modulated data over GSM voice (was Re: Data over normal GSM call)

Harald Welte laforge at
Mon Apr 14 21:02:54 CEST 2008

Just to give you a "summary judgement":

Running any kind of voice-encoded data over a regular voice channel of a
GSM phone is _extremely_ unlikely to work.

There are a number of different codecs in use.  Which codec is
determined by the network.  There is echo cancellation at potentially
multiple locations during the call.  There might be one or multiple
transcoders of the voice codec along the road.

If you can manage to design a modulation and coding scheme that survives
all (or even most) of the stages above, I think you have achieved
something great.  I doubt you will get more than 300bps though :)

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

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