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Re: Descepancy in 7-to-8 bit decoding
From: Pascal Quantin <pascal.quantin () gmail com>
Date: Sun, 12 Jan 2014 12:01:17 +0100

Hi Ajay,

the corresponding character in the bitstream is code point 64 that
corresponds to a '@' character in ASCII but that translates to character '
*¡*' in the GSM 7bit alphabet (see 3GPP 23.038 chapter 6.2.1). The '@'
character in GSM 7bit alphabet is code point 0.
So my understanding is that your decoder is wrong as it seems to use the
ASCII table instead of the GSM 7 bits one and that Wiresahrk output is
correct.

Best regards,
Pascal.


2014/1/12 Ajay Garg <ajaygargnsit () gmail com>

Hi all.

(This might actually serve as a follow-up to
http://www.wireshark.org/lists/wireshark-users/201311/msg00014.html, but
since the issue this time is a bit different, I preferred starting a new
thread, so as not to clutter up things).


We captured some USSD network traces, and found that the following *7-bit
encoded* message was being transferred over the wire :

*c2301b249dbb663617ce056abee56510391d668354b1584c151bb914532a111883c15fcdb43b0422be41d2612d070abbc9a0b25bfdce83a65422681866b3e720fa1b1474e741ce32fdfe96af41c0180cfe6abb40d6201324c391c3f9b90b44350d4141389c9d7701*



Wireshark shows the decoded-message to be


*Bal Rs.36.8. More dial *111*1#.STD¡10p/Min! Do RC59 and enjoy STD Calls
to Any Network ¡10p/m. VAL 28days. T&C Apply.*

while our 7-to-8 bit decoder shows this to be


*Bal Rs.36.8. More dial *111*1#.STD () 10p/Min! Do RC59 and enjoy STD Calls
to Any Network @10p/m. VAL 28days. T&C Apply.*

(Note the difference of "¡" and "@").


The wireshark rpm being used is wireshark-1.6.16-2.fc17.i686


Is this another case of using a special charcater ("*¡*" in the present
case), when some particular byte-stream is received?
Or am I being plain stupid?



We will be grateful for some pointers, that could help clear our doubts !!

Looking forward to a reply




Thanks and Regards,
Ajay

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