Debugging a cellular modem

Communicating directly with a modem with the AT command set

Most modems support AT commands which control the modem and query for information.   On a Chromium OS device with the developer mode enabled, you can use the socat command to communicate with the modem.  You need a Chromium OS device with a test image installed (or you can use gmerge socat if you are familiar with the dev server) .  For example:
  • Type CTRL+ALT+T to enter the crosh shell.
  • Type shell to enter the bash shell.
  • Figure you which USB device to use with: ls /dev/ttyACM* /dev/ttyUSB*
  • Stop modem manager if it is running: sudo stop modemmanager
  • Type sudo socat - /dev/ttyACM1,crnl or sudo socat - /dev/ttyUSB0 to communicate with the modem.
  • Ensure that the modem is enabled with the command:  "AT+CFUN=1"
For example:
crosh> shell
chronos@localhost ~ $ sudo stop modemmanager
chronos@localhost ~ $ ls /dev/ttyACM* /dev/ttyUSB*
/dev/ttyUSB0 /dev/ttyUSB1 /dev/ttyUSB2
chronos@localhost ~ $ socat - /dev/ttyUSB1
ATZ
OK
AT+CFUN=1
OK
ATI
Manufacturer: Qualcomm Incorporated
Model: ......
Revision:....
ESN: 0x80d...
+GCAP: +CIS707A, ....

chronos@localhost ~ $
Depending on the modem, you can try different AT commands, e.g. "AT+CGMI", "AT+CGSN", etc. 

A notable exception:  Gobi Modems

While modems based on the Qualcomm Gobi chipset support AT commands, Chromium OS uses the Qualcomm connection management API to control these modems.

Using minicom

One can also use minicom to communicate with the modem.  This program is available in test images (or you can use  gmerge socat if you are familiar with the dev server) .
  • Type CTRL-ALT-T to enter the crosh shell.
  • Type shell to enter the bash shell
  • Figure you which usb device to use with: ls /dev/ttyACM* /dev/ttyUSB*
  • Stop modem manager if it is running: sudo stop modem-manager
  • Type sudo minicom -s
  • Ignore, "WARNING: configuration file not found"
  • Use the down arrow key and enter to choose the menu Serial port setup
  • Type A to edit the Serial Device, changing it to /dev/ttyACM1 or /dev/ttyUSB1
  • Press enter to return to the top level configuration menu
  • Use the arrow keys and enter to select Exit which will exit configuration and let you communicate with the modem.
  • Use CTRL-A x to exit and CTRL-A z for help

Collecting Additional Logging

Run the following crosh command to enable debug logging in modem manager at runtime (without restarting modem manager). 
crosh> modem set-logging debug

On a Chromium OS device with the developer modem enabled, you can restart modem manager with debug logging enabled. 
crosh> shell
chronos@localhost ~ $ sudo restart modemmanager MM_LOGLEVEL=DEBUG

Testing Download Speeds

Chromium images include the sftp client, and also wget and curl for downloading.  If you have a test build, it contains iperf too.  You will need to enable developer mode on the platform before you are able to access the shell. 
  • Flip the dev switch to enable developer mode.
  • Reboot the system
  • Wait for system to wipe stateful partition.
  • When booted, press CTRL+ALT+F2 (right arrow) to enter VT2 shell.
  • Use username chronos or root. No password necessary.
  • You can now use sftp, curl, wget or iperf for testing performance.