It is my understanding that the CSMA/CD function is handled by the switch
internally before the output frame is sent to the device.
We recently had a thread on late collisions that mentioned as a possible
source a NIC set to full-duplex that was connected to a half-duplex hub.
Since CSMA/CD was disabled, the NIC would transmit anytime it felt like it
without checking first. This could cause a late collision if the subnet
already had a packet in progress.
Vern Stitt, AE, ASE, CCNA, MCSE
"woody" <[EMAIL PROTECTED]> wrote in message
8fbsfi$ec0$[EMAIL PROTECTED]">news:8fbsfi$ec0$[EMAIL PROTECTED]...
> Actually CSMA/CD is still required. When communicating through a switch,
> the TX (towards the switch) is guaranteed the full bandwidth towards the
> switch. However the inbound (RX from the switch) may be carrying traffic
> from more than one originating host. As such, collisions may still occur
if
> more than on of these hosts tries to talk to the same destination at the
> same time.
>
> A basic switch (without VLANS and fancy Layer 3 functionality) creates
> bandwidth domains but NOT broadcast domains. As such it will still flood
> broadcasts and multicasts to all ports - again creating opportunity for
> collisions that require CSMA/CD arbitration.
>
> Full duplex increases throughput as less traffic is forwarded to end
devices
> (cuts out unicasts not addressed to the attached devices) but does not
> guarantee collisionless connectivity. CSMA/CD is required to handle the
> collisions that do occur.
>
> Keith
>
>
> ""Joe Martin"" <[EMAIL PROTECTED]> wrote in message
> 8fbq4s$80t$[EMAIL PROTECTED]">news:8fbq4s$80t$[EMAIL PROTECTED]...
> > Full duplex transmission requires a point to point connection between
two
> > devices. This is achieved using a switch. Since the connection is
> between
> > two and only two devices at a time, this allows them to transmit and
> receive
> > at the same time. Thus a collision would never occur and CSMA/CD is
> > unnecessary.
> >
> > JOE
> > CCNP, CCDP, and a few other things...
> >
> >
> > "Dan West" <[EMAIL PROTECTED]> wrote in message
> > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > > Sorry to ask such a simple question--but the CCNA book
> > > is still unclear as to what's going on.
> > >
> > > Half-duplex ethernet uses CSMA/CD for arbitration on
> > > the link. Does full duplex use it as well for
> > > arbitration? The book makes it sound like if you are
> > > running full-duplex that the CSMA/CD is not necessary.
> > > It mentions half-duplex looping a duplicate frame onto
> > > the recieve wire from the transmit wire.
> > >
> > > Thanks.
> > >
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