26.2.4. Syntax of the +CMGR AT Command in SMS PDU Mode

The syntax of the +CMGR AT command in SMS PDU mode is the same as that in SMS text mode:

+CMGR=index

In the above line, index is an integer specifying the location of the SMS message to be read from the message storage area.

26.2.5. Format of the Information Response of the +CMGR AT Command in SMS PDU Mode

If the GSM/GPRS modem or mobile phone reads the SMS message from message storage successfully, it will return an information response to the computer / PC. In SMS PDU mode, the information response of the +CMGR AT command has the following format: (Optional fields are enclosed in square brackets.)

+CMGR: message_status,[address_text],TPDU_length<CR><LF>SMSC_number_and_TPDU

Before we discuss each of the fields that appear in the +CMGR information response, let's see an example that gives you some idea of how an actual +CMGR information response should look like:

+CMGR: 0,,62
07915892000000F0040B915892214365F700007040213252242331493A283D0795C3F33C88FE06C9CB6132885EC6D341EDF27C1E3E97E7207B3A0C0A5241E377BB1D7693E72E

OK

The message_status Field

The first field of the information response of the +CMGR AT command, message_status, is an integer that indicates the status of the SMS message. It can be one of the following four values:

• 0. It refers to the message status "received unread".

• 1. It refers to the message status "received read".

• 2. It refers to the message status "stored unsent".

• 3. It refers to the message status "stored sent".

The address_text Field

The second field of the information response of the +CMGR AT command, address_text, is a string that contains the text associated to address in the phonebook, where address is the phone number encoded in the TPDU of the SMSC_number_and_TPDU field. For example, if the phone number encoded in the TPDU is "91234567" and the text "Alice" is associated to the phone number "91234567" in the phonebook, address_text will be "Alice". The AT command +CSCS (command name in text: Select TE Character Set) can be used to specify the character set for displaying address_text.

Note that address_text is an optional field. Some GSM/GPRS modems and mobile phones (examples: most Nokia products, including my Nokia 6021) leave this field empty.

The TPDU_length Field

The third field of the information response of the +CMGR AT command, TPDU_length, is an integer that indicates the length (in octets. 1 octet = 8 bits) of the TPDU contained in the SMSC_number_and_TPDU field. In the earlier example command line, the value of the SMSC_number_and_TPDU field is:

07915892000000F0040B915892214365F700007040213252242331493A283D0795C3F33C88FE06C9CB6132885EC6D341EDF27C1E3E97E7207B3A0C0A5241E377BB1D7693E72E

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

040B915892214365F700007040213252242331493A283D0795C3F33C88FE06C9CB6132885EC6D341EDF27C1E3E97E7207B3A0C0A5241E377BB1D7693E72E

The TPDU is coded in hexadecimal format. Each character represents 4 bits, i.e. 1/2 octet. The TPDU has 124 characters and so there are totally 62 octets. That's why the value of the TPDU_length field is 62.

The SMSC_number_and_TPDU Field

The fourth field of the information response of the +CMGR AT command, SMSC_number_and_TPDU, specifies the SMSC number and the TPDU (Transfer Protocol Data Unit) in hexadecimal format. If the SMS message to be read is an incoming SMS message, the TPDU will be of the type SMS-DELIVER. If the SMS message to be read is an outgoing SMS message, the TPDU will be of the type SMS-SUBMIT.

26.2.6. Some Explanation about the Decoding of the SMSC_number_and_TPDU Field Value of the +CMGR AT Command

This section provides some explanation about the decoding of the SMSC_number_and_TPDU field value of the +CMGR AT command so that you can extract the information you need from it. Let's consider the SMSC_number_and_TPDU field value in the earlier example command line:

07915892000000F0040B915892214365F700007040213252242331493A283D0795C3F33C88FE06C9CB6132885EC6D341EDF27C1E3E97E7207B3A0C0A5241E377BB1D7693E72E

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

07915892000000F0

040B915892214365F700007040213252242331493A283D0795C3F33C88FE06C9CB6132885EC6D341EDF27C1E3E97E7207B3A0C0A5241E377BB1D7693E72E

26.2.6.1. Incoming SMS Messages and Outgoing SMS Messages

The SMS message discussed above is an incoming SMS message. The first part indicates which SMSC the SMS message was received from, and the second part is a TPDU of the type SMS-DELIVER. If the SMS message retrieved by the +CMGR AT command is an outgoing SMS message, the first part indicates which SMSC will be used to send the SMS message, and the second part is a TPDU of the type SMS-SUBMIT, whose format is different from that of an SMS-DELIVER TPDU.

The format of the SMSC_number_and_TPDU field value of an outgoing SMS message is the same as that mentioned in the earlier section "Some Explanation about the Coding of the SMSC_number_and_TPDU Parameter Value of the +CMGS AT Command". Thus, we will not discuss about it again here. In the following sections, we assume the SMS message retrieved by the +CMGR AT command is an incoming SMS message and the TPDU is of the type SMS-DELIVER.

