[mdlug] Curious - Phone Tapping Tech

[Michael Mol]

On Thu, Jun 21, 2012 at 6:58 PM, Aaron Kulkis <akulkis00 at gmail.com> wrote:
> Michael Mol wrote:
>> On Thu, Jun 21, 2012 at 1:32 PM, Aaron Kulkis <akulkis00 at gmail.com> wrote:
>>> Robert Adkins II wrote:
>>>> I am curious to know if phone tapping technology has "caught up" with
>>>> Holywood. In terms of being able to take a band and wrap it around a bundle
>>>> of phone lines and call that a tapped line.
>>>>
>>>
>>> No.  Kirchoff's Current Law makes this impossible.
>>>
>>> Every current travelling "outbound" on one wire of a twisted pair
>>> has an equal and opposite "inbound" current on the other half of the pair,
>>> making the net current going through the loop = Zero, and therefore, no
>>> way to detect ANYTHING  within the cable bundle.
>>
>> That assumes the two halves of the pair are coaxial, which they're not.
>>
>
> The only assumption I'm making is that the signal is carried on 2 wires,
> and that the detector ("tap") wraps around both wires.
>
> This sort of thing is covered very thoroughly in a college level
> 2nd semester physics course (Electro-magnetics optionally with optics)

And degrees of approximation are 1st-semester; I took that series,
too. It's been almost ten years, but still.

You're applying a 1st-degree of approximation to the problem. Reality
doesn't usually stop at the 1st approximation.

If Kirchoff's Law had the practical consequence you seem to believe it
does, then crosstalk wouldn't be a problem between two pairs in the
same cat3 cable. In reality, it is. As I noted earlier, I could hear
my parents' conversation on one line when I had the second line
off-hook. And, yes, these were two different circuits; they just
happened to share a cat3 cable part way to the telco.

-- 
:wq