Celular carriers with NB-IoT and CAT-M support PSM or eDRX?

[tedhavelka]

2022-04-12

Hello Devzone Community,

I have a couple questions regarding availability of nRF9160 LTE modem settings PSM and eDRX. Question (1) If my local celular carrier or Mobile Virtual Network Operation (MVNO) supports both NB-IoT and CAT-M, does this mean they will also support the range of PSM mode settings for which nRF9160 LTE modem can be configured?

Question (2) when app firmware sends an AT command to the LTE modem command handler, of the form AT+CPSMS to set PSM, what range of time periods are typical for the modem to receive a reponse from the celular network regarding PSM requested? How much time must pass in order to sensibly query the modem again to see whether settings were granted?

Searching Nordic's Devzone, I find multiple posts which indicate that PSM and eDRX are not guaranteed available in all geographies. Rather, these parameters depend on celular service providers. Following posts all speak to this external dependency on an important power savings mode of operation:

* https://devzone.nordicsemi.com/f/nordic-q-a/42433/edrx-psm-mode-always-on/165548#165548
NS Team Martin Lesund writes "Modem will retain those settings and negotiate values based on those settings with the network whenever that is needed."

*  https://devzone.nordicsemi.com/f/nordic-q-a/48027/edrx-mode-psm-mode-sleep-mode-standby-mode-active-mode-power-consumption
NS Team Martin Lesund writes "4.) You can request the intervals from the network. It is usually easier to get permission to increase the intervals"

*  https://devzone.nordicsemi.com/f/nordic-q-a/68307/enabling-edrx-or-psm-mode-with-iot-1nce-net-sim
NS Håkon writes "it might be that the network doesn't allow roaming sim cards to enter eDRX/PSM modes. You could try contacting Telekom and ask if they plan to open these features for roaming SIMs."

* https://devzone.nordicsemi.com/f/nordic-q-a/52441/how-to-use-psm-and-edrx-mode-in-custom-code/211688#211688
NS Håkon writes "If a network is available, you can test PSM by requesting this in AT command CPSMS, and then reading the parameters for the link by entering "AT+CEREG=5" and "AT+CEREG?" to read the parameters that the network has set."

Question (3) in cases where celular network does not support PSM or eDRX, is the next best strategy for app firmware to turn off the modem altogether for some period, as in a battery based system needing to extend battery life?

I realize this last option to power off the modem is not ideal. This means using more power to turn the modem on and re-establish a celular link, and likely a high level link at the protocol being used over celular. Asking these questions to determine whether nRF9160 SiP is a practical component and system to use in battery based products. The low low current spec of ~1.4uA to 3.0uA is amazing, but may not be available if issues with cel carrier support preclude its use.

- Ted

[Albrecht Markus Schellenberger]

Hello Ted,

Question (1) If my local celular carrier or Mobile Virtual Network Operation (MVNO) supports both NB-IoT and CAT-M, does this mean they will also support the range of PSM mode settings for which nRF9160 LTE modem can be configured?

Generally, PSM is a 3GPP standard. If the network operator allows PSM for a connecting device and in which value range is up to the operator itself. You have to check with your service provider here.

Question (2) when app firmware sends an AT command to the LTE modem command handler, of the form AT+CPSMS to set PSM, what range of time periods are typical for the modem to receive a reponse from the celular network regarding PSM requested? How much time must pass in order to sensibly query the modem again to see whether settings were granted?

The nRF9160 modem will negotiate PSM (meaning active time and tracking area update) with the network it is trying to connect to. It will use the values configured with AT+CPSMS and the network will either accept those values, assign different ones or reject them (meaning that PSM is not allowed).

You can check the PSM status using AT%XMONITOR right after the network connection has been established successfully. AT+CEREG can be used as well, although you always should cross-check the <Periodic-TAU> values with %XMONITOR. Anyhow, if there is no active time, PSM is disabled independent of the <Periodic-TAU> values that will be shown.

Question (3) in cases where celular network does not support PSM or eDRX, is the next best strategy for app firmware to turn off the modem altogether for some period, as in a battery based system needing to extend battery life?

Turning off the modem causes NVM wear, so my recommendation is to set it into flight mode using AT+CFUN=4.

I hope I could answer you questions :-)

Regards,

Markus

[tedhavelka]

Hello Albrecht,

Regarding PSM and eDRX questions thank you much.  You have answered my questions very clearly!  You also corrected my understanding of AT command `AT+CFUN=4`, setting modem into flight mode.  I mistakenly thought this command powered off the modem completely.

Follow up questions:

Question (1)  with modem in flight mode can nRF9160 SiP draw single microamps of current, e.g. 3uA to 4uA?

Question (2)  Going in and out of flight mode can an nRF9160, Zephyr based application approach the same power use efficiencies in battery based designs, as it could using PSM and eDRX?

In two recent posts I made -- 85747, 86265 -- Devzone engineers Didrik and Håkon provided key Kconfig and related settings for a low power sample app to run on the nRF9160DK.  Using this known dev kit and same code we each measured between 3.5uA and 4.5uA.  But the code makes no explicit calls to the LTE modem.  Is it the case that with CONFIG_NRF_MODEM_LIB=y and CONFIG_NRF_MODEM_LIB_SYS_INIT=y in one's Zephyr app prj.conf file, the LTE modem is configured for flight mode by default?

This last question reinforces or re-asks in a way whether flight mode enables single-microamp current use.  That is, with all Zephyr threads able to go to sleep for extended time periods.

- Ted

[Albrecht Markus Schellenberger]

Hello Ted,

tedhavelka said:

Question (1)  with modem in flight mode can nRF9160 SiP draw single microamps of current, e.g. 3uA to 4uA?

Yes. Referring also to the Modem current consumption table.

tedhavelka said:

Question (2)  Going in and out of flight mode can an nRF9160, Zephyr based application approach the same power use efficiencies in battery based designs, as it could using PSM and eDRX?

Going into flight mode means in practice to shut down the modem without wearing its memory. So while the modem is in flight mode it is very power efficient, yes. Otherwise it depends on the use case. Every time the modem goes into normal from flight mode, it has to search for a network and establish a connection before it is able to transmit/receive any data. Dependent on the environment, this can be quite power consuming. Contrary to this, PSM/eDRX does not need almost any power at all to wake up for data transmission/reception, as the network connection already is established and configured. Hence, there is no universal answer here, you have to decide that individually based on your specific use cases.

tedhavelka said:

Is it the case that with CONFIG_NRF_MODEM_LIB=y and CONFIG_NRF_MODEM_LIB_SYS_INIT=y in one's Zephyr app prj.conf file, the LTE modem is configured for flight mode by default?

On initialisation, the modem normally is in offline mode. But there is not really a difference between those two in this situation, so in a way you can call it flight mode as well :-)

Regards,

Markus