LTE: Network detach on UDP send

[JordanYates]

I have a collection of nRF9151 devices using PSM mode.

Each time the application attempts to send UDP data while in sleep mode, they get a detach request (from the network I assume), and the data payload is lost.
In response to the command the device re-attaches a second or two later, and returns to PSM sleep.
This process then repeats each time the device attempts to send (every 5 minutes).

The same software and SIMs work fine on a different subset of devices.
I have attached a modem trace of this behavior and would appreciate any insights that can be provided.

Possibly related, and shown in the same trace file, attempting to create a new PDN context with AT+CGFCONT=1,"IPV4V6" is occasionally working, and more often failing with the EPS error 50 (Cause #50 – PDN type IPv4 only allowed). I can't see any reason why this would only sometimes work.

vs

/cfs-file/__key/communityserver-discussions-components-files/4/1738825059_5F00_nrf_5F00_modem_5F00_trace.zip

The modem firmware version is v2.0.2

[emdi]

Hello,

I have created a ticket to investigate this. What I can see so far from the trace is that there is an AT notification coming right after the PDN activation failure, precisely:

`+CGEV: RESTR 3, 1, 1, ..., ...`

That AT notification has this format:
`+CGEV: RESTR <cause>,<validity>,<cid>,<remaining_time_regular>,<remaining_time_exceptional>`

where:

<cause>
Restriction cause
1 – Radio Policy Manager (RPM). Procedure restricted by RPM.
2 – Throttling. Procedure restricted by 3GPP or operator-specific throttling.
3 – Invalid configuration. Procedure restricted by invalid context configuration.

<validity>
Validity of restriction
1 – Permanent restriction. Enabling requires a power-off, Universal Integrated Circuit Card (UICC) change, or a configuration change.
2 – Temporary restriction. Enabling requires, for example, back-off timer expiry.

Full documentation is here.

[JordanYates]

I saw that error as well, but the failure parameters don't make much sense to me.
According to the docs, it is an invalid configuration error, as a permanent restriction.
Yet the exact same commands a few minutes later work without a problem?

[Charlie]

Hi Jordan,

It seem the issue is more realted to the SIM card/Network configuations.Please read the following analyses from our modem development team. The mentioned questions are worth to verfiy and check with your network opeator. 

In the log it seems that the first wake up from PSM is triggered by SIM activity. SIM card requests to establish a PDN connection with APN “teal”. The ESM cause #50 is normal when there is no support for IPv6. The PDN connection is successfully established and SIM card requests to disconnect soon, after which PSM is again entered.

While in PSM sleep, SIM card requests again to establish PDN connection with APN “teal”. This time the request is rejected by modem because Radio Policy Manager (RPM) allows to establish PDN connection to the same APN only certain number of times per hour. The number (F4 in the figure below) is configured to the SIM card. I cannot see from the log what is the number in this case and I don’t know how many PDN connection establishments there have been before the log starts.

More info of the Radio Policy Manager can be found from here: https://www.gsma.com/newsroom/wp-content/uploads//TS.34_v7.1.pdf

A bit later, there appears uplink data to be sent, hence modem initiates Service Request procedure to establish data radio bearers. Once the RRC connection has been established, network sends Detach Request to UE asking to re-Attach. There is no more details in the PDU (causes etc.) so it is difficult to say the reason for this. It could be related to the prevented PDN connection establishment, i.e. network was waiting for the SIM card to connect but because it did not connect (it was prevented by RPM), network decided to require re-Attach. It could be something else, too.

Has the customer tried using the SIM card in some other network? Is the result the same?

Might be also good to ask the operator how often it is expected that the SIM card connects the network? In this log the second connection was triggered about 2 minutes after the first one was completed, so this seems quite a lot if it continues like this - It triggers PSM exit on every attempt. Also, it would be informative to know what is the RPM F4 value in the card. If it doesn’t exist in the SIM card, the default value to be used by modem for F4 is 30.

Best regards,

Charlie