Power Saving Mode

Description

Power Saving Mode (PSM) is a power efficiency feature defined in 3GPP that allows a User Equipment (UE), particularly Machine-Type Communication (MTC) devices, to enter a state of extremely low power consumption while remaining registered in the network. When a UE activates PSM, it effectively powers down most of its radio and baseband circuitry, entering a deep sleep state analogous to turning off. Critically, the network retains the UE's registration and subscription context, including its IP addresses if assigned. The UE remains in this state for a duration defined by an Active Timer (managed by the Mobility Management Entity or Access and Mobility Management Function) and a longer, UE-requested PSM timer.

The operation is controlled by timers negotiated between the UE and the network during Attach or Tracking Area Update procedures. The UE indicates its desired PSM parameters, including a requested 'Active Time' and a 'Requested Periodic TAU/RAU Timer'. The network authorizes these values, returning the 'Active Timer' (T3324) and the 'Periodic TAU/RAU Timer' (T3412). After a data transfer or signaling procedure, the UE starts the Active Timer. During this active time, the UE remains reachable for mobile-terminated services. Once the Active Timer expires, the UE enters PSM. It will remain in this deep sleep, unreachable by the network, until its internal Periodic TAU/RAU Timer expires or it has uplink data to send.

To exit PSM, the UE must initiate an action. This is typically triggered by: 1) The expiration of its internal Periodic Update timer, upon which it performs a Tracking Area Update (TAU) or Routing Area Update (RAU), restarting the timer negotiation cycle. 2) The generation of uplink data from the application layer, which causes the UE to exit PSM, perform a Service Request procedure to resume the user plane connection, send the data, and then re-enter PSM after its Active Timer expires. This model is ideal for IoT sensors that report data periodically or on an event-driven basis, as the device controls its reachability and can sleep for days, months, or even years, conserving battery life.

Purpose & Motivation

PSM was created to address the critical challenge of battery life for massive Machine-Type Communication (mMTC) devices in cellular IoT networks. Traditional mobile devices and smart phones are designed for frequent interaction and need to be reachable, requiring periodic listening for paging messages, which consumes significant power. For IoT devices like utility meters, environmental sensors, or asset trackers that may only need to send a small report once per day or less, this constant radio activity is wasteful and results in impractical battery replacement cycles.

PSM solves this by fundamentally shifting the paradigm of reachability. Instead of the network being able to contact the device at any time (increasing power drain), the device becomes unreachable during its deep sleep, placing the onus on the device to 'wake up' when it needs to communicate. This allows the device's power consumption to drop to levels nearly equivalent to a powered-off state, while maintaining its network registration. It directly addresses the key IoT requirement of ultra-long battery life—often a decade or more—enabling feasible large-scale deployments of battery-powered sensors and actuators. It was a foundational power-saving technology introduced alongside other features like extended Discontinuous Reception (eDRX) to make LTE and subsequent technologies viable for the massive IoT market.