I have several Supreme 20 modems, with data-only enabled SIM cards. There are four on the server side and one on each client. The server modem calls the client modem at regular intervals while client modem calls the server modem when there is some important update at its end. Average duration of the call is 30 seconds, after that, the caller hangs up. All modems are driven with AT commands over serial port.
System can work for a couple of months at a time, but sometimes one of the modems stops receiving calls and is unable to dial out. It can happen to any modem on the server or client’s side.
There is no indication that anything is wrong with the problematic modem:
LED is blinking
+CEER reports no error
+CREG reports “0,1”
+COPS reports that modem is in home network
+CSQ reports that there is a good signal
With WIND and CMEE reporting turned on I have tried to dial a working modem. After the ATD command I got no more messages (ERROR, WIND, BUSY, NO CARRIER, etc.) from the modem. After five minutes of nothing happening, I have issued the ATH command and checked for CEER. And there was no error.
When trying to call the problematic modem with a working modem, the working modem also didn’t respond with anything to ATD command. It just waited silently until ATH.
Even the reinserting of the SIM card on the problematic modem did not fix the problem. The only thing that worked was issuing AT+CFUN=1 command or power-cycling the modem.
Currently I have modems on the server side set to auto-reset twice a day, but would like a better solution for clients.