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

[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.





> 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.

Of course, please feel free to ignore, if the last query is out of the
scope of this mailing-list :)





> Best regards,
> Pascal.
>
>
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Thanks and Regards,
Ajay

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