26.2.6.2. 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, i.e. 1/2 octet. 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 from which the SMS message was received is "+85290000000". If the value of the second sub-field is 0x81, the SMSC number assigned to the third sub-field may be "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 from which the SMS message was received 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 from which the SMS message was received. 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.

26.2.6.3. The TPDU Part (SMS-DELIVER TPDU)

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

04 0B 91 5892214365F7 00 00 70402132522423 31 493A283D0795C3F33C88FE06C9CB6132885EC6D341EDF27C1E3E97E7207B3A0C0A5241E377BB1D7693E72E

The First Sub-field: First Octet of the TPDU

The first sub-field is the first octet of the TPDU. It tells us several things:

• the type of the TPDU

• whether the SMSC has messages that are waiting to be sent to us

• whether a reply path exists

• whether a user data header exists in the TPDU

• whether a status report is requested by the sender of the SMS message

The value 0x04 means:

• the type of the TPDU is SMS-DELIVER

• the SMSC has no message that is waiting to be sent to us

• no reply path exists

• no user data header exists in the TPDU

• no status report is requested by the sender of the SMS message

The Second Sub-field: Length of the Sender Phone Number

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

The Third Sub-field: Type of the Sender Phone Number

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

• 0x81. It means the sender phone number is formatted using the typical ISDN / telephony numbering plan (ITU E.164/E.163) but it is not sure whether the sender phone number is an international number, a national number or a number of other types. For example, suppose there is an incoming SMS message that was originated from the phone number "+85291234567". If the value of the third sub-field is 0x81, the sender phone number assigned to the fourth sub-field may be "85291234567" (country code included) or "91234567" (country code omitted).

• 0x91. It means the sender 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 there is an incoming SMS message that was originated from the phone number "+85291234567". If the value of the third sub-field is 0x91, the sender phone number assigned to the fourth sub-field should be "85291234567".

The Fourth Sub-field: Sender Phone Number

The fourth sub-field specifies the sender 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 Fifth Sub-field: Protocol Identifier

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

The Sixth Sub-field: Data Coding Scheme

The sixth sub-field specifies the data coding scheme. The value 0x00 indicates that the text in the SMS message body is encoded according to the "GSM 7-bit default alphabet" text coding scheme.

The Seventh Sub-field: Service Center Time Stamp

The seventh sub-field specifies the time and data at which the SMS message arrived at the service center (i.e. SMSC). The value 0x70402132522423 represents 12 April 2007, 23:25:42 GMT+8 hours. Here's how the value 0x70402132522423 is decoded:

• Starting from the left, group the digits of the value 0x70402132522423 into pairs, like this: 70 40 21 32 52 24 23.

• Swap the digits in each pair and you will get 07 04 12 23 25 42 32. Starting from the left, the values represent year, month, day, hour, minute, second and time zone respectively. Note that the unit of time zone is a quarter of an hour.

The decoding of the time zone field requires a bit more explanation. The binary representation of the hexadecimal value 0x32 is 00110010. It can be divided into three parts, like this:

0 011 0010

The first part represents the plus-minus sign. If the value of the first part is 0, the sign is "+". If it is 1, the sign is "-". The second part represents the most significant digit of the time zone. "011" is equal to 3 in decimal. The third part represents the least significant digit of the time zone. "0010" is equal to 2 in decimal. Overall, "00110010" represents GMT+32 quarters of an hour = GMT+8 hours.

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 0x31 means there are 49 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 below:

0x493A283D0795C3F33C88FE06C9CB6132885EC6D341EDF27C1E3E97E7207B3A0C0A5241E377BB1D7693E72E

represents the text message "It is easy to read text messages via AT commands.". Below shows how to convert the hexadecimal value to text:

• Group the digits of the hexadecimal value into pairs, as shown below. Each pair represents one octet of data.

  49 3A 28 3D 07 95 C3 F3 3C 88 FE 06 C9 CB 61 32 88 5E C6 D3 41 ED F2 7C 1E 3E 97 E7 20 7B 3A 0C 0A 52 41 E3 77 BB 1D 76 93 E7 2E

• The binary representation of the first octet is 01001001. With the character set "GSM 7-bit default alphabet", each character is represented by 7 bits. The seven least significant bits, 1001001, represent the first character. The hexadecimal value of 1001001 is 0x49, which represents the character "I" according to the character set "GSM 7-bit default alphabet". To find out which character is represented by a certain hexadecimal code, see "Appendix: GSM 7-bit Default Alphabet Table (with Character Codes of ISO 8859 Latin 1)" of this SMS tutorial.

• The binary representation of the second octet is 00111010. Now move the most significant bit of the first octet to the rightmost position of the second octet. The result is 001110100. The seven least significant bits, 1110100, represent the second character. The hexadecimal value of 1110100 is 0x74, which represents the character "t" according to the character set "GSM 7-bit default alphabet".

• Similarly, move the two unused bits at the leftmost position of the second octet to the rightmost position of the third octet. The binary representation of the third octet is 00101000 and so the result is 0010100000. The seven least significant bits, 0100000, represent the third character. The hexadecimal value of 0100000 is 0x20, which represents the space character according to the character set "GSM 7-bit default alphabet".

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

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