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 <[EMAIL PROTECTED]> http://openmoko.org/ ============================================================================ Software for the world's first truly open Free Software mobile phone _______________________________________________ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
