> On Jun 27, 2018, at 12:36 PM, Eric Smith via cctalk <cctalk@classiccmp.org>
> wrote:
>
> Collision detection was the reason (or at least _a_ reason) why the spacing
> of taps on the 10BASE-5 "thick" Ethernet cable was required to be an exact
> multiple of 2.5m. It was never clear to me why this was not also a
> requirement for 10BASE-2 "thin" Ethernet.
Yes, to avoid false alarms. The purpose of the spacing rule is to ensure that
there is enough signal integrity that you do not get spurious collision
indications due to reflections off the impedance variations along the cable.
On a segment with few transceivers, there is enough margin that the rule
doesn't matter. This is why 10Base-2 doesn't have that rule: the station count
limit is low enough that it isn't needed.
A tap is a stub, and on a transmission line a stub is an impedance change -- a
small one if the stub is short in proportion to the wavelength. Similarly,
connectors will show up as small bumps because the devices aren't ideal.
If these bumps are spaced at multiples of the wavelength, the reflections that
occur at any impedance variation will combine to form larger reflections. If
enough of these add up, the reflected signal can appear like another
transmitter to the collision sense circuitry. So the spacing rule (for taps)
and the cable section length recommendations (distance between connectors) are
both set to place these perturbations at points that are NOT multiples of the
wavelength or small fractions thereof.
(Sometimes people say that the spacing rule is there to place things at
multiples of the wavelength; that is exactly backwards.)
So the spacing rule doesn't have anything to do with detecting real collisions,
but it is necessary to ensure no false collosions.
paul
