Data over normal GSM call

thomasg thomas at
Sat Apr 12 12:56:31 CEST 2008

That's not all, there are even more factors:
Mic -> A/D converter -> Codec (depends on net and quality: up to 5 different
codecs possible) -> Radio -> Air-Interface -> Radio ->Codec -> Voice Quality
Enhancer (VQE - who knows what this will do with your data, maybe just
dropping it at all) -> and the same way backwards, with different codecs.
And most important: there is no TCP or similar - you just can't know what
the other side will get.

On Fri, Apr 11, 2008 at 9:37 PM, Ian Stirling <OpenMoko at>

> Diego Fdez. Durán wrote:
> <snip>
> >
> > Can't you initiate a voice call between to FreeRunners and then use the
> > mic and mixer devs to modulate the data as sound?
> >
> Broadly yes.
> The problem is:
> A human making mobile - mobile call looks something like this.
> 1 microphone
> 2 analog-digital converter
> 3 GSM encoder
> 4 radio transmission
> 5 Add errors due to sunspots
> 6 radio reception
> 7 GSM decoder
> 8 ISDN-like 8KHz 8 bit sampling over mobile companies network
> 9 Public switched telephone network
> And then back again in reverse - with all the codec senses flipped.
> Any analog modem signal has to get through the GSM codec - twice - and
> survive bit errors. GSM is designed so that single bit errors make
> audibly similar output - but not similar numerically.
> The GSM codec is basically designed to throw away anything that is not
> voice-like.
> If you have a perfect radio channel, and can get at the digital data
> that would normally go to/from the codec (step 3) it won't help much for
> 'normal' modems - as you still have the GSM encode/decode cycle at the
> network side.
> In reality, what you have to do to push any sort of data through this
> link is to basically have a data-driven vocoder at one side, that
> gabbles - but makes sounds that could in principle be made by human
> throats, and a voice recogniser at the other end. This can get
> 1300bits/sec.
> CSD - 'data' GSM calls are different.
> Instead of using the normal GSM codec, they use a special codec that is
> designed for error-free data transmission.
> This can be broken out and transmitted over the normal phone network and
> end up in a device like an ISDN modem.
> Some ISPs 'normal' modems actually support this by default, so dialing
> just works.
> However, though this would be really nice to use - it's not free, and in
> many countries/telcos costs significant amounts to enable.
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