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Re: [Discuss-gnuradio] software implementation of GSM

From: Eng. Firas
Subject: Re: [Discuss-gnuradio] software implementation of GSM
Date: Wed, 6 Jun 2007 12:38:30 -0700 (PDT)

Hi Joshua,

I followed installation instruction with almost no errors. But, when running
Wireshark, I get nothing displayed with the GSM interface. My BTS signal is
very high, and I located the offset of the frequency correction burst (which
is by the way almost the same as your default one). Any suggested checking
point ?


Joshua Lackey-2 wrote:
> gssm-v0.1
> Groupe Special (Software) Mobile
>  or
> The Global Software System for Mobile communications
> ---
> Okay, calling gssm "The Global Software System for Mobile
> communications" is a bit of a stretch as all it does is monitor GSM
> control channels.
> What this package does is use the USRP and various daughterboards to
> capture live data, GNU Radio and custom modules to demodulate and decode
> the GSM packets, and then Wireshark to display the data.
>       Get it here:            http://thre.at/gsm
>       Install instructions:   http://thre.at/gsm/index.html#install.
>       Talk about it here:     address@hidden
>       More here:              http://wiki.thc.org/gsm.
> ---
> This package monitors GSM base station control channels. It uses the
> USRP and various daughterboards to capture live data, GNU Radio and
> custom modules to demodulate and decode the GSM packets, and then
> Wireshark to display the data.
> This version of gssm decodes most of the control channels. The control
> channels contain the information necessary for a mobile to communicate
> with a base station. The control channels gssm currently decodes are:
>       FCCH    The frequency correction channel.
>       SCH     The synchronization channel.
>       BCCH    The broadcast control channel.
>       PCH     The paging channel. Downlink only, used to page mobiles.
>       AGCH    The access grant channel. Downlink only, used to
>               allocate an SDCCH or directly a TCH.
>       SACCH   Slow associated control channel.
>       SDCCH   Stand-alone dedicated control channel.
> gssm displays the decoded data using Wireshark. Not only does this give
> us a very nice graphical front end to examine the dissected packets, but
> Wireshark already has quite a bit of code to dissect GSM data.
> Unfortunately, the current implementation of Wireshark does not dissect
> packets unique to the wireless interface. Up to now, there was no reason
> to include code to dissect these packets. I include a patch for
> wireshark-0.99.5 which adds partial Um packet dissection capability
> and a new custom ethertype to interface with the USRP.
> While gssm has basic functionality now, it really is alpha-quality
> software and there are a number of enhancements which must be made
> before it becomes truly useful.
>       1. The Mueller and Muller clock recovery method doesn't always
>       handle the quarter-bits present in a GSM burst. A more reliable
>       method must be implemented. Until then, this software will
>       suffer from a large number of receive errors even with a high
>       signal-to-noise ratio.
>       2. Wireshark dissects most GSM packets except those specific to
>       the Um interface, the wireless interface between the mobile and
>       the BTS, the Base Transciever Station.
>               a. I've only implemented a small portion of the Um
>               interface. Much more work must be done to complete this.
>               b. Only the Bbis frame type is implemented. When packets
>               arrive in Wireshark which are "malformed" or with
>               strange protocol descriptors, it is because they were
>               sent using some other frame type.
>               c. The interface between gssm and Wireshark is extremely
>               hacky, to say the least. It would be nice to eventually
>               standardize a GNU Radio interface for Wireshark. I also
>               want to clean up my Um interface and submit that there
>               as well.
>       3. You need to find your local GSM tower by hand. Once you've
>       found it, you need to edit the python script and enter the
>       information by hand. It would be very nice if this information
>       were automatically generated.
>       4. The code is designed to support all frequency bands but I
>       haven't implemented anything but U.S. support.
>       5. This code is receive-only and currently can only monitor
>       tower to mobile transmissions.
>       6. Lots more.
> ---
> This code is being adopted by the GSM Scanner Project and any updates to
> this code will be found there. Questions and suggestions can certainly
> be sent to me, but they also should be directed to the mailing list --
> address@hidden Also, check out the wiki at
> http://wiki.thc.org/gsm.
> The current version of this code can be found here:
> http://thre.at/gsm/gsm-v0.1.tar.bz2. Updates and bug-fixes will be
> located at the GSM Scanner Project, http://wiki.thc.org.
> --
> Joshua Lackey, Ph.D.  (address@hidden)
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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