GSM Modem Watchdog 🔒
Micromedia International has developed an embedded application for Sierra Wireless' GSM modem. This application offers voice and watchdog functionality. This document presents the watchdog functionality. The Alert software can check the status of the serial port and the modem. Alert can call users when the modem is down. Thanks to the watchdog, the GSM modem can check the link with Alert and restart or call if the link is broken or if Alert has stopped.
How does the watchdog work?
The GSM modem needs to check the Alert status. To do this, it periodically sends a frame to Alert and waits for an answer. The watchdog dialog is available in Alert since the version 3.6 Rev 1 Build 27.
Watchdog dialog
This dialog is called watchdog. As the modem doesn’t receive any answer to the sent frame, it sends a SMS to a preconfigured phone number.
The next picture shows the watchdog dialog.
The modem sends +WD unsolicited response every T0 (60 seconds by default). The Alert software sends back an AT command witch is answered by an OK. This is the normal behavior.
When things go wrong, that means no AT from Alert during a T1 (30 s by default) timeout, the +WD is sent again. If Alert send an AT back, then the T0 seconds cycle is restarted.
But, when no AT is answered during the T1 timeout, the modem resend the +WD message. After the Nth +WD attempt, then the modem can reboot or send a predefined SMS to a preconfigured number. As shown on next diagram.
In the following example, the number of attempts (N) is 2.
When no reboot is done, or after reboot, if Alert is not answering, the SMS is sent. The modem goes into a SOS state.
The SOS state
In this state the modem will send SMS to warn that Alert is out of order. We can specify a number of SMS to send or the modem can send SMS without limit. The SMS recipients and the SMS message and the sending period need to be configured first (See next part: Watchdog Configuration).
To stop this SOS state, an AT command must be sent to the modem. If such a command is received, the watchdog dialog is re-established and no more SOS SMS are sent. This can occur on modem reboot, if Alert is still alive.
Disabling the watchdog feature will stop the SOS SMS as well.
The reboot
The reboot can be activated independently of the SOS SMS. When both SMS and reboot are configured, the GSM modem reboots first and then send the SMS (if necessary) after reboot.
After reboot, if the watchdog fails again, the modem goes into the SOS state without rebooting.
Configuration Watchdog
Before using the watchdog, the user must configure:
- The watchdog timeouts
- The SOS message (SMS recipients, SMS message, SMS period)
- The modem reboot
Configuration
The following picture displays the Alert configuration box for the GSM watchdog.
Check or uncheck the Watchdog active checkbox to enable or disable the watchdog on the GSM modem.
The Polling group displays the watchdog polling parameters:
- Period is the time between two watchdog requests in seconds.
- Answer timeout gives the maximum time in seconds to wait for the request answer.
- Attempts number is the maximum number of requests without answer before rebooting and/or entering the SOS state.
The options in the On watchdog failure group describe the modem behavior when Alert is not responding to the watchdog requests.
If Reboot modem is checked, the modem will reboot after watchdog failure.
If Send SMS is selected, the modem will send a given number of SMS. Number of SMS gives the number of sent SMS. (A value of 0 means no limit). The messages are sent to Phone numbers every SMS period in minutes. The message text is given by the Message to send field.
You can specify up to 10 recipient numbers separated by “-“.
When validating by OK, all the configuration is sent and saved on the GSM modem.