Question

I’m working on a program in C# that can send a SMS from my computer using my GSM modem, and I got most of my program to work.

I can send most Hayes AT commands like: “AT” and “AT+CGMI;+CGMM”, which returns the message “OK”, so I know I can communicate with the modem.

However I DO have problems sending a standard 7-bit encoded message from the modem to my cell phone.

With the help of PDUspy I have a somewhat confident that I’m encoding my message right.

However the following code fails miserably:

public string SendEncodedSms(string reciever, string message)
{
  string response = GetResponse("AT+CMGF=0");
  if (isFine(response))
  {
    string encodedBody = "000100" + EncodedReceiver(reciever) + "0000";
    encodedBody += EncodeToSeptet(message);

    int cmgs_header = encodedBody.Length / 2;

    port.WriteLine("AT+CMGS=" + cmgs_header.ToString() + "\r\n");
    port.WriteLine(encodedBody + (char)26);

    return ReadResponse(300);
  }
  else throw new ApplicationException("Cant go into SMS PDU mode");
}

There five function calls inside:

• GetResponse() is function that works as a wrapper when communicating with the modem – always making sure each command ends with a “\r”, and returns OK or Error depending on the response from the modem.

• isFine() is a simple function that check whether or not the response from GetResponse() contained a “OK” message.

• ReadResponse() returns any response from the modem whether that is OK or any error messages and whatever there also might be embedded between the AT command and the status message.

• EncodeReceiver() encode the receivers phone number in reverse nibble notation. E.g. 12345678 becomes 21436587.

• EncodeToSeptet() Encodes the message from 8bit notation to 7bit notation.

All functions have been confirmed to work fine using PDU spy and responses sent to debug window.

Calling SendEncodedSms("<my phone number>", "test") returns:

AT+CMGS=17

0001000A91xxxxxxxxxx000004F4F29C0E

+CMS ERROR: 304

I replaced my encoded phone number for privacy issues… 😉

• Is there anybody who can give me a hint what I am failing at?

UPDATED:

Link hinted I was calculating cmgs_header wrong.

After a bit of cleaning code I got:

public string SendEncodedSms(string receiver, string message)
{
  if(isFine("AT+CMGF=0"))
  {
     string encodedBody = EncodedReceiver(receiver) + "0000";
     encodedBody += EncodeToSeptet(message);

     /* The +2 in calculation is a hack. Its probleby two of the octets 
      * in 000100 that should be a part of length calculation. But need 
      * to verify in against severeal sites.
      */
     int cmgs_header = (encodedBody.Length / 2) + 2;

     encodedBody = "000100" + encodedBody;

     // Rest is as before.

    ...

  }

  else ...
}

Answer 1

Maybe this helps you. Why don’t you send the SMS in text mode?