RE: Handle Weak LTE Connection

Achim Kraus — Mon, 26 Aug 2024 16:40:45 GMT

Did you try to get signal level and quality, e.g. executing

"AT+CEINFO?"

Especially, if that shows low signal levels (<-130dBm), low quality (< 0dB) and high repetition (e.g. NB-IoT down 128, up 32) then two consequences will show:

- the RTT gets up to 15s (and more)

- the IP message loss gets also high (depending on the message size, in my experience 250 bytes may result in 10% lost)

that may cause TCP to drop the connections. If you want/need to stick to TCP, you will need a (much) better antenna.

Even if you use a UDP based protocol, you need to ensure, that the transmission timeouts are larger enough, that the lost ip-messages are retransmitted (and even with 3 retransmissions, message will get lost) and that message is rather small. Also consider, that the signal level/quality may change within a day (usually better at night) and within weeks (e.g. weather).

Just a statistic from one of my test devices (UDP, CoAP/DTLS 1.2 CID):

250 bytes, 15s timeout , max 3 retransmission.

RTT: 12450 ms, CT: 13273 ms
CE: down: 128, up: 32, RSRP: -126 dBm, CINR: 2 dB, SNR: 3 dB

> 0*1471, 1*113, 2*3, 3*3, failures 8

1471 within that 15s.

113 with one retransmission after 15s

3 with a second retransmission after 15s and 30s

3 with a third retransmission after 15s, 30s, 60s

8 gave up after 3 retransmissions.