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Posted by =?ISO-8859-1?Q?Tom=E1s_=D3_h=C on March 28, 2008, 2:03 pm
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I understand that you can't use an RJ45 splitter as an ethernet
"double adapter".
But could someone please explain to me why? I'm not thick, I'll
understand. . .
I was under the impression that a very basic hub was a "dumb device"
that just connected certain wires together; I thought you'd be able to
make one using a wire stripplers, an ethernet cable, and a few RJ45's.
Or does a hub need some sort of processing?
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Posted by P.Schuman on March 28, 2008, 2:41 pm
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it's not like an extension cord... it's an intelligent electronic device.
The bits are formed into an Ethernet frame and transmitted on the media.
The receiver gathers the bits, buffers them, and re-creates the full frame.
The hub acts as a go-between to perform the same functions.
If you connect 2 computers together on the same wire at the same time,
as you are attempting - then what happens when both transmit at exactly the
same time,
or both receive at exactly the same time...... the bits would be all
shuffled together creating garbage.
That's why the Ethernet standard for twisted pair wires requires a hub in
the middle.
The original coax standard, using T-'s - did exactly that... share the
media.
However, the network card would first "listen" to see if anyone else was
transmitting,
and if not - then it would transmit. If collisions happened, then each
station would back off,
and after waiting a random amount of sub-second time - try to transmit
again.
With the Ethernet twisted pair standard, this has changed somewhat.
Hubs are powered, electronic devices that perform all the magic - they are
dumb in one sense -
but they are intelligent... not just splicing the wires together - else they
would not be powered.
If you need more - do a search on Ethernet wiring -
Tomás Ó hÉilidhe wrote:
> I understand that you can't use an RJ45 splitter as an ethernet
> "double adapter".
>
> But could someone please explain to me why? I'm not thick, I'll
> understand. . .
>
> I was under the impression that a very basic hub was a "dumb device"
> that just connected certain wires together; I thought you'd be able to
> make one using a wire stripplers, an ethernet cable, and a few RJ45's.
>
> Or does a hub need some sort of processing?
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Posted by =?ISO-8859-1?Q?Tom=E1s_=D3_h=C on March 28, 2008, 4:34 pm
If you were Registered and logged in, you could reply and use other advanced thread options
> If you connect 2 computers together on the same wire at the same time,
> as you are attempting - then what happens when both transmit at exactly the
> same time,
> or both receive at exactly the same time...... the bits would be all
> shuffled together creating garbage.
I was under the impression that all network hosts perform CSMA/CD. Are
you saying that only hubs do? (I'd like to leave switches out of the
conversation for now)
When you connect two computers together with a cross-over cable, is
the reason you don't get collisions = that you have separate pairs for
sending and receiving?
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Posted by Albert Manfredi on March 28, 2008, 5:12 pm
If you were Registered and logged in, you could reply and use other advanced thread options > P.Schuman:
>
> > If you connect 2 computers together on the same wire at the same time,
> > as you are attempting - then what happens when both transmit at exactly =
the
> > same time,
> > or both receive at exactly the same time...... the bits would be all
> > shuffled together creating garbage.
>
> I was under the impression that all network hosts perform CSMA/CD. Are
> you saying that only hubs do? (I'd like to leave switches out of the
> conversation for now)
>
> When you connect two computers together with a cross-over cable, is
> the reason you don't get collisions =3D that you have separate pairs for
> sending and receiving?
When two computers are connected with a cross-over cable, you can
indeed get collisions. The point is, both computers can detect the
collision, because the Tx pair of one goes to the Rx pair of the
other. A computer would detect the collision when it sees energy in
its Rx pair, while at the same time it is transmitting. So with two
computers set up this way, no problem.
Now parallel multiple computers on a single RJ-45 connection. How do
you do that? All Tx spliced together, all Rx spliced together?
Okay. let's try that. One of the computers starts transmitting. How do
the others hear that transmission? Certainly not from the signal
present on their Tx pairs, right? So how?
Bert
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Posted by Robert Redelmeier on March 28, 2008, 5:29 pm
If you were Registered and logged in, you could reply and use other advanced thread options > When two computers are connected with a cross-over cable, you
> can indeed get collisions. The point is, both computers can
> detect the collision, because the Tx pair of one goes to the Rx
> pair of the other. A computer would detect the collision when
> it sees energy in its Rx pair, while at the same time it is
> transmitting. So with two computers set up this way, no problem.
This is not a collision because no data gets mangled.
This is just normal full duplex operation.
-- Robert
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