Power Saving Mode (PSM) Part I

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What is PSM in NB-IOT?

• It is a mode through which the UE notifies the network of its indefinite dormant state.
• Power Saving Mode (PSM) was introduced in 3GPP Release 12 24.301-5.3.11, to improve battery life of IOT devices.
• PSM is similar to power-off, with the exception that the UE remains registered with the network and there is no need to re-attach or re-establish the PDN connection.





Is UE(device) reachable in PSM?

• UE is unreachable in PSM for mobile terminating services.
• UE(device) is reachable for mobile terminating services only for the period of an Active Time after a mobile originated event like data transfer or signaling.  E.g. After a periodic TAU/RAU procedure.





Does PSM support CS domain? / Limitations of PSM.

• CS domain is not supported on the network side by PSM UE.
• PSM should only be used by UEs using the PS domain, SMS and mobile originated IMS.


Note: A UE that uses mobile terminated IMS or CS services other than SMS should not use PSM as neither IMS nor the CS domain provide support for mobile terminated CS voice or IMS services to UEs that are in PSM.



What is the advantage of PSM?

• UE gets the advantage of having more power management control which is required for its application.
• It gives UE flexibility to manage its power which is essential for a wide range of IoT applications.
• PSM implementation prevents network congestion.








How Network congestion could be minimized using active PSM for multiple UEs?

• Using PSM, we can manage the timers of all the devices, and adjust the wake-up periods to be offset as much as possible. This way will make sure that all the devices do not wake at the same time and minimize network congestion.





Which timer is supported by PSM?

• PSM is activated by UE by including two timer values in the Attach or Tracking Area Update (TAU).
• The first timer - T3324, defines the time the UE should stay active after idle mode following the Attach or TAU procedure.
• The second timer - extended T3412, defines the extended time for an UE to send periodic TAU.
• The difference between these timers (T3412-T3324) is called PSM time.




How does UE request PSM? / What is the maximum time for sleep mode and reachability of UE?

• The process to request PSM is simple: UE includes a timer with the desired value in the attach request message with TAU or routing area update.
• According to 3GPP Release 13 for T3412, the maximum time a device can sleep is approx. 413 days.
• The maximum time for which a device is reachable is 186 minutes (equivalent of the maximum value of the Active timer T3324). 

          What are T3324 and T3412?

T3324 (Active Timer):

• T3324 is a timer which determines for how long the UE will monitor paging before entering in PSM.
• Its value is requested by the UE during Attach and TAU procedures and allocated by the MME.
• The T3324 active timer defines the time for which the device will remain reachable for mobile terminated services on transition from Connected to Idle mode.
• UE is not reachable by the Network in PSM and all modules are turned off. UE exits PSM after T3412 expires (TAU) or with a Mobile Originated transfer.
• The MME allows '0' value to be set for the T3324 timer, in which case the UE enters the Power Saving Mode immediately.

T3412 (Extended Timer):

• The T3412 timer is also called the periodic Tracking Area Update (TAU) timer.
• Periodic tracking area update is used to periodically notify the network the availability of the UE.
• The value of timer T3412 is sent to the UE by the network in the Attach Accept message or Tracking Area Update Accept message.
• The UE shall apply this value in all tracking area included in the list of tracking areas assigned to the UE, until a new value is received.
• Using T3412 extended timer, a longer periodic TAU timer is possible. When the T3324 value IE is included by the UE and the UE supports extended periodic timer value in the MS network feature support IE, it can also include the T3412 extended value IE.