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<font size="-1"><font face="Helvetica, Arial, sans-serif">At 100
megabits/sec, a bit is about ten feet long, so you can visualize this
phenomenon quite easily. Messages need to have a minimum length such
that collision information can propagate to the transmitter before he's
done transmitting his message. Ethernet truncates collided messages to
the minimum message size - 512 bits - as there's no sense in beating a
dead frame.</font></font><br>
<br>
On 6/3/2010 2:10 PM, Noel Chiappa wrote:
<blockquote cite="mid:20100603211038.354956BE635@mercury.lcs.mit.edu"
type="cite">
<pre wrap=""> > From: John Day <a class="moz-txt-link-rfc2396E" href="mailto:jeanjour@comcast.net"><jeanjour@comcast.net></a>
>> To get the -CD to work semi-reliably they had to limit the network's
>> physical size, and increase the minimum packet size
> Ensuring that all receivers could hear all transmitters (which also
> requires bounding the length) is what makes the main difference. by
> greatly reducing the probability of a collision.
Maybe we're saying the same thing, but there's a 'simultaneity' issue as
well.
If you have three stations (A, B and C) all in a line (effectively, which is
what a wire gives you), with some distance between them, then: If A transmits
a relatively short message to B at the same time as C transmits a relatively
short message to B, C's message doesn't start to get to A until after A is
done transmitting its message (and vice versa at C). So neither A nor C sees
a collision - but at B, in between them, the two messages _do_ collide.
In other words, for the -CD to work 'right', a transmitter has to occupy the
'whole' shared medium for long enough that other any station trying to
transmit will definitely see a collision (in cases where they start to
transmit basically simultaneously).
That's the reason Ethernet has restrictions on i) physical size and ii)
minimum message length - messages which are too short may produce the
scenario above (and if you make the network larger, you need to make the
minimum message size larger too, because end-end propogation time then
becomes larger).
Noel
</pre>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Richard Bennett
Research Fellow
Information Technology and Innovation Foundation
Washington, DC</pre>
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