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I have a Telit GL865 GSM/GPRS module which is connected to a SBC (running ARCH Linux.) via Serial Port. For GPRS connection, PPP connection as indicaten on this page.

I want to read Cell Tower Signal Strength by the following AT Command: AT+CSQ?

But I can not use AT commands (AT+CSQ?) if PPP connection is active. First I detach/release PPP connection. Than I can use AT commands. Reconnection to the GPRS tooks about 2-3 minutes. But I do not want to wait for this delay.

Is there any way to check Cell Signal Strength Level or running this AT command (AT+CSQ?) without closing/disconnecting the PPP Connection?

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Try the following:

Suspend the PPP connection by sending the +++ escape sequence:

(wait one second) +++ (wait one second)

If the modem comes back with OK, you are back in command mode. If it comes back with NO CARRIER, then this method will not work as it has hung up.

Assuming it comes back with OK, you can now send your AT+CSQ command (or any others).

When done, use this to re-enter the PPP data state:

AT+CGDATA=”PPP”,1

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It is possible to issue AT commands whilst a PPP connection is live by setting up multiple multiplexed serial streams over a single physical serial port.

The standard which permits this is GSM 27.010 serial multiplex mode, which is implemented by the modem ("CMUX mode" - telit has a guide to this mode which you can find online). Linux also supports the GSM 27.010 standard using a "line discipline" implemented in the n_gsm kernel module. You can use the ldattach command to put the modem into the correct mode, and then attach the line discipline to the serial port (there are also various other similar commands which do the same thing, but also do the CMUX setup on the modem automatically, instead of you having to specify the AT command via ldattach arguments).

Once the line discipline is attached, you'll then have multiple virtual serial ports /dev/gsmtty1, /dev/gsmtty2 etc. which you can use simultaneously. Put the ppp connection on /dev/gsmtty1, and execute the AT+CSQ command on /dev/gsmtty2 without any interruption to the ppp session.

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