There is no DTE or DCE in Ethernet-world. Those are serial communication
terms that do not apply here.
And hubs are definitely layer one devices, not layer two. To be considered
layer two, they would have to be aware of layer two addressing schemes,
which they are not. They are, however, very aware of the physical
necessities of ethernet communication on a 10 Mbps/Half duplex LAN, and they
do just a bang up job at it.
Regardless, there's no topological difference between a 10baseT LAN
connected to a single hub, and 10base5 LAN devices connected to a single,
long cable. And I hope no one will try to make the case that the cable is a
layer two device.
In Ethernet, your transmit pair must connect to the receiver's receive pair.
Hub and switch ports are wired internally to be crossover ports, or MDI-X,
so that any ethernet hosts connected to it will have their transmit pair
connected to everyone else's receive pair.
This means that if you connect a hub to a hub, a hub to a switch, or a
switch to a switch using a straight cable you are connecting two ports that
are both crossed-over. This will connect the transmit pair of the first
device to the transmit pair of the second, and consequently the receive
pairs will also be connected. This obviously will not work! That is why if
one of these devices is a hub with an MDI/MDI-X switch, you would set it to
MDI to undo the internal crossover. This matches up the pairs correctly and
communication will proceed.
If you have two switches, then you have no MDI/MDI-X button to press and a
crossover cable is required to match up the transmit and receive pairs
correctly.
I hope that makes sense. As usual, I need to go to bed and get some rest.
I tend to ramble when I'm sleepy...
Zzzzzzzzzzzzzzzz,
John
> DTE to DTE uses cross-over cables.
> Is this correct?
>
>
> ""Bradley J. Wilson"" <[EMAIL PROTECTED]> wrote in message
> 031901c0608a$b9d87d20$ca01010a@bwilson">news:031901c0608a$b9d87d20$ca01010a@bwilson...
> > Okay...I've been digesting the responses I've received about this
issue,
> and
> > what I've come up with can be reduced to two comments:
> >
> > 1) The fact that flipping the MDI/MDI-X switch didn't initially work
leads
> > me to believe that I wasn't using it right. The fact remains that
> > connecting a hub to a switch must be accomplished using a crossover
cable.
> >
> > 2) Why is that? It's true that connecting devices at different layers
of
> > the OSI model requires a straight-through cable, and that connecting
> devices
> > at the same layer requires a crossover cable. So isn't it true that
> > switches are layer 2, and hubs are layer 1? No. Hubs are actually
layer
> 2
> > devices, and here's why: while a hub may not understand or care about
> source
> > and destination MAC addresses, checksums, or what's in the "type"
field,
> it
> > *does* know the rules of CSMACD communication, and has to play by the
same
> > rules as other layer 2 devices. Therefore, hubs can be considered
Layer 2
> > devices, and thus must be connected to switches with a crossover cable.
> >
> > Anyone see anything wrong with my synopsis above? Let me know. Just
be
> > careful with your caps lock key. ;-)
> >
> > Thanks,
> >
> > BJ
> >
> >
> >
> >
> > _________________________________
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> >
>
>
> _________________________________
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