Send and Read SMS through a GSM Modem using AT Commands

Send and Read SMS through a GSM Modem using AT Commands

Introduction

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..

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 button.
• The Properties dialog box comes out. Enter the correct port settings for your mobile phone or GSM/GPRS modem. Then click the OK button.
• To find the correct port settings that should be used with your mobile phone or GSM/GPRS modem, consult the manual of your mobile phone or GSM/GPRS modem.
• Type "AT" in the main window. A response "OK" should be returned from the mobile phone or GSM/GPRS modem.
• If “OK” returns, it means your mobile phone or GSM/GPRS modem is connected successfully.

After successful connection of the GSM /GPRS modem with PC, you are ready to run this application. Download the attached project and run the application.

Sending SMS through GSM Modem using AT Commands

Port Settings

In this tab, you will have to do port settings which will be the same as you did in the hyper terminal and then click the OK button. If the modem is connected successfully, a message box will appear with the message “Modem is connected”.

public SerialPort OpenPort(string p_strPortName,

       int p_uBaudRate, int p_uDataBits,

       int p_uReadTimeout, int p_uWriteTimeout)

{

    receiveNow = new AutoResetEvent(false);

    SerialPort port = new SerialPort();

    try

    {

        port.PortName = p_strPortName;                 //COM1

        port.BaudRate = p_uBaudRate;                   //9600

        port.DataBits = p_uDataBits;                   //8

        port.StopBits = StopBits.One;                  //1

        port.Parity = Parity.None;                     //None

        port.ReadTimeout = p_uReadTimeout;             //300

        port.WriteTimeout = p_uWriteTimeout;           //300

        port.Encoding = Encoding.GetEncoding("iso-8859-1");

        port.DataReceived += new SerialDataReceivedEventHandler

                (port_DataReceived);

        port.Open();

        port.DtrEnable = true;

        port.RtsEnable = true;

    }

    catch (Exception ex)

    {

        throw ex;

    }

    return port;

}

Send SMS

In the second tab, you can send SMS:

public bool sendMsg(SerialPort port, string PhoneNo, string Message)

{

    bool isSend = false;

    try

    {

        string recievedData = ExecCommand(port,"AT", 300, "No phone connected");

        recievedData = ExecCommand(port,"AT+CMGF=1", 300,

            "Failed to set message format.");

        String command = "AT+CMGS=\"" + PhoneNo + "\"";

        recievedData = ExecCommand(port,command, 300,

            "Failed to accept phoneNo");

        command = Message + char.ConvertFromUtf32(26) + "\r";

        recievedData = ExecCommand(port,command, 3000,

            "Failed to send message"); //3 seconds

        if (recievedData.EndsWith("\r\nOK\r\n"))

        {

            isSend = true;

        }

        else if (recievedData.Contains("ERROR"))

        {

            isSend = false;

        }

        return isSend;

    }

    catch (Exception ex)

    {

        throw new Exception(ex.Message);

    }

}

Read SMS

In the third tab, you can read SMS:

public ShortMessageCollection ReadSMS(SerialPort port)

{

    // Set up the phone and read the messages

    ShortMessageCollection messages = null;

    try

    {

        #region Execute Command

        // Check connection

        ExecCommand(port,"AT", 300, "No phone connected");

        // Use message format "Text mode"

        ExecCommand(port,"AT+CMGF=1", 300, "Failed to set message format.");

        // Use character set "PCCP437"

        ExecCommand(port,"AT+CSCS=\"PCCP437\"", 300,

        "Failed to set character set.");

        // Select SIM storage

        ExecCommand(port,"AT+CPMS=\"SM\"", 300,

        "Failed to select message storage.");

        // Read the messages

        string input = ExecCommand(port,"AT+CMGL=\"ALL\"", 5000,

            "Failed to read the messages.");

        #endregion

        #region Parse messages

        messages = ParseMessages(input);

        #endregion

    }

    catch (Exception ex)

    {

        throw new Exception(ex.Message);

    }

    if (messages != null)

        return messages;

    else

        return null;

}

Delete SMS

In the fourth and last tab, you can count the number of SMS and delete SMS as well.

public bool DeleteMsg(SerialPort port , string p_strCommand)

{

    bool isDeleted = false;

    try

    {

        #region Execute Command

        string recievedData = ExecCommand(port,"AT", 300, "No phone connected");

        recievedData = ExecCommand(port,"AT+CMGF=1", 300,

            "Failed to set message format.");

        String command = p_strCommand;

        recievedData = ExecCommand(port,command, 300, "Failed to delete message");

        #endregion

        if (recievedData.EndsWith("\r\nOK\r\n"))

        {

            isDeleted = true;

        }

        if (recievedData.Contains("ERROR"))

        {

            isDeleted = false;

        }

        return isDeleted;

    }

    catch (Exception ex)

    {

        throw new Exception(ex.Message);

    }

}

Thanks & Regard
Sham :)