SARA-R42 - Application note
UBX-20050829 - R02
PSM, eDRX and deep-sleep mode
Page 26 of 58
C1-Public
8
PSM, eDRX and deep-sleep mode
8.1
Power saving mode (PSM)
3GPP has a standardized PSM to minimize power consumptions for devices that require sporadic
access to the cellular network. Instead of switching off the module at the end of the data transfer, if
PSM is enabled with the
+CPSMS
AT command, then a device can negotiate long lasting time periods
(up to 413 days) during which keepalive notifications can be postponed and the module can stay
registered to the network.
Meanwhile, pending mobile terminated traffic indications, such as SMS or small data packets, are
likely to be stored at the network side. This allows devices to skip paging reception and enter the
minimum functionality PSM deep-sleep mode. To retrieve the pending mobile terminated (MT) traffic
from the application server (AS), applications are suggested to implement a pull mechanism: the
application client sends a mobile originated (MO) pull message to the server after the NW-buffered
MT small trigger reception, the application server completes the MT data delivery upon the MO pull
message reception.
The host application can preconfigure PSM on the module before registration or activate it at runtime
(see the
+CPSMS
AT command description in the SARA-R4 series AT command manual
). The
former solution is preferred because the second one triggers an extra PSM negotiation with the
cellular network. The application-configured PSM settings are NVM persistent and used at any
registration procedure, periodic or due to mobility in other areas, which is seen as a new negotiation
opportunity. The requested PSM settings can be configured via the
+CPSMS
AT command, whereas
the PSM settings assigned by the network, which are the used one, can be discovered via the
+CEREG
URC and the
+UCPSMS
AT command. If PSM is not granted by the network, then the module can not
use PSM. In this case, for example, the host application could power on and off the module with the
desired periodicity in order to save power.
Two timers determine the entrance into PSM deep-sleep mode, namely:
•
T3324, the active timer, which starts when the network releases an existing RRC connection and
is reset if a new RRC connection is established before the timer has expired. At T3324 expiry, the
module can enter the PSM deep-sleep mode and it enters PSM deep-sleep as soon as the module
completes any pending activity. If the eNB needs to send any MT data, then it always pages it
before the active timer expiry. In case the eNB does not allow the UE to use PSM feature it does
not grant it not assigning the active timer.
•
T3412, the periodic TAU (Tracking Area Update) timer, which starts when the network releases
an existing RRC connection and is reset if a new RRC connection is established before the timer
has expired. It comes with two timer values: the legacy one and, in case of PSM, the optional
extended timer value, used whenever present. By default, the cellular network sets the T3412
duration to almost 1 hour, but if PSM is supported, its duration can be extended to up to 413 days.
When PSM is granted by the network context retention is possible. At PSM deep-sleep exit no LTE
attach is needed, previously activated EPS bearers and IP addresses are locally restored. The module
can directly start a MO data or signaling transmission, e.g. a TAU procedure, when it exits PSM
deep-sleep due to an early wake-up trigger or autonomously at periodic T3412 timer expiry.
8.1.1
GSMA recommendations
The selection of the requested T3412 timer value is a trade-off between power consumptions, which
can be minimized setting long timer values, and the tolerated MT data delay, which is the limit factor
for the T3412 timer duration. When using PSM it is recommended to request a T3412 timer extended
value of at least 4 hours. In case no MT data is considered in the application design power can be
minimized requesting the longest possible 413 days timer duration.
