Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

Syed Maysum Abbas Zaidi — Wed, 29 Jul 2026 12:22:13 GMT

Hi,

Apologies for not mentioning it before, our internal 4KB test that succeeded was using the socket-based path with DECT_MTU set to 4096. The direct DLC API (nrf_modem_dect_dlc_data_tx()) is currently not optimized for large SDUs and the modem memory configuration does not support it well. This has been noted internally, however we cannot confirm if or when this will be optimized.

In the meantime, we would suggest switching to the socket-based approach or continuing with your current workaround of reducing the PDU size. Thanks.

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

0x41 — Tue, 28 Jul 2026 09:20:12 GMT

Hi,

I've changed DECT_MTU as suggested. I have same crash, with non-socket based communication.

nrf_modem: Modem has crashed, reason 0x4, PC: 0x561bc

Kind Regards,
Alessandro

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

Syed Maysum Abbas Zaidi — Mon, 27 Jul 2026 07:11:42 GMT

Hi,
I would like to update that our team managed to send 4KB with the modified ncs dect nr+ stack by changing DECT_MTU to 4096. Can you try changing it and see if it helps. Thanks

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

Syed Maysum Abbas Zaidi — Mon, 20 Jul 2026 12:17:30 GMT

Thanks for confirming. We will surely get back to you on our findings soon. Appreciate your cooperation.

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

0x41 — Mon, 20 Jul 2026 12:07:27 GMT

Hi,

Holidays are holidays, and they're saints.

However in my application I proceeded to reduce the single data pdu length and continue the development, but I will available to test different configuration if this can help you and future developers.

"In the meantime can you confirm that in your original setup (using nrf_modem_dect_dlc_data_tx() with ~3200 bytes) you have also tried it with disabling modem trace as well. As till now we identified that the modem runs out of internal memory blocks during tracing of the large received SDU. "

Yes, initially I've encountered the crash without modem trace enabled.

Kind Regards,
Alessandro

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

Syed Maysum Abbas Zaidi — Mon, 20 Jul 2026 10:44:04 GMT

Hi,

Thanks for trying that out and for the detailed feedback.

You're right that NCS samples (dect_shell, hello_dect) are limited to 1280 bytes (DECT_MTU), so they can't reproduce the issue. I believe our internal 4KB test used some modifications, so I have reached out to internal team and will share more details on that after receiving their response. However just to inform that it could take some time as some of our resources are out of office due to summer break.

In the meantime can you confirm that in your original setup (using nrf_modem_dect_dlc_data_tx() with ~3200 bytes) you have also tried it with disabling modem trace as well. As till now we identified that the modem runs out of internal memory blocks during tracing of the large received SDU.

Best Regards,
Syed Maysum

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

0x41 — Fri, 17 Jul 2026 10:05:03 GMT

Hi, sorry for the response delay.

First of all, thanks for keeping me updated.

I've tried with nordic samples, the first tried is dect_shell, in which I'm not able to send and succesfully receive data higher than 1280 bytes, so nothing bigger than DECT_MTU.
After inspecting l2_shell source code, it's clear that maximum data to send/receive is equal to DECT_MTU. Same for hello_dect sample.

In <skd_home>/nrf/subsys/net/lib/dect/l2_shell/src/dect_l2_shell.c


#define DECT_TX_CMD_MAX_SEND_DATA_LEN DECT_MTU
...
recv_len = zsock_recvfrom(dect_shell_rx_sockfd, rx_data_buf, DECT_MTU, 0,
(struct sockaddr *)&src, &fromlen);

I don't see modem crash by the way.
How do you manage to send 4KB with socket-based stack?

Kind Regards,
Alessandro.

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

Syed Maysum Abbas Zaidi — Wed, 08 Jul 2026 15:00:18 GMT

Hi,

Thanks for waiting. We can see that the modem crashes when it runs out of large internal memory blocks while processing the received large SDU.

On our side, data up to 4KB went through successfully using the NCS DECT NR+ stack. We understand you need DLC segmentation directly, but is it possible for you to try NCS DECT NR+ socket-based stack to confirm the issue doesn't occur there? It uses the same DLC layer internally.

