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Q39: Why are the pairs shuffled around, instead of laid out logically?
A39:Well, they are logical, if you know the background. Ethernet was
originally designed to run over the same cabling used by the phone system
(AT&T created this as StarLAN), so this design left pins 4/5 available
for a phone.If you plug an RJ-11 (4 wire) into this, the middle two pins
(2/3) of the RJ-11 would connect to the middle two pins (4/5) of the RJ-45,
just perfect for a 1-line phone. If you plug an Ethernet cable into it,
pins 1/2 will be used as one pair,and pins 3/6 will be used as another.
So, there are no conflicts.
USOC
USOC is an old standard, used for voice cabling. For 1 & 2 line phones
(which use pins 4/5 and 3/6), 568A or 568B will work just as well as USOC.
But for Ethernet (pins 1/2 and 3/6), USOC won't work. An Ethernet NIC
transmiting signals on pins 1/2 will get troubles because 1/2 aren't a
pair (not the same color,and not twisted together). So unless one know
the cable plant is only for analog voice application and not for any digital
voice or ISDN data application, he should avoid using USOC.
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