toddi@nsr.bioeng.washington.edu (Todd Inch) (05/23/90)
In article <7909@accuvax.nwu.edu> julian@bongo.uucp (Julian Macassey)
writes that it is illogical to get crosstalk between twisted pairs in
a multi-pair cable considering that all the pairs are in one cable on
their way to the CO without any problems.
Thanks for saving me the typing, I couldn't agree more.
It is critical, however, to make sure that each line is actually on
one "pair". For instance, the Blue/White and White/Blue striped wires
form a pair and should be used for one line. Use the Orange/White
PAIR for the next line, etc.
Note the difference between two-pair and four-conductor wire: you can
run two lines on two pair, one line on each pair. You shouldn't do
this on the four conductor because the Red/Green aren't twisted
separately from the Black/Yellow. There *are no* pairs in that stuff.
Yes, I learned this the hard way.
If you're not sure about the cable, strip off a foot or two of the
outer sheathing. You should be able to see if they're twisted pairs
or not.
Also, the further you run on non-paired wire, the worse off you are.
For very short distances there shouldn't be a problem.
I'm not sure the four-conductor stuff is twisted at all. It's hard to
tell since the outer sheathing isn't "hollow" and can't be stripped
off easily without untwisting the conductors. If it's not twisted and
you run two of these cables next to each other, you'll still have the
same cross-talk problem.
Anybody know for sure, or does it depend on the particular cable?
All this applies to the flexible flat "modular" style (usually silver
colored) cable, too. Don't use that stuff for "permanent" wiring -
just between the phone sets and the wall.
Todd Inch, System Manager, Global Technology, Mukilteo WA (206) 742-9111
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