Verifying nRF9160 going into PSM

Question

Hi everyone 
 We're developing an IoT device with an nRF9160 embedded in it. We're trying to make sure we're implementing PSM. 
 We set our preferred values for TAU as you can see in the next image. And in our code we call lte_lc_psm_req(true); when configuring the modem. 
 
 We understand that the network is overriding the TAU values. 
 
 From this I can tell that TAU is 180 min and Active Timer is disabled. 
 
 My questions are: 
 1. Will my modem go into sleep mode immediately? 
 2. How can we verify that my modem is sleeping? 
 
 Any information that will help us broaden our knowledge will be appriciated. 
 Regards.

user100109 · Accepted Answer

Hello Zabdiel, 
 zabdielfavela said: What happens when we send data before the periodic TAU timer is up? Will the TAU timer restart or will it continue with the remaining time from the previous cycle? 
 The T3412 (Periodic Tracking Area Update) value is a negotiation between the UE and the network indicating after which time the UE has to wake up from sleep mode to transmit a tracking area update. If the UE does so before that time has passed, because it e.g. will transmit user data, then there is no need for a tracking area update within that cycle and the timer will be reset and start over again when the UE enters sleep mode. You might find this link interesting, as it describes the basis of PSM and eDRX. 
 zabdielfavela said: We’ve come up with an indirect way to detect modem transition into PSM. First we subscribe to RSRP notifications. Next, we query the signal connection status (CSCON). And if the connection state is IDLE, then we judge sudden signal values of 255 as modem entering deep sleep. Do you think our method detects PSM accurately? 
 My recommendation would be to use modem sleep notifications, %XMODEMSLEEP . Additionally, you could subscribe to signal quality notifcations, %CESQ , as the signal strength values will switch to 255 when the UE enters PSM. 
 zabdielfavela said: If we receive valid timer values, should we assume PSM is supported? 
 Yes. 
 zabdielfavela said: If not, is there a way to verify PSM is supported? 
 No. You have to get in touch with the carrier in this case. Regards, 
 Markus