Reliable MQTT connection in poor LTE signal? BSD library hangs.

[GJR]

Here is an issue I am having when using MQTT in a poor (RSRP < 15) LTE signal environment 

Condition:

nRF Connect SDK v1.3

modem firmware v1.2

TLS Socket is configured as non-blocking.

Modem does not notify any CEREG status when MQTT dropout.

 

Under this condition, when MQTT dropped, MQTT client can only attempt re-opening the socket until it's successful.

With this circumstance, frequently attempt re-opening the socket will cause the BSD library to hang. The device needs to be restarted

 

Is there any reliable method for re-connecting MQTT in a poor LTE signal environment?

[Martin Lesund]

Hi GJR,
Could you please take a modem trace of this behavior so we could study it in more detail?

[GJR]

Hi Martin,

Thank you for your reply. Attached are two modem traces and further details below.

Board: nrf9160DK
nRF Connect SDK v1.3
modem firmware v1.2.1

Test setup:
1) boots up the device in low 4G signal zone.
2) Once mqtt is connected, block 4G signal by shielding the device.
3) Once mqtt dropped, attempting mqtt connect with 1Hz rate until successful.
4) Modem trace is recorded since device boots up until BSD lib hangs and no trace is generated.

2 tests were performed
test1.bin - MQTT (TCP/TLS) sockets with AT CMD requesting RSRP and Battery Voltage @1Hz

test2.bin - MQTT(TCP/TLS) socket without AT CMD

Many thanks for your help with this.

Kind regards,
George

test1.bintest2.bin

[Martin Lesund]

Hi George,
Could you do a test with the latest MFW 1.2.2 and NCS v1.4.0-rc1?