nightrecon at verizon.net
Thu Jul 16 06:47:32 UTC 2009
David Kelly wrote:
> Since when does one have CSMA/CD when configured as full duplex? All
> full duplex ethernet connections are point to point, machine to
> machine, or machine to switch. There is no multiple access on full
> duplex. No chance of collision.
You are running Ethernet, right? CSMA/CD is part of the Ethernet framing
protocol. It is present in the protocol independent of simplex/duplex, etc.
As such the timing windows contain non-infinite discreet value ranges. It is
integral to Ethernet and does not get 'switched off' or disappear just
because a link is full-duplex.
> So I'm thinking at 5,000' the problem is one of echo cancelation and
> signal loss, not one of ethernet protocol.
These other electrical parameters are indeed important. Let's not forget
near-end crosstalk, et al. If you have an oscilloscope and decide to try
this, take a look at what's called the "eye pattern". Then compare it with a
circuit that is within correct functional parameters. You will immediately
see a difference, and these are electrical effects of the medium. Excessive
phase jitter and the NICs on either end will be unable to decode anything.
As far as they are concerned there is only random 'noise' present.
These physical parameters drive the limitations designed into the Ethernet
protocol. There are maximum distances in fiber just as there are in copper.
If we could simply ignore these things and do whatever we want why would
they need exist in the first place?
They exist because the propagation speed in the medium is not instantaneous.
This makes the problem time. The furthest apart two nodes can be located is
the time it takes for the smallest Ethernet packet to get from one end to
the other. When a NIC transceiver is in the process of transmitting a packet
it is also listening at the same time and calculating a CRC. It knows when a
collision has occurred when the CRC does not match on both TX and RX. If
they are too far apart in time, and both NICs key up at the same instant
neither will ever know the collision has not yet occurred. Both will assume
no collision has occurred and queue up the next packet, and so on and so
forth. The problem is time, and time is directly related to the propagation
speed of the medium.
This relationship to time is present in the Ethernet protocol. The
misconception present is that "with full duplex there is no chance of
collision" meaning that CSMA/CD is somehow magically turned off or excluded.
It is not. But none of this will matter. The electrical parameters of 5,000
feet of UTP will ensure that Ethernet doesn't even enter the picture as
neither NIC on either end will ever be able to identify or decode any
> David Kelly N4HHE, dkelly at HiWAAY.net
> Whom computers would destroy, they must first drive mad.
Ain't that the truth? Hi Hi Hi. Just trying to hint at not wasting your time
with something that won't work. By the way - I'm KD3FO
More information about the freebsd-questions