From: Jay Hennigan (jay@west.net)
Date: Mon Jan 03 2005 - 13:02:55 GMT-3
On Mon, 3 Jan 2005, Drew Whitaker wrote:
> Had someone ask me today, "Why are the Ethernet UTP receive pins (3&6)
> separated while the send pins (1 & 2) are together?" I answered,
> "Just because," but I don't think that was sufficient. ;-) Anyone
> know why it's pins 3&6 and not pins 3&4?
The original 568 design design was based on compatibility with legacy
telephone equipment. A conventional RJ-14 phone jack uses the center
two pins as line 1 and the outer two pins as line 2. This keeps things
balanced when using flat cordage as the spacing is identical between
the line conductors, reducing crosstalk.
An 8-pin jack will accommodate a 4-pin RJ-14 plug, with the center
pins as 4 and 5 and the outer as pins 3 and 6. Extra pairs 1 and 2
and 7 and 8 don't follow the telephone scheme, or they would be paired
2 and 7 and 1 and 8. In fact there is a jack spec paired this way and
some early Merlin telephones use it.
For twisted pairs, it's easier to pair 1 and 2 and also pair 7 and 8,
but 3 and 6 and 4 and 5 were kept paired for backwards compatibility
with phone gear. In fact early UTP ethernet specs called for the same
cable to carry both voice and data with the voice on 4 and 5. Even
the color code for 568A matches legacy WECo tlephone standards.
Another change from telephone gear is that the cords don't "roll" the
conductors. A telephone cord pinned left-to-right on one end is rolled
right-to-left on the other (as are Cisco console cables). Ethernet
cords are not rolled.
-- Jay Hennigan - CCIE #7880 - Network Administration - jay@west.net WestNet: Connecting you to the planet. 805 884-6323 WB6RDV NetLojix Communications, Inc. - http://www.netlojix.com/
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