Re: Query regarding processing of "USSD String" message, arriving in "GSM 7 bit" coding

[Ajay Garg <ajaygargnsit () gmail com>]

On Tue, Nov 19, 2013 at 4:37 PM, Pascal Quantin <pascal.quantin () gmail com>wrote:

> 2013/11/19 Ajay Garg <ajaygargnsit () gmail com>
>
> > Hi Pascal.
> >
> > Thanks for the electrifyingly-quick reply :)
> >
> >
> >
> > On Tue, Nov 19, 2013 at 4:10 PM, Pascal Quantin <pascal.quantin () gmail com
> > > wrote:
> >
> > > Hi Ajay,
> > >
> > > 2013/11/19 Ajay Garg <ajaygargnsit () gmail com>
> > >
> > > > Hi all.
> > > >
> > > > We are capturing some GSM-MAP messages over SCTP, and doing some
> > > > diagnostic testing.
> > > >
> > > > In some packets, we receive a USSD string, encoded in GSM-7-bit format.
> > > > I will take a particular example, to illustrate the issue.
> > > >
> > > >
> > > > The string (GSM 7-bit encoded) is
> > > >
> > > > #################################################
> > > >
> > > > c2301b249dbb6439d7cc060219e5e53248186687dde332483a77c96aaeda0d640fb3d3e4343d0f82dd5e30d94b068bd15ca0e65b5e0691d3613648158bc554b1914b4195d100
> > > > #################################################
> > > >
> > > >
> > > >
> > > >
> > > > Wireshark shows the corresponding string as
> > > >
> > > > #################################################
> > > > Bal Rs.29.36  Free Balance Rs.25.57 validity 07/02/2014. More dial
> > > > *111*1#.
> > > > T24@
> > > > #################################################
> > > >
> > > >
> > > >
> > > >
> > > > Now, we wrote a 7-to-8-bit-ASCII-converter locally at our end, and we
> > > > get the decoded message as
> > > >
> > > > #################################################
> > > > Bal Rs.29.36  Free Balance Rs.25.57 validity 07/02/2014. More dial
> > > > *111*1#.
> > > > T24
> > > > #################################################
> > > >
> > > > As is seen, there is no "@" character at the end.
> > > >
> > > >
> > > >
> > > > On some more head-banging, we realised that our local decoding seems to
> > > > be in fact correct, and there seems to be something wrong with the decoded
> > > > Wireshark message.
> > > > The last byte of the message is "00", and there is no way it can be
> > > > decoded to a "@".
> > > >
> > > > In fact, if we replace the last "00" with "80", we get the "@" at the
> > > > end (at our local decoding setup, that is).
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > We will be really grateful, if we could get some light on the "@"
> > > > appearing at the end in the Wireshark's decoded USSD-string.
> > > > We will be happy to share more information about our local
> > > > decoding-code, but I guess we are missing something obvious :P
> > > >
> > > >
> > > > Looking forward to a reply :)
> > > >
> > > >
> > > >
> > > > Thanks and Regards,
> > > > Ajay
> > >
> > > '@' is usually displayed due to the padding bits at the end of the
> > > buffer. I already fixed such bugs in the past in SMS decoding.
> >
> > Ahhh.. ok.
> >
> >
> >
> >
> > > Could you indicate us with Wireshark release you tested, whether it
> > > happens with 1.10.3?
> >
> > We tested with 1.6.7, on an Ubuntu 12.04-LTS.
>
> I don't remember backporting the fixes in 1.6 branch. Moreover I just
> looked at the GSM MAP dissector and the bug is probably still present. To
> be verified.
>
> > >  Moreover if you can share a pcap file, I will happily have a look at
> > > it.
> >
> > I don't think that is required now; your reply suffices, and is
> > absolutely to-the-point :)
> >
> >
> >
> > Just one last query, is there a particular reason why someone would send
> > padding-bits ?
> > I ask this, because we get the "padding bits" in only some USSD-strings,
> > and not all USSD-strings.
>
> Depending on the number of characters you have in the buffer, you will end
> up with unused bits to finish the current byte.

Hmm.. I am sorry I don't quite get this :-\

Of course, I understand that there might be unused bits in the last byte,
but in the present case, we are getting a full byte (8 bits) as a
padding-byte/padding-bits.

Anyhow, thanks a ton .... thanks a lot for the quick resolution to the
subject-of-this-email :)




> Pascal.
>
>
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Thanks and Regards,
Ajay

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