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

[Achim Kraus]

At least in the past, one side effect of using PSM was the "shutdown" of the SIM. In the past it was also said, that this writes to the SIM, which is usually limited to 500.000 write cycles. If this is still the case, then your approach is in danger of killing your SIM in about 3 years.

[droberson]

Hey Achim,

Following up on this, our eSIM OEM for this project never gave us specific numbers on the impact of PSM shutting down the eSIM every 3 minutes in regard to NVM wear of the eSIM. However, your comment about eSIM flash endurance is spot on it seems.

Our email chain with our eSIM OEM is marked as "limited distribution", so I will summarize their comments since it likely applies to any eSIM.

Modern domestic grade eSIMs have a 1 M cycle flash endurance and can last 3-5 years. Industrial grade eSIMs can have a 16 M cycle flash endurance and can last 7-10 years. Powering the eSIM on and off every 3 minutes via PSM is considered excessive and is expected to churn flash cycles from their comments. Whether this will shorten the life of the eSIM below 3 years is unknown since it is device / modem dependent according to them.

In summary, our customer has chosen to stick with the domestic grade eSIM for now. For safety, based on our eSIM OEM's comments, I would use an industrial grade eSIM for applications that power the eSIM on and off for short duty cycles such as every 3 minutes using PSM. If your duty cycle is much longer, a domestic grade eSIM is probably fine.

Thanks,

Derek

[Achim Kraus]

Thanks for the update!

droberson said:

Whether this will shorten the life of the eSIM below 3 years is unknown

I just calculated that by:

every 3 Minutes := 20 per H := 480 per Day and 175200 per Year. With the limit of 500.000 that about 3 Years.

If your eSIM has 1 Million instead of 500.000, then it's about 5.7 Years.

[droberson]

Hey Achim,

I assumed this was how you calculated it, and I was looking for a similar calculation from our eSIM OEM but they couldn't provide it. They couldn't say how many times the eSIM writes to flash on power up, and that this can vary between modem, carrier, etc. They just said that their domestic grade eSIM lifespan averages 3-5 years without providing the math to back this claim up. Frustrating in my opinion, especially from a well known large player in the eSIM space.

Thanks,

Derek

[Achim Kraus]

Ah, I see.

I just assumed, that on PSM sleep/wakeup it writes once ... but that's just an assumption.