27 Jun
2018
27 Jun
'18
9:59 a.m.
On Jun 27, 2018, at 12:36 PM, Eric Smith via cctalk
<cctalk at 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