Why not always use Proprietary PSM instead of regular PSM?

[droberson]

To preface:

• nRF9151DK
• Modem firmware: mfw_nrf91x1_2.0.2
• nRF Connect SDK / Toolchain: 2.9.0
• Terminal app: PuTTY
• Carrier: Verizon and T-Mobile
• Device application will always initiate the data connection

Hello,

I have a few questions about PSM. I am new to PSM, eDRX, and CAT-M in general.

Question 1: Why not use proprietary PSM all the time for devices that always initiate the data connection instead of using standard PSM on the network?

It seems like using proprietary PSM all the time instead of network PSM would eliminate cases where poor cellular coverage or roaming results in the network not granting PSM mode to the modem.

Question 2: Is there documentation from Nordic that details which PSM method is better, and the pros and cons of each?

The only documentation I see is in the AT command reference guide, which is minimal: https://docs.nordicsemi.com/bundle/ref_at_commands_nrf91x1/page/REF/at_commands/intro_nrf91x1.html 

The application for the product I am currently working on will send a chunk of data to a server every 3 minutes. The modem should enter its lowest power state between these 3-minute intervals after sending the data.

Thanks,

Derek

[dejans]

Hi Derek,

Proprietary PSM is used when network does not allow regular PSM. Please refer to proprietary PSM documentation. In proprietary PSM mode it is expected that device always initiates data connection.

Best regards,
Dejan

[droberson]

Hey Dejan,

Thanks for the link! I came across this link at some point and lost / forgot about it. Been doing a lot of research the past few days.

In regard to my use case detailed above, it looks like the potential downsides from your link of using Proprietary PSM are the following:

1. If the cellular network notices the device is in proprietary PSM mode, it can "perform local deregistration for the device, which would lead to a new registration and extra signaling when the device wakes up. The operator can take some unwanted actions if a large number of devices are constantly unreachable when there is downlink data to be sent to the devices."

2. The Tracking Area Update (TAU) timer T3412 is typically "shorter when PSM is not used."

Edit: I will need to test that if when PSM is disabled, that the TAU timer doesn't get set to less than 3 minutes. Otherwise, this will require a shorter PSM interval or re-registration each time the modem wakes up to send data.

This leads to the following additional questions:

Question 3: In regard to downside #1 highlighted above, if there is no "downlink data" pushed from a client server, then #1 is of no concern, correct?

Question 4: What happens when the modem fails to enter PSM mode due to poor cellular connectivity or roaming, does the modem use the proprietary PSM mode as a fallback? If so, is this operation automatic so long as PSM is enabled or is there some other configuration needed by me?

Thanks,

Derek

[dejans]

Hi Derek,

It is important that no downlink traffic is coming to the device. Device behaves as unreachable from the network side and any potential downlink traffic could cause new registration and extra signalling. As long as there is no downlink data, there should be no concern.

Proprietary PSM is intended to be used in specific use cases where ordinary PSM is not allowed by the network (home or roaming) and connection is always initialized by the device.

When modem fails to enter PSM mode, it can automatically fallback to proprietary PSM but this needs to be configured separately. This can be done either by enabling CONFIG_LTE_PROPRIETARY_PSM_REQ or by calling lt_lc_proprietary_psm_req(). To use proprietary PSM, PSM needs to be configured using AT+CPSMS and proprietary PSM must be enabled using AT%FEACONF.

Best regards,
Dejan