Data over normal GSM call

Harald Welte laforge at
Mon Apr 14 21:00:34 CEST 2008

On Sun, Apr 13, 2008 at 07:57:02PM -0500, Hans L wrote:
> Although the transfer of audio data over cell phone audio is very
> lossy, there is at least one standard, proven way of transferring data
> by phone: DTMF(

please note though, that if you send DTMF over your mobile phone, the
DTMF tone is generated by the network, _NOT_ inside the phone.

Your phone is required to generate the tone, too.  But that tone is only
played through the ear speaker to the user, it is not sent over the
voice channel.

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.

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

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