[rescue] OT: Networking Topology Question

[Devin L. Ganger]

On Tue, Jul 03, 2001 at 01:34:42PM -0400, Ken Hansen wrote:
 
> They are all really still in the same "collision domain", aren't they?

No.  They're in the same broadcast domain, but each connection between a
switch and another device is its own collision domain, since no one
transmission can block another transmission that doesn't involve both of
the same hosts in the same direction (which are serialized anyway by the
physical layer drivers).
 
> Perofmance might be worse because every time a client prepares to send
> traffic, they will see an "open" line, and send the first packet, which
> will be held at the switch to be passed on to the server - if they were
> all on a hub, they would at least see the line as busy and not bother
> with the first packet.

No, because the switch has an advantage that collision detection does
not -- it knows when the transmit line to a client is open and when it
is not.  In fact, in full-duplex mode, it is the sole device
transmitting to any given device -- and can effectively make maximum use
of the transmit bandwidth by serializing the various packets as they
come in.

Additionally, by breaking up the collision domains, even on half-duplex,
you've now reduced the number of machines that can interfere with a
given transmission (which requires a random back-off and then a
re-transmission, which is more costly timewise than a buffer that holds
a packet in-transit until it can be serialized) to exactly two.  With
full duplex, of course, collisions become impossible, as the Ethernet
drivers serialize all the various packets to the same destination...

-- 
Devin L. Ganger <devin at thecabal.org>
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