nRF9160 send larger MQTT payloads than 2048 bytes to save battery power (need to send 200KB)

[Johan]

Hi,

I want to optimize the transmission efficiency and thinking of sending larger packets than 2048 bytes of MQTT would help.

We have limited battery power, and looking at the MQTT traffic towards AWS I see that this is happening:

1) MQTT-packet is sent.

2) Waiting for response (several seconds)

3) Receiving an ACK.

4) goto 1 (is the fastest way I can do it now, send the next packet of max 2048 bytes).

If I can change the max mqtt packet size to for instance 16K, that would mean an improvement I'm guessing.

Changing it though, makes the example code crash, going somewhere into non program space. It doesn't seem to allow changing of that size, even though it's available in the menu <Project -> configure nRF SDK...etc -> searching for MQTT -> finding maximum MQTT PACKET SIZE set to 2048>

I tried 4096, 8192, 16384.

I get for bigger sizes, crash in the code.

For smaller increases like 4096, I get an error from the socket_write instead, so it's then in the modem software I'm guessing.

I really want to send bigger packages, to improve battery life.

Who can help with this? 

IS this bugs or crash by design, or fail by design, even though the MQTT PACKET SIZE is allowed to be changed by design?

Johan

[Martin Lesund]

Hi,
From our extensive investigation on the Telia LTE-M network in Sweden with the nRF9160, we have found that the issue is caused by the network side.

The RLC connection release is not happening, and after that, the new connection cannot be created. This leads to long pauses in data transfers. 
We have tested against the same infrastructure that Telia uses and have not seen this in other networks.

I would recommend contacting Telia Sweden to get this solved.

Best regards,

Martin L.