TELECOM Digest OnLine - Sorted: Re: Help Needed With WAV File

Re: Help Needed With WAV File

Robert Bonomi (
Thu, 11 May 2006 22:32:56 -0000

In article <>, <>

> I would like to recover the bits of the digital data from a
> conversation between two modems.

> Why do you think this is virtually impossible for a standard modem
> connection with an analog-side speed above 2400 BPS?

Because it _is_. <grin>

Because I know how such things work.

Because you -- the eavesdropper -- cannot tell which parts of the
'hiss' belong to which end of the conversation.

Because *BOTH* ends use the _same_ frequencies. At the *same* time.

At either end, by "knowing" what _you_ are putting on the wire, you
can 'deduce' what the other end is doing, by 'cancelling' your signal
from what you 'hear' on the wire.

WITHOUT that 'insider' knowledge, your best bet is "Miss Cleo".

This is also why there is all that hissing going back and forth
_before_ the modems 'connect' -- they are each 'seeing' what the line
conditions look like, *without* 'near-end' interference, when the
far-end modem does 'known things' on the line. *Then* they both start
talking, still saying 'known things', to verify that they can filter
out their own noise and recover what the far end is saying.
*ONLY*THEN* do they announce 'connect', and let "user" data flow
across the connection.

With _extremely_high-end_ equipment that has been precision calibrated
and the artifacts of gear identified and compensated for, *and* with a
non- trivial amount of testing with known data on the suspect phone
circuit, one _may_ be able to decode the bit-stream.

It is far simpler to play "man in the middle" by using a pair of your
own modems, and intercepting the call start-up. one end (a) talks to
your 1st modem, "thinking" it is talking to the far end (b), wile your
2nd modem is talking to that far end, in it's place. Now, you've got
the recovered bit-stream passing between your 1st and 2nd modems, in
decoded digital form, with the two halves of the conversation
separated. reading _that_ is 'trivial', as they say.

of course, going _that_ way means that you run afoul of any number of
laws forbidding 'wiretapping', and/or interception of wire
communications, but you're not going to let a little thing like _that_
stop you, are you? After all, you have the same legal issues with
taking a recording of the modem 'conversation' in the first place.

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