Intercepting Audio == Freedom from Service Providers

Bryan Fink bryan.fink at
Sat Jan 13 15:25:42 CET 2007

On 1/13/07, Andreas Kostyrka <andreas at> wrote:
> > If your phone can receive data over a voice channel, you have less of
> > a reason to pay your service provider for a separate "data package".
> > This would be especially nice for someone like me who only subcribes
> > to the "bare bones" plan, but still has minutes left over at the end
> > of the month and has to pay $.50 for the five text messages received.
> > (Sure, only useful when communicating with other OpenMoko users, but
> > still a possibly useful feature.)
> The problem is, that sending raw data is probably not possible.

Ha!  Possible, schmossible.  With that attitude you'll fail before you begin.

> *) trying to send data via the audio channel is doomed on two counts:
> first you need a software modem, and these are not free software, plus
> you would need to take into account the fact that the GSM module does
> apply all kinds of lossy compression on the audio data.

A) Software modems can be written - that's the point of an open source
phone.  See B for one example.  I'm sure someone much more clever than
me could come up with something much better.

B) Even with lossy compression, there are still plenty of methods of
sending data.  I've mentioned one already - Morse code.  I've seen so
many term-long class projects implement Morse code encoder/decoders
over crappy transmission channels, it's not even funny.  Simply find a
carrier and transmission rate that the gsm module doesn't mangle "too
bad", and you're golden.

"But Morse code only sends text!" you claim?  Simple: uuencode your
raw data first (or use any other binary-to-text encoding, of course).
Throw some parity/hashes on the end to catch errors.

I never said it would be fast, but it *is* possible...

> The question still staying, how will one access the gsm audio streams?

...assuming that is possible, of course. ;)


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