[P1] Questions about Airport

[Allan Hise]

On Thu, 9 Jan 2003, Brian Pearce wrote:

> > Aren't all iBooks from the last year or so able to detect crossover vs.
> > normal cables (or would that be hub/switch port vs NIC port?) and
> > switch
> > to the appropriate connection.
> > In other words, if I recall correctly, I don't think you even need a
> > crossover cable for these two machines.
>
> This is true for the later-model iBooks; I wasn't sure it was the case
> with the eMac, though.

Only one needs to switch. On your RJ-45, there are 4 twisted pairs.
10Base-T/100Base-TX uses only 2 of the 4. The pairs used are on pins 1 &2
and on pins 3 & 6. It's been several years since I worked on ethernet, so
I can't remember which one is RX and TX on which side by default, but the
goal is to have the TX on the computer connect to the RX on the hub and
vice-versa.

Plug in two normal ports with a normal cable and TX connects to TX. On
hubs & switches, the connections are reversed, so TX goes to RX.

Using a crossover cable, one the pair that is on pins 1 & 2 will connect
to pins 3 & 6 on the other side instead of pins 1 & 2 like on a normal
cable. Hence the crossover. It has the same effect as switching the
connections on a hub and will allow two PC-side ethernet ports to talk.

If the iBook can auto switch, than the problem is solved. The connection
on the iBook flips to the hub configuration and the eMac and the iBook can
talk with a normal cable.

My question... Back in the day I worked on this stuff, the auto-switchover
feature wasn't around. So I wonder how the ethernet interface decides when
to switch. And what happens when they both can auto switch? I assume that
they start in the default mode and try to switch when there is no link
detected. But if they both switch simultaneously, it will never end. I
assume there is some pseudo-random factor involved like the inter-packet
gap timing...

Ooh, this has gotten pretty long. Sorry for rambling...