25.5. Syntax of the +CMGS AT Command in SMS PDU Mode

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In SMS PDU mode, the syntax of the +CMGS AT command is:

+CMGS=TPDU_length<CR>SMSC_number_and_TPDU<Ctrl+z>

Before we discuss each of the parameters, let's see an example that gives you some idea of how an actual command line should look like:

AT+CMGS=42<CR>07915892000000F001000B915892214365F7000021493A283D0795C3F33C88FE06CDCB6E32885EC6D341EDF27C1E3E97E72E<Ctrl+z>

The TPDU_length Parameter

The first parameter of the +CMGS AT command, TPDU_length, specifies the length (in octets. 1 octet = 8 bits) of the TPDU (Transfer Protocol Data Unit) assigned to the SMSC_number_and_TPDU parameter. In the earlier example command line, the value assigned to the SMSC_number_and_TPDU parameter is:

07915892000000F001000B915892214365F7000021493A283D0795C3F33C88FE06CDCB6E32885EC6D341EDF27C1E3E97E72E

It can be divided into two parts. The following part is the TPDU:

01000B915892214365F7000021493A283D0795C3F33C88FE06CDCB6E32885EC6D341EDF27C1E3E97E72E

The TPDU is coded in hexadecimal format. Each character represents 4 bits, i.e. 1/2 octet. The TPDU has 84 characters and so there are totally 42 octets. That's why the value assigned to the TPDU_length parameter is 42.

The <CR> Character

<CR>, which represents the carriage return character, follows the TPDU_length parameter. When the GSM/GPRS modem or mobile phone receives the carriage return character, it will send back a prompt formed by these four characters: the carriage return character, the linefeed character, the ">" character and the space character. If you don't understand what this means, don't worry. This should be clear to you when you see the example in the section "Example Demonstrating How to Use the +CMGS AT Command to Send SMS Text Messages in SMS PDU Mode".

The SMSC_number_and_TPDU Parameter

The second parameter of the +CMGS AT command, SMSC_number_and_TPDU, specifies the SMSC number and the TPDU in hexadecimal format. Entering the <Esc> character will cancel the +CMGS AT command. If you don't understand what this means, see the example in the section "Example Demonstrating How to Use the +CMGS AT Command to Send SMS Text Messages in SMS PDU Mode".

The <Ctrl+z> Character

When you finish entering the value for the SMSC_number_and_TPDU parameter, you have to enter the <Ctrl+z> character to mark the end of the value. The GSM/GPRS modem or mobile phone will then attempt to send the SMS message to the SMS center.

25.5.1. Some Explanation about the Coding of the SMSC_number_and_TPDU Parameter Value of the +CMGS AT Command

This section provides some explanation about the coding of the SMSC_number_and_TPDU parameter value of the +CMGS AT command. Let's consider the SMSC_number_and_TPDU parameter value in the earlier example command line:

07915892000000F001000B915892214365F7000021493A283D0795C3F33C88FE06CDCB6E32885EC6D341EDF27C1E3E97E72E

The above value can be divided into two parts, as shown below. The first part contains information about the SMSC to be used for sending the SMS message. The second part is the TPDU.

07915892000000F0

01000B915892214365F7000021493A283D0795C3F33C88FE06CDCB6E32885EC6D341EDF27C1E3E97E72E

25.5.1.1. The SMSC Part

The SMSC part can be further divided into three sub-fields, like this:

07 91 5892000000F0

The First Sub-field: Length of the Second and Third Sub-fields

The first sub-field specifies the length in octets of the following two sub-fields. There are 14 hexadecimal digits in "915892000000F0" and each hexadecimal digit represents 4 bits. So, there are totally 7 octets. That's why the value of the first sub-field is 0x07.

The Second Sub-field: Type of SMSC Number

The second sub-field specifies the type of the SMSC number assigned to the third sub-field. Two values are commonly used. They are 0x81 (129 in decimal) and 0x91 (145 in decimal):

• 0x81. It means the SMSC number is formatted using the typical ISDN / telephony numbering plan (ITU E.164/E.163) but it is not sure whether the SMSC number is an international number, a national number or a number of other types. For example, suppose the SMSC number provided to you by the mobile network operator for sending SMS messages is "+85290000000". If the value of the second sub-field is 0x81, the SMSC number assigned to the third sub-field can be either "85290000000" (country code included) or "90000000" (country code omitted).

• 0x91. It means the SMSC number is formatted using the typical ISDN / telephony numbering plan (ITU E.164/E.163) and it is an international number. For example, suppose the SMSC number provided to you by the mobile network operator for sending SMS messages is "+85290000000". If the value of the second sub-field is 0x91, the SMSC number assigned to the third sub-field should be "85290000000".

The Third Sub-field: SMSC Number

The third sub-field specifies the SMSC number for sending the SMS message. 0x5892000000F0 represents the phone number +85290000000. Here's how the value 0x5892000000F0 is obtained:

1. Starting from the left, group the digits of the phone number 85290000000 into pairs, like this: 85 29 00 00 00 0.

2. As the last group has only one digit, we add an "F" to make up a pair. The result is 85 29 00 00 00 0F.

3. Swap the digits in each pair and you will get 58 92 00 00 00 F0.

Note

It is possible to tell the GSM/GPRS modem or mobile phone to use the SMSC number specified by the AT command +CSCA (command name in text: Service Centre Address) for sending SMS messages. Just assign the value 0x00 to the first sub-field and omit the second and third sub-fields, i.e. the SMSC part becomes:

00

25.5.1.2. The TPDU Part (SMS-SUBMIT TPDU)

The TPDU part can be divided into nine sub-fields, as shown below. This is a TPDU of the type SMS-SUBMIT.

