Data over normal GSM call

joerg at joerg at
Tue Apr 8 21:01:59 CEST 2008

Am Di  8. April 2008 schrieb Tilman Baumann:
> Tilman Baumann wrote:
> > Maybe CSD is billed like a voice call. I never used it, but i don't 
> > remember any special charges for that.
> I dug deeper. Seems like this is true.
> But this raises the question if the Neo can do CSD. Probably it does, 
> since the GSM module does not appear to be somehow intentionally crippled.
> 9.6 kBit/s is no fun though...

Right, the Neo calypso modem speaks "AT" on the internal serial interface. I 
don't think there is any problem with the very much Hayes standard 
AT-commands for CSD. Nearly every phone could do this even 1999. IIRC there 
even is a 14400B/s mode (at least for networks/areas with support for 
EFR-codec), and I'm quite sure this isn't charged differently for 
mobile-originated calls.
For Mobile-Terminated calls however, you usually need a second special "data" 
number linked to your simcard. Otherwise the wouldn't know 
whether or not to set the data-flag for the connection, what means they send 
the POTS-modem's sound over GSM "as is", instead of recoding it to digital 
prior to sending it over the air. This might not apply for MT-ISDN-calls, 
with data-service flag set for the originating device.


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