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

Harald Welte laforge at
Tue Apr 15 07:33:23 CEST 2008

On Mon, Apr 14, 2008 at 10:15:06PM +0200, Adrian-Ken Rueegsegger wrote:
> Harald Welte wrote:
>> 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 :)
> Somebody already has. May I point you towards the paper "Real Time End  
> to End Secure Voice Communications over GSM Voice Channel" by N.N.  
> Katugampala, K.T. Al-Naimi, S. Villette, and A.M. Kondoz [1].

While stating it being unlikely, I have no dout that it can be done,
given enough effort.  But it requires a lot of work by somebody with
lots of knowledge and experience in signal processing, baseband digital
communications, modulation schemes, the intrinsic properties of the
various codecs involved.

Oh, and even if you do that, you still have to manage to implement it
all based on fixed-point math, make it run next to all the other things
on the ARM920T CPU core :)

Not to forget the testing on the various codecs that are in use.  GSM
networks are getting more diverse every year.  With some carriers now
operating their BTS's behind IP links (Abis-over-IP), I only can make
vague guesses what additional jitter, delay (and potential transcoding)
this might imply to the network.

All I wanted to point out is that it is unlikely that general-purpose
developers like most community members (including myself) can come up
with something that works in any reasonable time frame.

> [2]. Nevertheless highly interesting to read :)

Yes, thanks a lot.

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

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