LTE-M and NB-IoT both support the power saving techniques Power Saving Mode (PSM) and extended Discontinuous Reception (eDRX) to minimize power consumption.
The reason for employing power saving techniques, instead of just turning off the modem, is that the UE stays connected to the network and doesn’t have to go through the power consuming attach procedure to re-establish the connection. Power saving techniques allow the UE to switch off the radio while still maintaining connectivity with the network, mitigating the need for the connection re-establishing procedures.
RRC modes
The Radio Resource Control (RRC) protocol is used for communication between the UE and the eNB. The technical details of the RRC protocol and the RRC connection is beyond the scope of this course.
We will only focus on the fact that the UE can be in one of two RRC modes: RRC Connected and RRC Idle.
RRC Connected mode
RRC Connected mode refers to the time when the UE’s radio is actively sending or receiving messages. During RRC Connected mode, the UE must maintain its synchronization with the network, which consumes a lot of power.
In RRC Connected mode, the radio is mostly used for application-specific activity:
Sending uplink (UL) messages from the UE to the eNB, i.e. reporting a sensor value
Receiving downlink (DL) from the eNB, i.e. configuration updates
Neighbor cell measurements
RRC Idle mode
RRC Idle mode refers to the state when the UE is switched on, but the radio is not executing high battery-consuming tasks. In RRC Idle mode, the UE does the bare minimum to stay connected to the network and to stay reachable, so the battery consumption is low compared to RRC Connected mode, but still high compared to being in power saving mode.
In RRC Idle mode, the radio performs keep-alive activities:
Periodically listening to paging messages to be informed when the eNB requires a status update or value reporting
Periodically performing tracking area updates (TAUs) to update the eNB about the UE’s location for reachability
Paging
Paging is the mechanism in which the network tells the UE it has messages for it. The UE will then decode the content and initiates the appropriate procedure.
The UE must monitor paging, which is the process of monitoring whether the network is sending paging messages. This happens while the UE is in RRC Idle mode.
Since continuous page monitoring consumes power, the UE sleeps between monitoring pages. This method is called Discontinuous Reception (DRX).
Timers
This is a short explanation of the function of timers in cellular connections. The three timers that will become relevant later in this section are:
Periodic TAU timer (T3412): Time that the UE will stay in power saving mode before waking up to send a Tracking Area Update (TAU) to the network.
Active timer (T3324): Time that the UE is monitoring paging in RRC Idle mode, before going into PSM.
RRC Inactivity timer: Time that the UE stays in RRC Connected mode after transferring data. After this time, the UE receives the event RRC Release from the network
Understanding the different timers, and the different connection statuses and modes is important for understanding and calculating the power consumption in your application, and for debugging your cellular application.
Note
It is important to keep in mind that although some of these timer values can be requested from the network, the network ultimately decides which value to give and doesn’t have to follow a requested value.
Power saving mode (PSM)
Power Saving Mode (PSM) is a state where the UE reduces its power consumption to a bare minimum to stay connected, by going into sleep mode. In PSM, the UE is not monitoring paging and as a result, becomes unreachable. To avoid the re-connection procedure when waking up again, the UE is allowed to keep its connectivity status.
For this to happen the UE must wake up regularly and send a Tracking Area Update (TAU) to the network. This period is decided by the Periodic TAU timer. A UE in PSM will wake up for one of two reasons, either triggered by the Periodic TAU timer to send a TAU, or by the application to send UL messages.
Either way, the device wakes up, enters RRC Connected mode, sends and receives data packets, then stays in RRC Connected mode until the RRC Inactivity timer has run out. Then it will go to RRC Idle mode for the Active timer period to monitor paging, before going back to power saving mode.
The image below explains what this process looks like.
The difference between using PSM and simply turning off the modem is that in PSM the UE can still be configured to wake up to update the network about its status and stay reachable for a while. The advantage of PSM is that the UE goes into deep sleep and the radio only consumes a few microamps. The UE does not lose its connection information on the network, meaning that upon UE wakeup, the connection is established relatively quickly. This reduces the power consumption compared a powered-off modem waking up and having to re-establish a connection with the network from scratch.
eDRX
As we mentioned earlier, DRX (Discontinuous Reception) is a method where the UE and the network negotiate cycles during which data transfer may occur. The UE wakes up regularly to check with the network if it has any incoming messages (pages), and otherwise remains in sleep mode. However, the maximum sleep interval for standard DRX configurations is typically around 2.5 seconds. This duration works well for smartphones but is generally too short for many IoT applications, where longer power-saving periods are needed.
Extended Discontinuous Reception (eDRX) builds on DRX by allowing the UE to remain in sleep mode for much longer periods before waking to check for paging messages. In this mechanism, the UE and the network agree on two parameters: the eDRX cycle, which defines the total duration of the sleep-and-wake period, and the Paging Time Window (PTW), which specifies how long the UE listens for paging messages during each cycle.
Note that the device is not reachable while it is sleeping, so the time it takes to reach the device depends on the duration of the eDRX Cycle.
The image below illustrates two different eDRX operation scenarios. In the example on the left, the UE wakes up during its Paging Time Window to monitor for paging messages but none are received, so it returns to sleep until the next eDRX cycle. In the example on the right, the UE wakes up and receives a paging message that triggers a data transfer. It then transitions to RRC Connected mode for the duration of the RRC Inactivity Timer, after which it returns to RRC Idle mode to monitor paging again before going back to sleep.
PSM and eDRX
Although we presented PSM and eDRX as two separate power saving techniques, they can also coexist as they implement changes to different parts of the connection process.
The image below shows the connection states when using both PSM and eDRX.
When configuring eDRX together with Power Saving Mode (PSM), it is important that the Active Timer (T3324) is long enough to cover multiple eDRX cycles. The eDRX cycle defines how often the UE wakes up to monitor paging, while the Active Timer determines how long the UE remains reachable in RRC Idle mode before entering deep sleep in PSM. If the Active Timer expires before the next eDRX cycle, the UE will enter PSM too early and become unreachable for downlink communication. Therefore, the Active Timer should ideally be set to a duration that includes several eDRX cycles.
Different use cases
The key difference between Power Saving Mode (PSM) and Extended Discontinuous Reception (eDRX) lies in how deeply the device sleeps and for how long it can remain in that state.
In PSM, the UE enters a deep sleep state where both the transmitter (TX) and receiver (RX) are turned off, but the device remains registered with the network. This allows for very long sleep periods with minimal power consumption. However, while in PSM, the UE cannot receive paging messages and must wake up, either on a timer (TAU timer T3314) or when triggered by the application upon sending of application data.
In contrast, eDRX allows the UE to stay in RRC Idle mode and periodically wake up to listen for paging messages. Only the receiver is activated during these short paging windows, making eDRX suitable for devices that need to be intermittently reachable while still conserving energy.
Even when the UE’s radio is sleeping, whether in PSM or during the eDRX sleep interval, the application processor can remain active to perform tasks such as reading sensors, logging data, or processing information locally.
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•Single-Slot DFU and RAM Load mode are both promoted to fully supported •Partition Manager is officially deprecated in favor of Zephyr's devicetree-based partitioning.
