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For details about AT commands refer this link . If you are using windows you can use hyperterminal to communicate with any modem. In this case the GSM modem. Find a method to connect your GSM modem to the PC  (Refer this Link  ). You can use a USB2TTL converter to directly interface your GSM modem to PC. After interfacing the modem go to device manager and find the com port no: assigned to your GSM modem. After identifying the port number open hyperterminal. You need to download Hyperterminal or copy the files from windows XP. You can also use putty for this purpose.  Type a name and click OK. Then select the com port you are using and press ok. Change the boadrate to 9600(you can choose other also. Higher boadrates may not work with some GSM modem) and let other options remain unchanged. After that type AT and press Enter. If you get a response showing OK. Then everything is fine. As a default typed character are not shown in the terminal. You can enable this by goi

AT commands are used for communicating with a modem. AT is the abbreviation of ATtention commands. Every command start with AT. Some of the useful AT commands are listed below. You can call a number, attend a call, hang a call, send sms, read sms, and the list goes on with the features of a particular provider. Some commands are specific to some models. GSM modems can be bought and can be integrated to Micro-controllers or directly to PC. Following list may be helpful to hobbyist or others working on GSM modems for project works. AT The response will be OK. This means that the modem is up and ready. ATD94xxxxxxxx This command will initiate a voice call to the no 94xxxxxxxx. The semicolon indicates that it is a voice call. If you want to run any USSD codes like *123# for balance enquirey, do the same without semicolon. ATD*123# Other Call control commands Command Description ATA Answer command ATD Dial command ATH Hang up call   Sending an SMS.