Overview
This short guide will help you understand how to troubleshoot issues/problems with the CAT-M/NB-IoT connection. A lot of information about the connection can be obtained using modem "AT" commands. By definition, "AT" means 'attention'. Each command string is prefixed with "AT", and a number of discrete commands can be concatenated after the "AT".
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note: Mobile network is managed by the ISP. If there are any issues with the connection, we advise contacting ISP beforehand and making sure that they support/allow the connection or the setting in the first place. If you decide to change the configuration with AT commands (without consulting with the ISP), you can do it at your own risk.
Initiating AT Commands
Option A
To send the command, you must use the CLI.
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In the example above, AT command "at+cpin?" is used and the answer "+CPIN: READY" is received.
Option B
The second way to send AT commands is through the ppp-client interface:
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You can get the same result as with Option A.
AT Commands
at
Simple at command checks communication between the module (device) and application.
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When "OK" is returned, it means the module successfully responded.
at+cpin?
This command is used to check the status of the SIM card.
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If you get the response "+CME ERROR: SIM not inserted" → double-check that the SIM card is properly installed in the SIM slot.
at+cfun?
The "CFUN" command is used to check the functional mode. There could be different responses to this command.
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You can manually set the mode to full functionality with the command at+cfun=1.
at+cops?
The at+cops? command is used to check the current network connection.
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The fourth part of the response represents the technology used. "8" stands for eMTC (CAT-M) and "9" for NB-IoT.
at+cops=?
The at+cops=? is used to get the list of all available ISPs. This commandcantake some time (up ~ 1 min) to display the results.
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To select ISP automatically, use the command at+cops=0.
at+creg?
at+creg? command is used to check the network's registration status.
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If you have more than 2 values returned, it means that network registration and location information unsolicited result code is enabled. The second value will show a two-byte location area code in hexadecimal format, the third value will show four-byte GERANE/E-UTRAN cell ID in hexadecimal format and the last value will show access technology of the serving cell ("8" for CAT-M and "9" for NB-IoT).
at+csq
The at+csq command returns received signal strength indication (RSSI) and channel bit error (BER) values. This command will return proper values only when the registration in the network is complete.
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As per the example/output "+csq: 25,99", the result translates to RSSI~63 dBm and BER parameter not detectable.
at+qnwinfo
The command returns information about access technology, ISP and band selected. This command will return proper information only when the registration in the network is complete.
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As per the example above, you can see that the current module uses "eMTC" (CAT-M), ISP is "24701" (numeric format, which stands for "LMT"), LTE Band 20, and channel ID "6300".
at+qcsq
The command at+qcsq returns information about the signal strength of the current ISP connection and the technology used.
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As per the example above, RSSI=-62 dBm, RSRP=-87 dBm, SINR=111(111/5-20)~2 dB, RSRQ=-12 dB.
at+qcfg="iotopmode"?
The command is used to check which technology is used to connect to ISP. It is possible for the modem to work only in CAT-M mode, only in NB-IoT mode, or in both.
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To speed up network searching, it is suggested to execute the command at+cfun=0 (to turn off the radio) and then use the described command. After that, execute at+cfun=1,1 (to turn on the radio back and reset the module to take effect).
at+qcfg="band"?
This command is used to check which bands/frequencies are currently accepted.
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To speed up network searching, it is suggested to execute the command at+cfun=0 (to turn off the radio) and then use the described command. After that, execute at+cfun=1,1 (to turn on the radio back and reset the module to take effect).
at+qgpscfg="priority",(0-1)
It is important to understand that data transfer over CAT-M/NB-IoT and GNSS can not work simultaneously (WWAN and GNSS Rx chains in the BG77 module share certain hardware blocks, meaning the module does not support concurrent operation of WWAN and GNSS.). That is why the at+qgpscfg="priority",(0-1) command was introduced.
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