Data over normal GSM call

Harald Welte laforge at
Wed Apr 16 16:48:22 CEST 2008

On Tue, Apr 15, 2008 at 11:17:19AM +0200, Stefano Cavallari wrote:
> On Monday 14 April 2008 21:00:34 Harald Welte wrote:
> > On Sun, Apr 13, 2008 at 07:57:02PM -0500, Hans L wrote:
> [...]
> > AFAIR, the DTMF key press is encoded and send over the Um (air)
> > interface to the BTS/BSC/MSC and then in the end converted into
> > actual audible sounds.
> >
> > Please see Page 22 of for a
> > graphical illustration how this works.
> This imply that there is a data channel available. Can you trasmit anything 
> else that DTMF? What is the bandwidth of this channel?

no, there is no data channel.  The START/STOP DTMF messages are part of
the layer 3 signalling messages (sent over a signalling channel that is
also used for call setup/teardown).

also, that signalling channel is between the MSC and the telephone.

- Harald Welte <laforge at>         
Software for the world's first truly open Free Software mobile phone
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