Necessary AT command sequence, prerequisites to obtain PSM?

[tedhavelka]

Hello Devzone Community,

My name is Ted, and I am continuing work on an nRF9160 based, battery powered custom board firmware.  I recently posted a couple of tickets here regarding LTE modem Power Saving Mode (PSM) requesting command, AT+CPSMS.  Also asked how to interpret the data returned by both AT+CEREG? and AT%XMONITOR.  So far I have limited success seeing my team's 9160 based hardware be granted any active, enabled PSM T3412 timer values.  ( Note the info message that appears on Online Profiler when hovering over periodic-TAU entry box. )  I seem only to be able to be granted a 54 minute extended periodic-TAU.  This leads to some questions:

Question 1 - is there an AT command sequence prior to, or before and after issuing AT+CPSMS="..." I must use in order to be granted a PSM value?

Question 2 - must an LTE modem have connection with a celular tower or provider in order for AT+CPSMS to be answered, or is a PSM request somehow queued until a celular connection is made?

Question 3 - is it common that a celular provider or Mobile Virtual Network Operator grant only one PSM value?

I find that no matter what extended periodic-TAU value I request, if I am granted anything at all the value granted is invariably 54 minutes.  Is this typical, or a sign of my doing something incorrectly?

- Ted

[Hakon]

Modem team responded;

Q1 and Q2:

If I look AT sequence in https://devzone.nordicsemi.com/f/nordic-q-a/87176/meaning-of-at-xmonitor-reported-psm-values:

s1 > at+cfun?

- DEV 0218 - found possible AT command.

modem reply: +CFUN: 0

 

s1 > at+cfun=1

- DEV 0218 - found possible AT command.

modem reply: s1 > at_ 

 

s1 > at+cfun?

- DEV 0218 - found possible AT command.

modem reply: +CFUN: 1

 

s1 > at+cpsms=1,,,"00100010","00000101"

- DEV 0218 - found possible AT command.

modem reply: s1 > at+cpsms?

- DEV 0218 - found possible AT command.

modem reply: +CPSMS: 1,,,"00100010","00000101"

 

s1 > at+cereg=5

- DEV 0218 - found possible AT command.

modem reply: s1 > at+cereg?

- DEV 0218 - found possible AT command.

modem reply: +CEREG: 5,5,"3558","02BDF917",7,,,"11100000","11100000"

 

s1 > at+cgdcont?

- DEV 0218 - found possible AT command.

modem reply: +CGDCONT: 0,"IP","globaldata.iot","10.165.91.12",0,0

 

s1 > at%xmonitor

- DEV 0218 - found possible AT command.

modem reply: %XMONITOR: 5,"","","310260","3558",7,12,"02BDF917",397,5035,41,40,"","11100000","11100000","01001001"

 

s1 > at%xsystemmode?

- DEV 0218 - found possible AT command.

modem reply: %XSYSTEMMODE: 1,1,0,1 

at+cpsms=1,,,"00100010","00000101" should be commanded before CFUN=1. Otherwise it's not negotiated in LTE attach procedure. If still XMONITOR or CEREG shows Active time "11100000", then PSM is not supported by the NW or SIM card.

Here's some general information about PSM that could be helpful;

• UE negotiates the PSM usage with the network and network can approve or reject it. The negotiation is done during the registration (i.e. Attach procedure) or if PSM is configured later then UE initiates TAU procedure for the negotiation.
• During the PSM negotiation basically two timer values are negotiated:
  • "Active time" timer, i.e. T3324. This timer defines how soon after RRC connection release UE can enter to PSM.
  • "Periodic TAU timer" i.e. T3412. This timer defines how soon after RRC connection release UE must initiate periodic TAU. In other words this value tells when UE must wake up from PSM (in case PSM was entered in the first place).
  • If network approves PSM usage during the negotiation, it must provide both of these timers to UE. **
• If client wants to activate PSM immediately after startup, the AT command that does the PSM configuration must be sent already before activating modem (+CFUN=1) so that the negotiation is can be done already during the Attach procedure.
• PSM support has no dependency to signal quality.
• We have seen that some carriers do not allow PSM usage for devices roaming in their network.
• Checking has PSM been successfully negotiated during Attach, i.e. taken into use:
  • Configure PSM with +CPSMS. For example: AT+CPSMS=1,"","","10101010","00100001"
  • Subscribe +CEREG notification: AT+CEREG=4
  • Activate modem with +CFUN=1.
  • Wait for +CEREG notification that indicates that registration, i.e. Attach procedure has been completed successfully. This can be recognized so that <stat> in +CEREG notification is either 1 (registered, home network) or 5 (registered, roaming). This indication contains also "Active time" and "Periodic TAU timer" values provided by network. These are the values that have been taken into use by modem and network. They may not be the same that were configured, because network can take into use also some other values if the requested values were not suitable for some reason. If both values are valid, then PSM usage has been agreed between modem and network. If either one is missing, then network did not allow PSM usage.

Does this answer your question?