Send and Read SMS through a GSM Modem using AT Commands
By Syeda Anila Nusrat, 30 Aug 2010
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There are many different kinds of applications SMS applications in the market today, and many others are being developed. Applications in which SMS messaging can be utilized are virtually unlimited. Some common examples of these are given below: Person-to-person text messaging is the most commonly used SMS application, and it is what the SMS technology was originally designed for. Many content providers make use of SMS text messages to send information such as news, weather report, and financial data to their subscribers. SMS messages can carry binary data, and so SMS can be used as the transport medium of wireless downloads. Objects such as ringtones, wallpapers, pictures, and operator logos can be encoded in SMS messages. SMS is a very suitable technology for delivering alerts and notifications of important events. SMS messaging can be used as a marketing tool.
In general, there are two ways to send SMS messages from a computer / PC to a mobile phone: 1. Connect a mobile phone or GSM/GPRS modem to a computer / PC. Then use the computer / PC and AT commands to instruct the mobile phone or GSM/GPRS modem to send SMS messages. 2. Connect the computer / PC to the SMS center (SMSC) or SMS gateway of a wireless carrier or SMS service provider. Then send SMS messages using a protocol / interface supported by the SMSC or SMS gateway. In this article, I will explain the first way to send, read, and delete SMS using AT commands. But before starting, I would like to explain a little bit about AT commands. AT Commands
AT commands are instructions used to control a modem. AT is the abbreviation of ATtention. Every command line starts with "AT" or "at". That's why modem commands are called AT commands. There are two types of AT commands: 1. Basic commands are AT commands that do not start with a "+". For example, D (Dial), A (Answer), H (Hook control), and O (Return to online data state) are the basic commands. 2. Extended commands are AT commands that start with a "+". All GSM AT commands are extended commands. For example, +CMGS (Send SMS message), +CMGL (List SMS messages), and +CMGR (Read SMS messages) are extended commands. If you want to get more information about AT commands, then you can get it on my other article on CodeProject here:http://www.codeproject.com/KB/system/IntroductiontoATcommands.aspx. Operating Modes
The SMS specification has defined two modes in which a GSM/GPRS modem or mobile phone can operate. They are called SMS text mode and SMS PDU mode. (PDU stands for Protocol Data Unit.) The mode that a GSM/GPRS modem or mobile phone is operating in determines the syntax of some SMS AT commands and the format of the responses returned after execution. I am using SMS text mode in this article.
How to Test GSM Modem Connectivity Using Hyper Terminal
First, find the best GSM modem that suits the needs. I tested this application with a Wavecom FASTRACK M1206. Understand the AT Command set required to communicate with the modem. Connect the modem to the computer according to the setup guide specified in the manual provided with the GSM modem. Put a valid SIM card into the mobile phone or GSM/GPRS modem. Connect your mobile phone or GSM/GPRS modem to a computer, and set up the corresponding wireless modem driver. Run the MS HyperTerminal by selecting Start -> Programs -> Accessories -> Communications -> HyperTerminal. In the Connection Description dialog box, enter a name and choose an icon you like for the connection. Then click the OK button. In the Connect To dialog box, choose the COM port that your mobile phone or GSM/GPRS modem is connecting to in the Connect using combo box. For example, choose COM1 if your mobile phone or GSM/GPRS modem is connecting to the COM1 port. Then click the OK...
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