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The following ASCII string is the ASCII representation of the hexadecimal values that will be
transmitted as a binary string. So, 'F''4' below means that the hex values 0xF and 0x4 will be
transmitted over the air. CMGS=19 is the length of the TPDU in octets, see below. Note that the
message is terminated with a Control-Z character (0x1A)
AT+CMGS=19
>07914483056100F511000B914410927856F40000000541E190F804<CTRL-Z>
07 = LENGTH OF SCA element including the TON/NPI
91 = TON/NPI,TON=9=Type of number, NPI=1=Numbering Plan Identifier
4483056100F5 SCA =SERVICE CENTER ADDRESS
I.E. the real phone number for the SCA is 44-385-016-005
•
Explanation of coding of phone numbers. One hex digit for each phone number digit will be
used. Each pair of hex digits is considered an octet (8 bit binary number). Then the two hex
digits are reversed in order to form a semi-octet. So, The phone digits 3,8 will become 0x3,
0x8, then 0x38, and then reversed to become 0x83. The ASCII characters '8' and '3', ie the
string '83' are then inserted into the full ASCII string above. Since there are an odd number
of digits, the last digit 5, is placed in the lower nibble of the last octet and then it top nibble is
filled with 1's. So '5' becomes 0x5, then 0x05, then 0xf5, and then 'f5', the last two
characters in the Service Center address.
Refer to ETSI 04.11 for more detail.
Looking at the rest of the message after the SCA we find the Transport Protocol Data Unit
(TPDU) which contains the destination device address (phone number) and the user data or
message. There are 38 characters, which represent 19 octets, which is the length of the TPDU.
11000B914410927856F40000000541E190F804 = TPDU
11 = 8 bits MTI,RD,VPF,SRR,UDHI,RP,MMS,SRI
00 = MR Message Reference
0B914410927856F4 = DA Transport Destination Address.
0B = Number of digits (11) in phone number
Note that the destination phone number is converted to an ASCII string in the same manner
as the SCA.
§
After the phone number is the user data control information and the user data.
The user data is encoded as a GSM characters.
•
Description of encoding the user data. The GSM character set is a method of encoding 7 bit
ASCII characters into 8 bit numbers. GSM only supports 128 characters with ASCII values
0x00 to 0x7f. Since these only need 7 bits to define them, and the SMS message is
transmitted as a series of 8-bit values, GSM uses a method of packing the series of 7 bit
values into 8 bit octets. To explain:
ABCDE is encoded as 41E190F804 as follows.
A
B
In ASCII hexadecimal is:
0x41 0x42 0x43 0x44 0x45
d15 Modem Integration and Application Developers Guide version draft 0.53
91=TON/NPI (as above)
4410927856F4 = Destination phone number 44-012-987-654.
0000000541e190f804
00 = PID Protocol ID
00 = DCS Data Coding Scheme
00 = VP Validity Period
05 = UDL User Data Length
41E190F804 = The user data 'ABCDE'.
C
D
E
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