[ih] principles of the internet

[Noel Chiappa]

    > From: John Day <jeanjour at comcast.net>

    >> 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