[mdlug] Curious - Phone Tapping Tech

[Michael Mol]

On Thu, Jun 21, 2012 at 8:14 PM, Aaron Kulkis <akulkis00 at gmail.com> wrote:
> 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.

Of course.

>
>> 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.

I don't know that I buy it; if we're talking about twist rate X and
twist rate Y for adjacent pairs X and Y, and we're talking about short
lengths having a greater difficulty than long lengths...then I think
what we're really talking about are harmonics.

Given twist frequency X and twist frequency Y, there will be places
where the two twists are aligned(+), unaligned, and alligned(-) (where
the original signal is eventually canceled out by opposing crosstalk).
This translates well into a discussion of constructive and destructive
interference waves. I expect that what you really see is that over
these 'short' runs, you've only seen aligned(+) and unaligned, and
over these 'long' runs, you see the crosstalk canceled out by
aligned(-).

I expect that if you extend the run further (presuming you don't run
out of signal due to line losses), you'll come back around to more
additive crosstalk.

And we're still back at my point that the application of Kirchoff's
Law here depends on idealized circumstances.

-- 
:wq