eDRX and PSM are power saving schemes for IoT devices connected to LTE-based low-power wide-area networks (LPWAN).
Because keeping power consumption low is a critical need for many IoT use cases, devices connected to LTE-M or NB-IoT (and LTE Cat-1 bis) networks can benefit from eDRX and PSM when enabled by the network operator and configured on the device level.
eDRX allows devices to stay idle during data reception, offering a balance between connectivity and power savings, while PSM enables a “deep sleep” mode for maximum power conservation but with delayed responses.
Let’s define and compare these features in the next sections, as well as provide examples and caveats about how they work in real-world use.
Table of Contents
What is eDRX?
Short for Extended Discontinuous Reception, eDRX is a power-saving feature designed to conserve the battery life of IoT devices connected to LTE networks (LTE-M or NB-IoT), specified in 3GPP Release 13 back in 2015.
eDRX lets devices take naps to save power, waking briefly (1ms) to check for messages, which uses much less energy.
Think of eDRX as scheduled breaks for your devices. Instead of constantly listening for new messages, which drains the battery faster, enabling eDRX puts the device on “standby” mode. This means the device is still reachable, and still listens and responds to downlink messages but with appropriate latency.
eDRX improves upon Discontinuous Reception (DRX) used in LTE systems, allowing for longer idle periods to further reduce energy consumption.
How does eDRX work?
eDRX operates within an Active Timer (T3324), which defines the overall reachable duration, ranging from seconds to minutes depending on network configuration and whether the device uses eDRX in NB-IoT or eDRX in LTE-M.
Within the Active Timer, the device cycles through repeating paging cycles (eDRX cycles). Each cycle consists of two parts:
- Paging Time Window (PTW): A short interval where the device actively listens for incoming data. The PTW itself is divided into even shorter periods of active listening (Paging Occasions or PO) interspersed with short sleep periods (DRX).
- eDRX (idle period): During this time, the device ignores paging and downlink control channels. It’s possible to have more than one eDRX cycle within the active timer
Depending on configuration, these cycles can repeat a few times during the Active Timer. This setup allows the device to save power while still being able to receive important messages periodically.
Key events:
Normal sleep cycles (without eDRX):
- Network controls sleep duration.
- Typically 1.28 or 2.56 seconds.
With eDRX enabled:
- Application controls sleep duration.
- Can be set up from a few seconds to minutes, which varies per network configuration and for NB-IoT or LTE-M.
- Only specific cycle lengths permitted.
During eDRX cycle:
- Receiver stays off.
- Device ignores paging/downlink channels.
- Brief check of Physical Control Channel at cycle end.
- Can still send data at any time.
The benefits and limitations of eDRX
| Benefits | Trade-offs |
|---|---|
| Helps extend battery life, especially for devices that don’t need constant, continuous updates. | Longer sleep periods save more battery but may slightly delay receiving new data. |
| Uplink latency is not affected, as devices can quickly switch to a connected state to send data. | The system aims to balance power savings with timely communication. |
| Application can choose sleep duration that matches its needs, reducing power use while maintaining connectivity. |
Practical considerations with eDRX
- Data handling: If data arrives between paging events, it’s temporarily buffered by the network.
- Network: eDRX must be enabled in the network by the operator, which should comply with general telecommunication standards.
- Device: eDRX duration can be configured on the device-level, usually through the SIM. It must fall within network-specified eDRX duration.
Best use cases for eDRX
eDRX is best suited for IoT devices that need to balance battery life with occasional downlink data reception. It may not be ideal for devices that need to respond instantly to incoming messages, and requiring frequent two-way communication constantly.
What is PSM?
Power Save Mode (PSM) is an LPWAN feature introduced in 3GPP Release 12 that optimizes power consumption for IoT devices on LTE networks. Instead of completely disconnecting between sending and receiving data, devices maintain their network connection while entering a deep sleep state.
Without PSM, devices must regularly wake up to respond to network “pages” that verify their presence. While each wake-up cycle uses minimal power, these frequent checks significantly impact battery life over time. PSM addresses this by allowing devices to negotiate extended sleep periods with the network, during which they won’t receive pages.
During PSM, devices maintain their connection configurations while hibernating, only waking to send data or when their agreed sleep time ends. This approach is more energy-efficient than repeatedly disconnecting and reconnecting to the network, enabling battery-powered devices to potentially operate for up to 10 years.
How PSM works for LTE-M and NB-IoT
Key events:
Device requests PSM during attach/TAU/RAU by proposing two timers:
- T3324: Active time or the period when device is reachable before sleep. This paging cycle, determined by the PTW and eDRX value, governs how frequently these checks occur.
- T3412: Time until next TAU (Tracking Area Updates). When T3412 expires, the device checks for messages or updates its location with the network.
- PSM period = T3412 – T3324 (defining the PSM duration).
Network response:
- Can approve or adjust requested timers.
- Maintains device’s connection state.
- Often has maximum PSM duration limits.
- Note: consult your network provider’s specific PSM duration limits as they vary by operator
During PSM:
- Device enters deep sleep.
- Radio is completely off, becomes unreachable.
- Device keeps network registration.
- Network buffers incoming data (minimum 100 bytes as per 3GPP).
- Early wake-up possible without needing reattachment.
- Development note: AT channel may be closed during PSM.
The benefits and limitations of PSM for IoT devices
| Benefits | Trade-offs |
|---|---|
| PSM avoids the overhead of frequent network attach and detach procedures, further conserving energy. | High downlink latency due to extended periods of unavailability. |
| Deep sleep can be configured for seconds and up to days, depending on specifications. | The inherent latency makes PSM unsuitable for applications requiring immediate two-way communication. |
Practical considerations with PSM in LTE-M or NB-IoT
- Data handling: If data arrives between paging events, it’s temporarily buffered by the network.
