What is Power Saving Mode?

Power Save Mode (PSM) is a feature found in low-power wireless devices that minimizes power consumption and thus maximizes battery life. This feature was introduced in 3GPP Release 12 and is available for all LTE device categories (especially useful for IoT).

Typically wireless modules operating under normal conditions send and receive data uninterruptedly - they are usually always powered on. PSM allows the device to be put to sleep between periods of data transmission and reception thereby lowering the power consumed by the device. PSM is similar to powering off the device while keeping the device registered with the network.

The process of re-connecting to the network consumes only a small amount of energy, however, this process needs to be done very often and can become significant over the lifetime of a device. PSM further extends the battery life of the IoT device by voiding the repeated re-attachment procedure.

PSM is a device-side mechanism to reduce the radio energy consumed during transmitting and receiving.

When the device becomes active again there is no need to re-attach or re-establish PDN (Packet Data Network) connections.

In an active PSM cycle, the radio is fully shut down, hence the device cannot receive messages and is completely unreachable during this period. Data packets that get sent to a device when it’s in an active PSM cycle must be stored by the network according to the 3GPP requirements. For this purpose, the network sets aside at least a packet of 100 bytes.

How PSM Works

The device or user entity (UE) periodically notifies its availability to the network via Periodic Tracking Area Updating (TAU). This procedure is controlled using the periodic TAU timer - timer T3412. T3412 has the TAU period’s value stored in it that informs the network that it is still registered. The device or UE also uses another timer – T3324 which stores the value of the duration for which the device remains reachable for data transmission, on transition from connected to idle mode. By 3GPP Release 13 standards the maximum time a device may sleep (T3412 maximum value) is set to approximately 413 days, and the maximum time a device may be reachable (maximum value of the Active timer T3324) is set to 186 minutes.

When a device initiates PSM with the network, it sends the two timer values (T3324 and T3412); The PSM cycle/time is the difference between these timers (T3412 minus T3324) as shown in the diagram. Through these timers, the device reports to the network how often and for how long it needs to be active to transmit and receive data. However, the final values are determined by the network and it may accept these values or set different ones. The network then retains the device’s state information and it remains registered with the network. If a device awakes and sends data before the expiration of the time interval it agreed with the network, a re-attachment procedure is not required.

For example, for a monitoring application, the radio module in a device might be configured by an application to enable PSM, negotiate a 24 hour time interval with the network and provide a daily status update to a centralized monitoring point. If the device’s monitoring application were to detect an alarm condition, irrespective of any agreed sleep interval, the application could wake the radio module instantly and send vital information to the centralized monitoring point without the need to execute a reattach procedure.