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[liberationtech] Efficient digital one-way communication

Michael Rogers michael at briarproject.org
Mon Mar 4 07:40:22 PST 2013


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On 03/03/13 14:25, Jens Christian Hillerup wrote:
> I basically just wanted to throw it out here. Does anybody have 
> experience in modulating data? Has this kind of digital one-way 
> communication been done in an activist setting before? Does it
> make sense to kick off a project aimed at creating a easily usable
> system capable of modulating and demodulating data at modest
> bitrates (>15KB/s)?

Hi Jens,

Last year I spent some time playing with audio encoding of data for
transmission over handheld radios. The state of the art here is dialup
modems - on a good day they can get 56,000 bits per second over a
channel designed for voice, but that requires advanced modulation and
error correction techniques. The radio hams have packet radio (AX.25
and APRS) running at 1,200-9,600 bps over long distances using simple
modulation and no error correction. Some early home computers used
audio cassettes for storage (300-1,200 bps, CUTS or Kansas City Standard).

If you want to support purely one-way communication (no acks), you'll
need to forward error correct the data. Hamming codes and parity
checks are simple to implement but they'll eat a lot of your
bandwidth; Reed-Solomon codes are more bandwidth-efficient but also
more complex.

Some Java code for modulation, framing and error correction is here if
you're interested:

http://briar.git.sf.net/git/gitweb.cgi?p=briar/sandpit;a=tree;f=src/net/sf/briar/sandpit/modem

http://briar.git.sf.net/git/gitweb.cgi?p=briar/sandpit;a=tree;f=src/net/sf/briar/sandpit/fec

Cheers,
Michael

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