- Network: The network infrastructure must support PSM functionality, which should comply with general telecommunication standards.
- Device-level: Not all modems support PSM, check with your module. Proper timer configuration is crucial for optimal performance. Device configuration must fall within network specifications.
Best IoT use cases for PSM
PSM could work well for IoT devices with sporadic uplink transmissions and minimal downlink frequency needs, such as smart meters and remote sensors. Careful configuration of the T3412 extended timer and PTW is crucial to optimize the balance between power savings and application requirements.
eDRX vs. PSM: A comparison
Both PSM and eDRX are power-saving modes for IoT devices using LTE networks, but they differ in how reachable the devices are to incoming messages and its impact on power consumption and responsiveness.
With PSM, the radio completely shuts down (both sending and receiving), though the device remains registered with the network. The application processor can still function, performing tasks that don’t require the modem.
In comparison with eDRX, the device’s sending capability is off, but it periodically turns on its receiver to listen for messages (paging). This allows for faster wake-up and shorter active periods compared to PSM. The application processor (reading sensor data or other processes that don’t involve the modem) can also function independently.
Similarities between PSM and eDRX
Goal:
- Both aim to maximize battery life by minimizing the device’s active time on the network.
Network registration:
- In both modes, the device remains registered with the network, eliminating the need for power-consuming reattachment procedures.
IoT focus:
- Both are primarily used in IoT applications where infrequent communication is acceptable to extend the battery life of devices.
Specified for LTE-based IoT networks:
- PSM and eDRX are both included in NB-IoT and LTE-M network specifications. However, network support and specific settings are up to individual network providers. For example, PSM in NB-IoT could support up to 310 hours while eDRX in NB-IoT supports up to 2.01 hours.
- It is also worth noting that LTE Cat 1 bis devices can use both eDRX and PSM, but no cat 1 bis networks outside of China reportedly support it at the time of writing.
Device:
- Check if modules support eDRX and PSM. These features must be enabled and disabled.
Differences between eDRX and PSM
PSM is like deep hibernation, while eDRX is more like dozing. PSM allows for much longer sleep periods, saving more power, but making the device less responsive. eDRX involves shorter, lighter sleep, balancing power savings with better responsiveness.
| Feature | eDRX | PSM |
|---|---|---|
| Sleep depth | Light sleep; periodic wake up | Deep sleep; effectively offline |
| Reachability | Intermittently reachable during PTW | Unreachable until wake-up or timer expiry |
| Downlink latency | Moderate; messages received during PTW | High; messages buffered until device wakes up |
| Uplink latency | Low; can send data during active periods | Low; can wake up and send data anytime |
| Sleep duration | Shorter, defined by eDRX cycle (PTW and eDRX value) | Longer, controlled by T3412 timer (up to ~13 days for NB-IoT) |
| Communication pattern | Periodic two-way communication | Primarily for infrequent uplink, sporadic downlink |
| Use cases | Devices needing occasional downlink data with tolerable latency (e.g., smart meters, environmental sensors with moderate reporting frequency) | Devices with infrequent uplink and minimal downlink needs (e.g., remote sensors reporting infrequently, asset trackers) |
Do PSM and eDRX align with your application’s goals?
When supported by the LTE-M or NB-IoT network provider, IoT devices can use both eDRX (light sleep) and PSM (deep sleep) for power saving. Settings for both can be configured, but the network might override them.
Different settings impact battery life and message delays. The ideal settings depend on how the device is used. For example, if minimizing delays is important, using longer eDRX checks with more frequent paging (listening) cycles could be the better setting.
One practical first step to understanding how eDRX and PSM can work for your IoT solution is to ask your network provider of choice if they support these low-power features and what are the specifications. Our fully integrated core network enables our SIMs to offer real-time access to deep network data of 680+ networks in 180+ countries.
Building energy-efficient IoT devices
eDRX and PSM are not the only ways to optimize power efficiency for your device. If low power consumption is a must-have for your IoT application, or if you’re looking for ways to make your devices more energy efficient, our low-power IoT white paper will walk you through other ways to connect devices to make them last longer.
You can access the free low-power IoT device design white paper with your email.
Resources:
- A. K. Sultania, P. Zand, C. Blondia and J. Famaey, “Energy Modeling and Evaluation of NB-IoT with PSM and eDRX,” 2018 IEEE Globecom Workshops (GC Wkshps), Abu Dhabi, United Arab Emirates, 2018, pp. 1-7, doi: 10.1109/GLOCOMW.2018.8644074. https://ieeexplore.ieee.org/document/8644074
- A. K. Sultania, C. Blondia and J. Famaey, “Optimizing the Energy-Latency Tradeoff in NB-IoT With PSM and eDRX,” in IEEE Internet of Things Journal, vol. 8, no. 15, pp. 12436-12454, 1 Aug.1, 2021, doi: 10.1109/JIOT.2021.3063435. https://ieeexplore.ieee.org/document/9367281
- S. R. Borkar, “7 – Long-term evolution for machines (LTE-M)”, in LPWAN Technologies for IoT and M2M Applications, Academic Press, pp. 145-166, 2020, ISBN 9780128188804, https://doi.org/10.1016/B978-0-12-818880-4.00007-7. https://www.sciencedirect.com/science/article/pii/B9780128188804000077
- 3GPP TS 27.007 version 18.7.0 Release 18, ETSI TS 127 007 V18.7.0 (2024-07). https://www.etsi.org/deliver/etsi_ts/127000_127099/127007/18.07.00_60/ts_127007v180700p.pdf
