Re: [school-discuss] Cat-5 question

[Tonnesen Steve <tonnesen@cmsd.bc.ca>]

On Wed, 14 Aug 2002, Mark Evans wrote:

> A possible alternative is GgOBboNn, but the only
> time you are likely to see this is on a crossover
> cable.


Just as a bit of historical explanation for the strange pairing in CAT5
wiring:


1 2 3 4 5 6 7 8
G g O B b o N n

As people have pointed out, the pairs are 12 36 45 and 78.

The 45 (Bb) pair lines up with the original tip and ring wires used by
POTS (Plain Old Telephone System) phones.  Note that the 45 pair is still
right in the center of a CAT5 jack.  You can still plug in a POTS phone in
a CAT5 jack with a single pair cord and it will work just fine.  This
system has been used since shortly after farmers used their barbed wire
fences to talk to their neighbours. 

Later a data pair was added.  To ensure that old two wire cords could
still be plugged in, this new pair was added to either side of the
original tip and ring pair.  The new data pair was used for a second phone
line, or as a data connection between a PBX phone switch and their
proprietary telephones.  [SIDENOTE: At the time, the color code used was
Red and Green for the inside pair and Black and Yellow for the outside
pair.  You might still see this colour coding on older jacks and cords.]

When the standard was expanded next, two new pairs were added, one on
either side of the 4 wire standard.  They couldn't continue the trend of
putting one wire on either side, because the length required to untwist to
make a 27 and 18 pair would be too long.  The 8 wire standard is still
compatible with 2 wire and 4 wire cables. 

Hope that explains why we have 36 and 45 pairs instead of just 12 34 56
78. :)


Steve Tonnesen