Release 12 of 3GPP added the Power Saving Mode (PSM) device feature that allows a device to enter a deep sleep mode and switch off most of its circuitry. This mode is different than power off because the device stays registered into the network so that it does not need to re-attach after wakeup. As with a power off state, the device will not be reachable by the network, but it may wake up if it has some data to transmit or when a parameter called Tracking Area Update parameter (TAU) expires.
The UE remains registered with the network and there is no need to re-attach or re-establish PDN connections. The UE will not be reachable in the PSM state and will save power during this deep sleep window.
The UE activates PSM by including two timer values in the Attach or Tracking Area Update (TAU). The first timer is the T3324, which defines the time the UE stays active after idle mode following the Attach or TAU procedure. The second timer is an extended T3412 which defines the extended time for a UE to send periodic TAU. Figure 1 illustrates device status versus active and extended timers (T3324 and T3412).
Figure 1 — Power Profile of Power Saving Mode activities
The UE requests for a T3324 Active Timer value during Attach and TAU procedures. The MME allocates the T3324 value to the UE. The T3324 active timer determines the duration during which the device remains reachable for mobile terminated transaction on transition from connected to idle mode. The device starts the active timer when it moves from connected to idle mode and when the active timer expires, the device moves to Power Saving Mode. The MME takes the UE requested value and MME local configuration into account for determining the Active Timer value. The MME includes the T3324 value IE in the ATTACH ACCEPT/TAU ACCEPT message only if the T3324 value IE was included in the ATTACH REQUEST/TAU REQUEST message. A UE using PSM is available for mobile terminating services only for the period of an Active Time after a mobile originated event like data transfer or signalling, for example after a periodic TAU/RAU procedure. The MME allows a value of ‘0’ for the T3324 timer. In this case the UE enters the Power Saving Mode immediately. Table 1 summarizes the units (increment) for the Active Timer T3324. The value is 5 bits in length and must be multiplied by the increment to get its real value.
Table 1 — T3324 Active Timer Unit and Value
The T3412 timer is also referred to as the periodic Tracking Area Update (TAU) timer. Periodic tracking area updating is used to periodically notify the availability of the UE to the network. The procedure is controlled in the UE by the periodic tracking area update timer (timer T3412). The value of timer T3412 is sent by the network to the UE in the ATTACH ACCEPT message and can be sent in the TRACKING AREA UPDATE ACCEPT message. The UE shall apply this value in all tracking areas of the list of tracking areas assigned to the UE, until a new value is received. A longer periodic TAU timer is possible using T3412 extended timer.
When the UE includes the T3324 value IE and the UE indicates support for extended periodic timer value in the MS network feature support IE, it may also include the T3412 extended value IE. Apart from the value requested by the UE, the MME verifies the local configuration into account while selecting a value for the T3412 extended timer. When the MME includes the T3412 extended value IE in the ATTACH ACCEPT message or TRACKING AREA UPDATE ACCEPT message, the MME uses timer T3412 extended value IE as the value of timer T3412. Table 2 summarizes the units (increment) for the Extended Timer T3412. The value is 5 bits length and must be multiplied by the increment to get its real value.
Table 2 — Automation Test of Power Saving Mode
Measuring the PSM Power Profile
To measure the power profile of a device in normal and PSM modes, one can use a Digital Power Meter to measure current consumption. This digital power meter will basically power the device, measure the dropped current, and then compute the power consumption and draw the power profile. A typical setup would monitor the device from an external computer, the power meter and the PicoLTE network emulator is depicted in “Error! Reference source not found.” This setup is not part of the current release, but will be provided in future releases as a reference design.
In a procedure containing several steps, the operations are numbered (1, 2, 3…). The diamond icon (♦) is used to indicate a procedure containing only one step, or secondary steps. Lowercase letters (a, b, c…) can also be used to indicate secondary steps in a complex procedure.
The abbreviation NC is used to indicate no connection.
Capital letters (A,B,C…) are used to identify any term marked as is on an instrument, such as the names of connectors, buttons, indicator lights, etc. Capital letters are also used to identify key names of the computer keyboard.
All terms used in software, such as the names of menus, commands, dialog boxes, text boxes, and options, are presented in bold font style.
The abbreviation N/A is used to indicate something that is not applicable or not available at the time of press.
The screen captures in this document are taken from the software version available at the time of publication. For this reason, they may differ slightly from what appears on your screen, depending on the software version that you are using. Furthermore, the screen captures may differ from what appears on your screen if you use different appearance settings.