AT Command is a command set used to control GSM machine, it is very similar with DOS command.
In DOS mode, you key in a DOS command (like DIR, DEL etc), DOS recognize it and execute the command and reply the result...
To control GSM machine, you "key in" AT command, the machine recognize it and execute the command and reply the result...
But the GSM machine don't has keyboard, so the command is injected to the machine thru serial port and the result answer we get from serial port too.
The AT Command set has hundreds of command, please check which completely explain the whole set of Nokia AT Commands.
If you like to get the answer of "What I have to do if I want to...?" questions, read the PDF file I attached.
I don't find any AT command tutorial, but there are SMS tutorial :
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How to Send SMS Messages from a Computer / PC?
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How to Receive SMS Messages Using a Computer / PC?