Power Saving Mode (PSM) and Extended DRX (eDRX) are energy-saving modes of mobile networks that can extend the battery life of mobile devices to ten years or more.
Mobile devices that support PSM operation inform themselves during the TAU period, TrackingAreaUpdate (TAU), according to an active timer. This determines how long the mobile device remains reachable for transactions and when it enters sleep mode. The subscriber device starts the active timer as soon as it switches from active to idle mode. In PSM mode, the terminal device cannot be reached, but it is still registered in the network. The terminal remains in PSM mode until a transaction is initiated from the mobile device using Mobile Originated( MO).