps the device reachable within one cycle. PSM gives longer sleep but no reachability. A device can ask for both, and the MME decides whether to grant PSM, eDRX or both (TS 23.682 clause 4.5.13.1).

### Data and SMS for a sleeping device

Downlink data cannot be delivered while the device sleeps. TS 23.682 clause 4.5.7 defines high latency communication for this case. The MME can ask the serving gateway to buffer downlink packets until the device is expected to wake, with the number of packets decided by the gateway. An application server can also ask, through the SCEF, to be told when the device becomes reachable. Both depend on operator configuration, so the robust design is for the device to contact the server when it wakes and poll for anything waiting. TS 23.682 recommends this for periodic downlink data.

Mobile-terminated SMS is not delivered while the device is in PSM. The SMS centre holds it and retries when the network reports the device reachable. TS 23.682 notes that the active time should be long enough for that delivery, for example two DRX cycles plus 10 seconds. See [SMS for IoT and SMS API](/technical/sms/).

### AT commands

TS 27.007 defines the module commands. Values are bit strings in the TS 24.008 encodings.

```
AT+CPSMS=1,,,"00111000","00100001"
```

This enables PSM and requests a periodic TAU of 24 hours (unit 001 = 1 h, value 11000 = 24) and an active time of 1 minute (unit 001 = 1 min, value 00001). `AT+CEREG=4` makes the module report the active time and periodic TAU the network actually allocated.

```
AT+CEDRXS=1,4,"0101"
AT+CEDRXRDP
```

The first line enables eDRX on LTE-M (access technology 4, E-UTRAN WB-S1) and requests 81.92 s. Access technology 5 is NB-IoT. `AT+CEDRXRDP` returns the requested value, the value the network provided and the paging time window.

### Practical points for IoT

- Networks cap what they grant. Deutsche Telekom, for example, accepts T3412 up to 310 hours and T3324 up to 186 minutes.
- GSMA advises roaming partners to keep IPX idle timers long enough for sleeping devices: 31 days for NB-IoT and 24 hours for LTE-M.
- Avoid relying on SMS sent to the device while it uses PSM or eDRX: it may wait or expire before the device listens.
- Always read back the granted values. Battery estimates built on requested values can be badly wrong.
- Expect differences between networks. The visited network grants or refuses these features when roaming, so a device may get PSM at home and nothing abroad. Check before you rely on it, as with [roaming](/technical/roaming/) generally.
- Keep the PDN connection in mind. The IP address survives PSM, but NAT bindings on the path can time out during a long sleep, so inbound traffic will not reach the device. See [CGNAT and static IP](/technical/cgnat-static-ip/).
- Choose T3412 slightly longer than the device's own reporting period, as TS 23.682 suggests, so regular uplink replaces the TAU instead of adding to it.
- Keep the active time long enough for any waiting SMS or downlink data to arrive.

## Myths

- **Myth.** A device in PSM is switched off and has to reconnect when it wakes.
  **Fact.** The device stays registered and keeps its PDN connections, so it does not attach again when it wakes. It simply does not listen for paging until its next periodic update or uplink.
- **Myth.** Data sent to a sleeping device is stored and delivered when it wakes.
  **Fact.** The core network can buffer a limited number of downlink packets, and only where the operator has set this up. Packets can be discarded. The reliable pattern is for the device to contact its server when it wakes and collect anything waiting.
- **Myth.** A device gets the PSM and eDRX timers it asks for.
  **Fact.** The device only requests values. The network decides, and it can grant different values or refuse the feature. Always read back the values the network allocated.
- **Myth.** If PSM works on the home network it will work when roaming.
  **Fact.** The visited network decides whether to grant PSM and eDRX and with which values. Support differs between networks, so test on each network a device will use.

## Sources

1. [Architecture enhancements to facilitate communications with packet data networks and applications (PSM, eDRX, high latency communication)](https://www.3gpp.org/DynaReport/23682.htm), 3GPP, TS 23.682
2. [Mobile radio interface Layer 3 specification (GPRS Timer 2 and 3, Extended DRX parameters)](https://www.3gpp.org/DynaReport/24008.htm), 3GPP, TS 24.008
3. [Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS), T3324 and T3412 negotiation](https://www.3gpp.org/DynaReport/24301.htm), 3GPP, TS 24.301
4. [AT command set for User Equipment (+CPSMS, +CEDRXS, +CEDRXRDP)](https://www.3gpp.org/DynaReport/27007.htm), 3GPP, TS 27.007

Checked 30 October 2026.