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". 

Please check the examples below to better understand how to initiate those commands and what information can be obtained using them.

Initiating AT Commands

Option A

To send the command, you must use the CLI.

Open the terminal and type in the following:

[admin@MikroTik] > system serial-terminal port=modem channel=2

Pay attention to the port and channel used. Select the port and channel that is used by the PPP-OUT interface. In our example, the port is "modem" and the data channel is "2".

Once the command is initiated it should show the following output where you can now type in the AT commands:

[admin@MikroTik] > system serial-terminal port=modem channel=2

[Ctrl-A is the prefix key]


at+cpin?
+CPIN: READY

OK 

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:

[admin@MikroTik] > interface/ppp-client/at-chat input="AT+CPIN\?"
number: 0
  output: +CPIN: READY

You can get the same result as with Option A.

AT Commands

AT

smh

at+cpin?

This command is used to check the status of the SIM card.

[Ctrl-A is the prefix key]


at+cpin?
+CPIN: READY

OK 

Status "+CPIN: READY" means that the SIM card is installed properly.

If the SIM PIN is used, you should get the answer "+CPIN: SIM PIN" → in this case, you can unlock it via the command at+cpin="xxxx", where xxxx is the pin code.

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.

[Ctrl-A is the prefix key]   


at+cfun?
+CFUN: 1

OK

When the status is "+CFUN: 1" it means that the device is set to full functionality. It is suggested to use this mode.

Among many other responses, for example, you can get "+CFUN: 4" which means that the device is set to flight mode where Tx, Rx, LTE, and GNSS services, are disabled.

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.

[Ctrl-A is the prefix key]      


at+cops?
+cops: 0,0,"LMT",8

OK

at+cops=?

The at+cops=? is used to get the list of all available ISPs. This command can take some time (up ~ 1 min) to display the results.

[Ctrl-A is the prefix key]      


at+cops=?
+cops: (1,"LMT","LMT","24701",9),(2,"LMT","LMT","24701",8),,(0,1,2,3,4),(0,1,2)

OK

You can manually set up the ISP that you wish to connect to via the command at+cops=mode,format,"ISP_name", where mode is the mode used (0→ automatic network selection, 1→ manual network selection), format is the ISP name format (0→ long alphanumeric format, 1→ short alphanumeric format, 2→ numeric format), and "ISP_name" is the ISP name (in the format that was selected). For example, to manually select the network from the list, use the command at+cops=1,1,"LMT".

To select ISP automatically, use the command at+cops=0.

at+csq

The at+csq command returns received signal strength indication (RSSI) and channel bit error (BER) values.

[Ctrl-A is the prefix key]           


at+csq
+csq: 25,99

OK

The first value is RSSI (dBm):

The second value is BER (%):

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.

[Ctrl-A is the prefix key]              


at+qnwinfo
+QNWINFO: "eMTC","24701","LTE BAND 20",6300

OK

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 and the technology used.

[Ctrl-A is the prefix key]                 


at+qcsq
+QCSQ: "eMTC",-62,-87,111,-12

OK

The first parameter returned is "eMTC" which shows the technology used by the module (CAT-M). If the network can not be determined (is uncertain) →  "NOSERVICE" will be returned.

The rest of the parameters show the signal strength of the connection.

When the technology is recognized as "NBIoT" or "eMTC" (CAT-M):

• the first numeric value displays received signal strength (LTE RSSI);
• the second value displays the reference signal received power (LTE RSRP);
• the third value displays the signal to interference plus noise ratio (LTE SINR) →  the value is displayed as the logarithmical value of SINR and the value is 1/5th of a dB (result can be in between 0-250 range, which translates to -20 dB to +30 dB range);
• the fourth value displays the reference signal received quality (LTE RSRQ).

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.

[Ctrl-A is the prefix key]   


at+qcfg="iotopmode"?
+QCFG: "iotopmode",2

OK

You can lock the SIM card to work only in the specific mode with the command at+qcfg="iotopmode",mode,effect, where mode is the mode (0→ only CAT-M, 1→ only NB-IoT, 2→ both), and effect is when to take effect (0→ to take effect after reboot, 1→ to take effect immediately).

at+qcfg="band"?

This command is used to check which bands/frequencies are currently accepted.

[Ctrl-A is the prefix key]   


at+qcfg="band"?
+QCFG: "band",0x0,0x100002000000000f0e189f,0x10004200000000090e189f

OK

The first value in the "qcfg" response shows a hexadecimal value that specifies the GSM frequency band. This value is not relevant for the Quectel BG77 module. This value can be ignored.

The second hexadecimal value specifies CAT-M frequency bands (which is 0x100002000000000f0e189f). If it is set to 0, it means not to change the frequency band. If, for example, it is 0x15, it means → 0x01(LTE Band 1)+0x04(LTE Band 3)+0x10(LTE Band 5).

The last hexadecimal value specifies NB-IoT frequency bands (which is 0x10004200000000090e189f). If it is set to 0, it means not to change the frequency band. If, for example, it is 0x15, it means → 0x01(LTE Band 1)+0x04(LTE Band 3)+0x10(LTE Band 5).

You can change the band with the command at+qcfg="band",GSM_band,CAT_M_band,NB_IoT_band,effect, where GSM_band is GSM frequency band (can be left 0x0), CAT_M_band is the CAT-M frequency band, NB_IoT_band is the NB-IoT band and effect is whether to take effect immediately (1) or to take effect after module reboots (0).

To setup module the way so that all bands are supported, use the command:

[Ctrl-A is the prefix key]     


at+qcfg="band",0x0,0x100002000000000f0e189f,0x10004200000000090e189f,1

OK

at+qgpscfg="priority",(0-1)

It is important to understand that data transfer over CAT-M/NB-IoT and GNSS can not work simultaneously (the BG77 module is designed the way that concurrent WWAN and GGNS operations are not supported). That is why the at+qgpscfg="priority",(0-1) command was introduced.

It is possible to change the priority with the commands:

[Ctrl-A is the prefix key]    

at+qgpscfg="priority",0

OK

at+qgpscfg="priority",1

OK

at+qgpscfg="priority",0 command sets the highest priority for GNSS.

at+qgpscfg="priority",1 command sets the highest priority for LTE (WWAN).

When you require to check your device coordinates via GNSS, you will need to set the priority to 0.

When you require to begin using WWAN traffic, you will need to set the priority back to 1 (as soon as you do that, GNSS coordinates will stop working).