[Gllug] Ethernet crossover wiring

[John Winters]

On Fri, 2006-02-24 at 16:24 +0000, t.clarke wrote:
> If you think Cat5 wiring is daft,   just have a look at the way RS232 25pin
> connectors are used   !!!

It's precisely the same mistake.  Pin 1 is wired to pin 1, pin 2 to pin
2 etc.  You thus can't use them to connect two identical pieces of kit
because if one of them is transmitting on pin 2 then the other has to be
listening on pin 2.

It's like building a road between two towns and saying that traffic will
enter and leave town A driving on the left hand side of the road, but
enter and leave town B driving on the right.  You then need a crossover
somewhere on the road between the two, and you need two different kinds
of road to connect towns depending on whether they're left-side towns or
right-side towns.

Once you recognise that a genuinely "straight-through" road is one where
the left hand side of the road as seen from town A is the same side as
the right hand side seen from town B, then everything works neatly.
Both towns can use the same convention and you need only one kind of
road.
        Left=>
        -------------------------------------------------------
      A                                                         B
        -------------------------------------------------------
                                                         <=Left

I'll stop ranting now - honest.

John

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