Electron LTE TCP connection issue

We are using Electron LTE-M device with ublox SARA R410 modem.
We are opening TCP socket and sending some data directly bypassing the Particle Cloud.
We cannot use the UDP Particle cloud path, because this data is too large. As such, we are resorting to this method. Other smaller data is going through the particle path with no issues.

With the TCP path, we are seeing intermittent drop off and data transfer fails. Our cloud log shows that we suddenly lost connection.
It seems like this issue happens more often with areas with lower signal strength. (below 50% but greater than 40%) However, we have seen issues with >60% signal strength, although not as often.

Has anyone seen similar issue?


That can certainly happen with TCP. In TCP, the standard retry algorithm starts at 3 seconds and doubles the back-off, with 3 retries, for a total of 12 seconds. I didn’t verify that the u-blox TCP stack uses the same values, but I also did not see a socket option for configuring it. In any case, it’s certainly possible that cellular data can be interrupted for periods of more than 12 seconds, even if it recovers, however the TCP connection will definitely fail. That’s one of the reasons the Particle cloud connection uses DTLS (UDP) instead of TCP. The Photon and P1 use TCP.