PDU GSM 7bit Encoding

Question

working on C# project and Huawei E1550 modem to send USSD codes using +CUSD command

after a lot of research and try and error i discovered that i should send the command encoded using "GSM 7bit"

i found online converter that do that : http://smstools3.kekekasvi.com/topic.php?id=288

so i search to find class/algorithm to implement it using c# and i find this : https://sites.google.com/site/freesmsuk/gsm7-encoding

the problem is if i encode one character only it encode it correctly "1" --> "31"

but when i encode string "*888#" the online converter generates "2A1C0E3702" while the class generates "2A38383823"

and the modem processed the online encoding not the class

what's the wrong with the algorithm? thanks in advance

Terence Eden Terence Eden · Accepted Answer · 2012-07-23T08:55:24

If I try to convert it, I get "2A1C0E3702"

Hex         2A          1C          0E          37          02      +++++++
Octets      00101010    00011100    00001110    00110111    00000010     
Septets      0101010     0111000     0111000     0111000     0100011    +++++++
Character    *           8           8           8           #

This is the seven bit alphabet. The spacing is purely for readability.

SEVEN_BIT_ALPHABET_ARRAY = (
'@', '£', '$', '¥', 'è', 'é', 'ù', 'ì', 'ò', 'Ç', '\n', 'Ø', 'ø', '\r','Å', 'å',
'\u0394', '_', '\u03a6', '\u0393', '\u039b', '\u03a9', '\u03a0','\u03a8', '\u03a3', '\u0398', '\u039e',
'€', 'Æ', 'æ', 'ß', 'É', ' ', '!', '"', '#', '¤', '%', '&', '\'', '(', ')','*', '+', ',', '-', '.', '/',
'0', '1', '2', '3', '4', '5', '6', '7','8', '9',
':', ';', '<', '=', '>', '?', '¡',
'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
'Ä',                                                                  'Ö',
                                                                 'Ñ',                               'Ü', '§', 
                         '¿',
'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
'ä',                                                                  'ö',
                                                                 'ñ',                               'ü',
'à')

I used the calculator at http://rednaxela.net/pdu.php to convert these.

It looks like it might be a character encoding issue. Make sure the characters you're converting are ASCII, not unicode.

Finally, are you sure you need to PDU encode your USSD? Most modems will work with

AT+CUSD=1,"*888#",15

PDU GSM 7bit Encoding

1 Answers