[mdlug] Curious - Phone Tapping Tech

[Aaron Kulkis]

Michael Mol wrote:
> 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. 

Actually, without the pairs being twisted pairs, and the twist
rates in each pair being different, then it would be a serious
problem.  In fact, cross talk in telephone cables is precisely
why twisted pair was invented.

> 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.

Short-runs are more susceptible to cross-talk because the
varied twist rates of the various pairs are designed to
cancel out cross-talk over long runs.  In short runs,
adjacent twisted pairs with similar twist rates still
exhibit lots of cross-talk, because both pairs flip
polarity of their magnetic fields at about the same
rate. But over a long haul, the adjacent pairs have
as much time in one relative orientation as another.