Best Regards,
Syed Maysum

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

Syed Maysum Abbas Zaidi — Thu, 25 Jun 2026 17:24:06 GMT

Hi,

Okay, We understand it. Thanks I will keep you updated on it.

Best Regards,
Syed Maysum

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

0x41 — Thu, 25 Jun 2026 14:42:55 GMT

Hi,

Thanks for the update.
The reason is.. my application requirements.

Since with only PHY (mfw-nr+_nrf91x1_1.1.0) layer it needs in someway to implement a "custom" pdu segmentation/re-assembly algorithm in order to fit data in subslot for bigger data, I was wondering that DLC layer will do this (and it does, as explained in DLC layer TS, it's also mentioned in Hands on with DECT NR+ API release v2.0).

I know that a possible solution can be to segment data chunks but with this I'm loosing a service provided by the DLC layer.
Also a strange behavior I've encountered is that ~2000 bytes chunk is received successfully.

Kind Regards,
Alessando.

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

Syed Maysum Abbas Zaidi — Thu, 25 Jun 2026 14:07:23 GMT

Hi,

Our Team is looking into it, just a question that is there reason for sending big data chunks (as a typical Ethernet MTU is 1500 bytes). Thanks.

Best Regards,
Syed Maysum

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

Syed Maysum Abbas Zaidi — Mon, 22 Jun 2026 16:52:50 GMT

Hi,

Thanks a lot for providing the modem trace. It has been forwarded to modem team for deeper investigation. We will respond back to you soon.

Best Regards,
Syed Maysum

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

0x41 — Mon, 22 Jun 2026 13:38:11 GMT

Hi,

I managed to get a full modem trace + coredump.
I traced with CONFIG_NRF_MODEM_LIB_TRACE_LEVEL_IP_ONLY , using trace-db 2.2.0 (my mfw is mfw-nr+_nrf91x1_2.0.0) with the help of cellular monitor desktop application with uart trace-backend.

I attach the full 65KB trace file for the FT device that receives 3300 bytes data, so the modem crash.
The content of pdus are all A.

devzone.nordicsemi.com/.../trace_5F00_ok.mtrace

King Regards,
Alessandro.

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

Syed Maysum Abbas Zaidi — Fri, 19 Jun 2026 10:52:10 GMT

Hi,

Yes you are correct, that CONFIG_NRF_MODEM_LIB_SHMEM_CTRL_SIZE cannot be directly assigned by the user sorry for pointing it out before.

It would be best if we could get a modem trace to analyze your issue properly, can you tell if you are facing any issues with the trace so we may try to help you on it. Thanks

Best Regards,
Syed Maysum

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

0x41 — Thu, 18 Jun 2026 14:49:09 GMT

Hi,

Thanks for your answer, but maybe is not the point of what I'm looking for, CONFIG_NRF_MODEM_LIB_SHMEM_CTRL_SIZE is not directly user assignable.

From .config in build directory it sets to 1832, value found inside <sdk home>/nrfxlib/nrf_modem/Kconfig.

In my project I don't re-define NRF_MODEM_DECT_SHMEM_CTRL_SIZE.

Regards,
Alessandro.

RE: Modem crash with 0x4 while using DECT nr+ MAC/DLC APIs

Syed Maysum Abbas Zaidi — Thu, 18 Jun 2026 14:05:51 GMT

Hi,

Yes , the error 0x4 is a modem-internal memory management fault, the modem runs out of internal memory during PDU reassembly on the FT side. One thing I would like to mention that the define NRF_MODEM_DECT_PHY_SHMEM_CTRL_SIZE was renamed to NRF_MODEM_DECT_SHMEM_CTRL_SIZE in 3.3.0. If your project still uses the old name, the control region size may get silently ignored so can you please look through all project files and if you find any old name then rename it to NRF_MODEM_DECT_SHMEM_CTRL_SIZE.

Moreover also ensure CONFIG_NRF_MODEM_LIB_SHMEM_CTRL_SIZE is set to at least 1832.

Best Regards,
Syed Maysum