From: Evrim ULU (evrim@envy.com.tr)
Date: Sat Jun 29 2002 - 03:26:31 EDT
Greg wrote:
> Well an older FSK 300 baud job, no problem at all. Do it with just a sound
> card and a poor gain tap. But modern 56k QAM with all the jiggery-pokerey,
> no doubt possible but not very practical for those without unaccountable
> public funding.
>
Heh:-) I obviously do not have funding like $70K. The machine that does this job
was $70K or something.
> Now demodulating the RFI from the serial cable/controller/modem interface,
> if you're close enough would be a lot easier.
>
> sorry
The limit on a normal phone line is 64K. But when there is noise on the line
(this is the usual case in fact) one bit is dropped and result is simply 56K
(53.3K says modem-howto of linux).
But i'm insisting on this.heh:-)
PCM says that there are 256(8 bits) different signals at a sampling rate of 8000
per second. 56K Modems uses amplitude modulation. Although modems do lots of
tricks like crc checking/data compression, there must be a way to demodulate the
traffic since it's a simple analog one. One can setup a test line & two test
modems which are not doing any compression for simple analysis. I'm not very
experienced with ADC's (only used adc0832/04/08 etc. before) but using an opamp
or so, max 48Volts can be scaled to 5V range and using a fast adc, one can
distinguish these signals. Then using the software and cpu power, i don't think
it will be a hard job to demodulate the traffic. In fact, i can employ a cluster
system for this since here i've lots of dual linux machines waiting to run mpich.
In addition to these, adc's of sound cards can be used since their sampling rate
is enough. (44khz or so) (btw, i don't know if a 64 bit sound blaster really
have a 64bit adc, somebody said they work differently than normal adc's)
My final thought about these theories and assumptions: Why one uses adc or
external devices? I know that there are soft modems that do not know anything
about crcs/compression. They are just adc circuits made specially for this
purpose. I think one may alter the kernel driver of lucent modem to gather
digital data of the analog line , then decrunch it to a certain level. Finally
of course, this data must be fed to pppd (modified) then dump the traffic using
tcpdump or so:-)
Ehe,warning: this message was full dreaming:-) But i'm going to continue to look
for alternative solutions. Any suggestions?
Thnx.
-- Evrim ULU evrim@envy.com.tr / evrim@core.gen.tr sysadm http://www.core.gen.tr ---------------------------------------------------------------------------- This list is provided by the SecurityFocus Security Intelligence Alert (SIA) Service. For more information on SecurityFocus' SIA service which automatically alerts you to the latest security vulnerabilities please see: https://alerts.securityfocus.com/
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