01 00 0B 91 5892214365F7 00 00 21 493A283D0795C3F33C88FE06CDCB6E32885EC6D341EDF27C1E3E97E72E

The First Sub-field: First Octet of the TPDU

The first sub-field is the first octet of the TPDU. It tells the GSM/GPRS modem or mobile phone several things:

• the type of the TPDU

• whether the SMSC should reject duplicated TPDUs

• whether a validity period exists in the TPDU, and the format of the validity period if it exists

• whether a reply path is requested

• whether a user data header exists in the TPDU

• whether a status report is requested from the SMSC

The value 0x01 means:

• the type of the TPDU is SMS-SUBMIT

• the SMSC should not reject duplicated TPDUs

• no validity period exists in the TPDU

• no reply path is requested

• no user data header exists in the TPDU

• no status report is requested from the SMSC

The Second Sub-field: Message Reference Number

The second sub-field specifies the message reference number. It is an integer in the range from 0 to 255. The value 0x00 tells the GSM/GPRS modem or mobile phone to assign a message reference number to the SMS message automatically.

The Third Sub-field: Length of the Destination Phone Number

The third sub-field specifies the length in digits of the destination phone number. The destination phone number specified in the fifth sub-field is "+85291234567", which has 11 digits. Thus, the value of the third sub-field is 0x0B (i.e. 11 in decimal).

The Fourth Sub-field: Type of the Destination Phone Number

The fourth sub-field specifies the type of the destination phone number assigned to the fifth sub-field. Two values are commonly used. They are 0x81 (129 in decimal) and 0x91 (145 in decimal):

• 0x81. It means the destination phone number is formatted using the typical ISDN / telephony numbering plan (ITU E.164/E.163) but it is not sure whether the destination phone number is an international number, a national number or a number of other types. For example, suppose you want to send an SMS message to the phone number "+85291234567". If the value of the fourth sub-field is 0x81, the destination phone number assigned to the fifth sub-field can be either "85291234567" (country code included) or "91234567" (country code omitted).

• 0x91. It means the destination phone number is formatted using the typical ISDN / telephony numbering plan (ITU E.164/E.163) and it is an international number. For example, suppose you want to send an SMS message to the phone number "+85291234567". If the value of the fourth sub-field is 0x91, the destination phone number assigned to the fifth sub-field should be "85291234567".

The Fifth Sub-field: Destination Phone Number

The fifth sub-field specifies the destination phone number. The value 0x5892214365F7 represents the phone number +85291234567. Here's how the value 0x5892214365F7 is obtained:

• Starting from the left, group the digits of the phone number 85291234567 into pairs, like this: 85 29 12 34 56 7.

• As the last group has only one digit, we add an "F" to make up a pair. The result is 85 29 12 34 56 7F.

• Swap the digits in each pair and you will get 58 92 21 43 65 F7.

The Sixth Sub-field: Protocol Identifier

The sixth sub-field specifies the protocol identifier. Its value should be 0x00 for normal cases.

The Seventh Sub-field: Data Coding Scheme

The seventh sub-field specifies the data coding scheme. With 0x00, we inform the GSM/GPRS modem or mobile phone that the text in the SMS message body is encoded according to the "GSM 7-bit default alphabet" text coding scheme.

The Eighth Sub-field: Length of the SMS Message Body

The eighth sub-field specifies the length of the SMS message body in septets (1 septet = 7 bits). The value 0x21 means there are 33 septets (or characters, since each character is represented by 7 bits according to the "GSM 7-bit default alphabet" text coding scheme) in the SMS message body.

The Ninth Sub-field: SMS Message Body

The ninth sub-field specifies the SMS message body. The value 0x493A283D0795C3F33C88FE06CDCB6E32885EC6D341EDF27C1E3E97E72E represents the text message "It is easy to send text messages.". Below shows how the hexadecimal value is obtained:

• Encode the SMS text message "It is easy to send text messages." according to the character set "GSM 7-bit default alphabet". For example, 0x49 is used to represent the first character "I" in the SMS text message, 0x74 is used to represent the second character "t", etc. To find the code representing a certain character, see "Appendix: GSM 7-bit Default Alphabet Table (with Character Codes of ISO 8859 Latin 1)" of this SMS tutorial.

  After encoding, the SMS text message "It is easy to send text messages." becomes:

  49 74 20 69 73 20 65 61 73 79 20 74 6F 20 73 65 6E 64 20 74 65 78 74 20 6D 65 73 73 61 67 65 73 2E

• With the character set "GSM 7-bit default alphabet", each character is represented by 7 bits. If the SMS text message is encoded as above, 1 bit is wasted in every octet. To prevent this from happening, the SMS specification requires us to pack the 7-bit characters in octets. The packing process is described below:

  • The binary representation of the first character is 1001001. Since the most significant bit of the first octet is unused, we fill it by the least significant bit of the second character, which is 0. (The binary representation of the second character is 1110100.) As a result, the first octet contains the binary value 01001001, which is 0x49 in hexadecimal format.

  • As the least significant bit of the second character has been moved to the first octet, the second octet now contains the binary value 111010 and the two bits at the leftmost position of the second octet is unused. We fill them by the two least significant bits of the third character, which are both 0. (The binary representation of the third character is 0100000.) As a result, the second octet contains 00111010, which is 0x3A in hexadecimal format.

  • Similarly, since the two least significant bits of the third character have been moved to the second octet, the third octet now contains the binary value 01000 and the three bits at the leftmost position of the third octet is unused. So, we fill them by the three least significant bits of the fourth character, which are 0, 0 and 1. (The binary representation of the fourth character is 1101001.) Now the third octet contains the binary value 00101000, which is 0x28 in hexadecimal format.

  • The packing process continues. This is illustrated in the following table.

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