[Shop-talk] running ethernet wires

[David Scheidt]

On Tue, Apr 7, 2009 at 2:53 PM, Wayne <wmc_st at xxiii.com> wrote:
> Randall wrote:
>>
>> I'm pretty sure the same thing holds for B. B Phones don't care about
>> pairing
>> nearly as much as Ethernet does; and the pairing is the only difference
>> between 568A and 568B.
>
> Correct. B Both 568A & 568B, as applied to modular connector wiring, only
> swap the 2nd & 3rd pairs (orange & green) around. B Both configurations are
> electrically identical, with "straight through" wiring. B Analog "POTS"
> phones only use the 1st blue pair and it's identically positioned B on both
> anyway. B 568B is preferred.
>

No.  T568B is not preferred.  It's probably more commonly used in the
US than T568A, and if you're already using it, keep doing so.  But the
current accepted practice is to use t568A on new installations; that's
what EIA recommends.  The issue with phone equipment is that using
568B puts pair three on pins 3 and 6.  (That's the second pair of
pins, counting from the center.)  That confuses people, in particular,
telco craft people, who are paid to be easily confused.  The
structured cabling that was (and still is, in dedicated phone
installs) used for phone wiring had pair 1 on the inner most pair of
pins, pair 2 on the second inner-most pair, pair 3 on the third
innermost pair, and pair four on the outer two pins.  (this is called
Universal Service Order Codes, USOC.)  T568A is compatible with the
two pairs that are commonly used in telco patch cabling.

I have no idea why T pushed 258A (which is what they called T568B when
they invented it, and why T568B is common in the US, but unknown in
the rest of the world) instead of something that was compatible with
what they'd done for USOC.





--
David Scheidt
dmscheidt at gmail.com