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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.
0000000541e190f804
00 = PID Protocol ID
00 = DCS Data Coding Scheme
00 = VP Validity Period
05 = UDL User Data Length
41E190F804 = The user data ‘ABCDE’.
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
C
D
E
In ASCII hexadecimal is:
0x41 0x42 0x43 0x44 0x45
In 8 bit binary is:
01000001 01000010 01000011 01000100 01000101
We truncate off the most significant bit, resulting in a series of seven bit values. The process is to
pack the now unused most significant bit(s) with enough least significant bits from the following
value to fill it up to eight bits.
‘A’
0x41
‘B’
0x42
‘C’
0x43
‘D’
0x44
‘E’
0x45
1000001
1000010
1000011
1000100
1000101
01000001
11100001
10010000
01011000
00000100
This results in the string of eight-bit hexadecimal numbers 0x41, 0xE1, 0x90, 0x58, 0x04. By
continuing this process a string of 160 ASCII characters can be stored as a string of 140 bytes
(octets).
The Service Center Address is described in ETSI GSM 04.11.
The Transport Protocol Data Unit is described in ETS GSM 03.40.
Note:
Sending and Receiving SMS messages interfere with each other, when the time in between is
too short, or when both operations are taking place at the same time.
g18 Modem Integration and Application Developers Guide version 2.0 Page 65