RE: CSMA/CD

["Trevor Sayle" <trevorsayle () comcast net>]

I though that CSMA/CD only operated at the MAC (Media Access Control)
layer of Layer 2 in the OSI model. Physical only interprets the signals
(RF, Electrical or even Light) to logical bit values for the
corresponding layer which would be the MAC layer of Layer 2.

-----Original Message-----
From: Adam Balogh [mailto:adam () vattnet net] 
Sent: Friday, August 22, 2003 1:40 AM
To: Security-basics () securityfocus com
Subject: Re: CSMA/CD

On Thu, 2003-08-21 at 20:12, David Nichols wrote:
> You are talking about two different things. Ethernet, and TCP/IP.
Even
> thought the combination of the two is the most common today, they are
not
> the same thing.  You can run TCP/IP over token ring, or IPX/SPX over
> Ethernet.  Basically Ethernet (or Token Ring) are layers 1 and 2 while
> TCP/IP (or IPX/SPX) are layers 3 and above.
> You wouldn't re-write the TCP/IP stack, but the device driver for the
> Ethernet card.  Even thatmay not work because the CSMA/CD may be done
> entirely in hardware on the card
>
> Working-on-my-CCNA-as-well-ly
> David Nichols
>
> ----- Original Message -----
> From: ". ." <ajfomania () hotmail com>
> To: <Security-basics () securityfocus com>
> Sent: Thursday, August 21, 2003 7:08 AM
> Subject: CSMA/CD
>
>
> > Hi there!
> > I'm currently reading a CCNA book, and I've got some questions I
can't
> find
> > an answer to.
> > The Carrier Sense Multiple Access with Collision Detection checks if
there
> > is any traffic on the wire and then starts to send the host's data,
if
> > someone tryes to send at the same time, the sending host sends the
> "trying"
> > host a jam signal that would make the trying host to wait.If the
trying
> host
> > get's 15 jam signals at one time, it times out.
> >
> > Some questions came up at this chapter. Is there any possibility to
write
> a
> > program that sends jam signals to other hosts within the same
broadcast
> > domain until they timed out and died ? Does this mean that IF you
where
to
> > write this program, actually needed to rewrite a part of the  tcp/ip
> > protocol stack ? Isn't this a big issue?
> >
> > Sorry for my bad english..
> >
> > Best Regards: Fredrik Wessberg
> >
> > _________________________________________________________________


When a nic receives a jam-signal it randomises a number between 1-65536
ns/us(don't recall/depends on nic). This is a delay it uses before it
resends the package. The chances that 2 nic's randomises the same number
15 times is indeed minimal. This hasn't really anything to do with
TCP/IP since this in layer 1 and 2.

/Adam





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Attend Black Hat Briefings & Training Federal, September 29-30 (Training), 
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