From: Eric Hughes <hughes@soda.berkeley.edu>
Date: Tue, 2 Mar 93 15:57:42 PST
To: cypherpunks@toad.com
Subject: tapping
In-Reply-To: <199303022155.AA04077@eff.org>
Message-ID: <9303022354.AA27572@soda.berkeley.edu>
MIME-Version: 1.0
Content-Type: text/plain



>> > uses a proprietary technique to reduce noise interference while picking up
>> > and transmitting data signals.  

>> > works with both shielded and unshielded twisted-pair copper cabling, is
>> > small enough to fit on laptop PC internal adapters, 

Phiber writes:
>Mike Watson rediscovers inductance, and the inductive tap.
>Film at 11.

Don't be so dismissive.  There is something interesting going on here,
even if it's not very complex.  This thing works with _shielded_ pair.
With twisted pair to begin with, you largely attenuate the inductive
signal.  (A very short lesson in physics: Current generates magnetic
fields.  Opposite travelling currents generate cancelling fields.
Fields do not completely cancel because the wires are not in exactly
the same place.)  Shielding a twisted pair further attenuates a
signal.

It sounds to me like it's an inductive tap with some sort of phase
locking built into it.  By the mentioning networks, it indicates to me
a digital signal.  I doubt this thing would tap a POTS line carrying
voice.

Eric