Imagine the following. You have some twisted pairs to run between
pieces of equipment. To cover the most likely end user connection
case, you flip the connector on the router/switch, so a
straight-thru cable can be used.
Computer TX ------------- RX Router/Switch
RX ------------- TX
Now, imagine what happens, when you line two computers up.
What do you notice ? There's a problem. How do we get the
TX of one computer, to go to the RX of the other computer.
Computer TX ??? TX Computer
RX ??? RX
The computer to computer case, needs a "twist" in the wiring,
and that's where the special crossover cable comes from. And
before MDIX came along, it meant stocking two kinds of cables
in your home, to be prepared for any computing scenario. (If
you don't connect computer-to-computer, perhaps you don't
need this one.)
____ ____
\ /
X
____/ \____
red blue
GbE interfaces have four twisted pairs, rather than just two
twisted pairs working. It's easier to insert a digital crossbar
inside, and line up what's needed that way. The GbE can see
signal transitions on any pair, and then all that's needed is
a negotiation protocol, to decide what to do on each end.
The wiring correction is implemented digitally, at the
crossbar level.
*******
In many ways, the situation is similar to RS232 connectors,
and DTE versus DCE. A different thing needing to be done,
when connecting computer to dialup modem, versus connecting
computer to computer. It's one of the reasons I keep a
whole *bag* full of RS232 connectors, and one RS232 cable.
I need connectors to convert from 9 pin to 25 pin, convert
males to females as required, insert a crossover, and finally,
a cable to stretch the distance. The connector collection
costs more than the modem did.
One other nice difference, is Ethernet 4 wire and 8 wire cables,
have no "ground" connection. Which removes the shock hazard. I've
received an electrical shock in the lab, while using RS232, and
that won't happen with the modern Ethernet. That's because
Ethernet is transformer isolated, on either end. You'll notice
no "ground metal" on the Ethernet connector, just a safe plastic
body, with the wire pairs for the signals themselves. The
person who invented transformer isolation for Ethernet, should
be given the Nobel prize in Physics, for a job well done
Paul
