Implementing eDRX and PSM: AT Commands

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.

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-M or NB-IoT networks, 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:

1. 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).
2. 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 several times during the Active Timer. This setup allows the device to save power while still being able to receive important messages periodically.

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.

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 quickly to incoming messages, requiring frequent two-way communication, and devices needing constant network access.

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

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 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 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)

However, telecom can be slow to innovate as many solutions must come from big organizations agreeing on what to make, how to build it, and how to regulate its use. The wheels of change are slow to turn in this industry.

How to use eDRX for IoT energy management

eDRX AT Commands

These AT commands provide comprehensive control over eDRX functionality on your device. Remember to consult the full specification for detailed information and parameter definitions (based on 3GPP TS 27.007 specification):

1. Setting eDRX Parameters:
   +CEDRXS=[<mode>,[,<AcT-type>[,<Requested_eDRX_value>]]]
   This command lets you configure eDRX settings, including enabling/disabling eDRX, specifying the access technology type, and requesting a specific eDRX value. More details on the parameters below.
   
2. Reading Current eDRX Settings: +CEDRXS?
   Use this command to retrieve the current eDRX configuration for each access technology type.
   
3. Querying Supported eDRX Settings: +CEDRXS=?
   This command returns a list of supported eDRX modes, access technology types, and eDRX values. It’s helpful for understanding the device’s capabilities.
   
4. Enabling Unsolicited Result Code: +CEDRXS=2
   This special form of the set command enables unsolicited result codes. This means the modem will automatically notify you (using the +CEDRXP URC) if the network changes the eDRX parameters.
   
5. Disabling eDRX and Resetting Parameters: +CEDRXS=3
   This command completely disables eDRX and clears all associated parameters, restoring them to manufacturer defaults if available.

Understanding the Parameters

• <mode>: Integer indicating whether eDRX is enabled (1 or 2) or disabled (0 or 3). Mode 2 additionally enables the unsolicited result code +CEDRXP. Mode 3 disables eDRX and discards all related parameters.
• <AcT-type>: Integer representing the access technology (e.g., GSM, LTE, NB-IoT).
• <Requested_eDRX_value>: String specifying the desired eDRX value. This value determines the length of the eDRX cycle. Refer to 3GPP TS 24.008 for the coding and valid range.

eDRX cycle times and Paging Time Window PTX

The device requests an eDRX cycle from the network, which confirms it and sets a Paging Time Window (PTW). If the requested cycle time doesn’t match official durations, the network adjusts it to the nearest allowed value.

Here are the available cycle times and PTW for eDRX in LTE-M and NB-IoT (from Twilio docs):

Feature	eDRX	PSM
"0000"	Light sleep; periodic wake up	Deep sleep; effectively offline
"0001"	Intermittently reachable during PTW	Unreachable until wake-up or timer expiry
"0010"	Moderate; messages received during PTW	High; messages buffered until device wakes up
"0011"	Low; can send data during active periods	Low; can wake up and send data anytime
"0000"	Shorter, defined by eDRX cycle (PTW and eDRX value)	Longer, controlled by T3412 timer (up to ~13 days for NB-IoT)
"0000"	Periodic two-way communication	Primarily for infrequent uplink, sporadic downlink
"0000"	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)

The SIM is a good example of this—in how it has evolved, and is still evolving.