The Natural Philosopher wrote:
On 24/05/2020 10:40, Roger Hayter wrote:
The Natural Philosopher wrote:

On 23/05/2020 12:13, The Natural Philosopher wrote:
On 23/05/2020 11:18, Tim Streater wrote:
The collision detect budget is critically dependent on the
DC resistance of the system; the high-frequency attenuation is much
less critical.

No, it isn't. This seems to have been a myth that got around

The rest is valid tho.


I want you to not accept what an expert has said, but explain WHY what
he says, is true?
Especially since I can find no other reference anywhere to 'DC' or
'cable *resistance* in any paper on CSMA/CD systems. Cable impedance,
yes, resistance no.

And today's systems still implement CSMA/CD but are universally coupled
in with transformers. That don't pass DC..

And DC won't propagate any faster than someone else's pulse train anyway.

AIUI collision detection is dome by sensing that the output on the wire
which you have 'grabbed' is not exactly what you are putting on it.

Cable *impedance* and of course attenuation matters but as long as they
are within tolerance that's OK, what is crucial however is that you
don't have excessive propagation delays and that is what screws you with
long cable runs. Two stations - one at each end - can transmit, see
their packets go clear and un buggered before detecting each others
transmission if the cable is too long.

In short.

- impedance is determined as your article says by ratio of inner to
outer conductor diameter, modified by the dielectric in between - air
cored polythene usually.

- Attenuation is a function of resistance, which given the above is a
function of cable core circumference and material - usually silver plate
on quality cables, or just copper. (fat cables lose less).

- propagation delay is determined by the dielectric and the cable length
- the nearer a vacuum the nearer the speed of light the signal travels.


As it happens for all cables and electronics in use then and now,
propagation delay is the limiting factor. Because you have to have a
line that is clear of all other travelling waves to transmit on.

Collision detection is a simple matter of receiving stuff you did not
transmit.

No DC involved at all.


Same as wifi today. No DC involved at all.

What causes dispersion then? Does not loss (resistance) slow higher
frequencies more and reduce pulser risetime? I have no recollection of
theory, and I am quite prepared to be wrong.

theoretically dispersion doesn't happen :-)

AFAICR its down to the effective dielectric constant not being constant
with frequency. Bit like chromatic aberration in glass.


Yes, at least indirectly.

It is due to the Velocity factor changing with frequency and Vf is related
to the dielectric constant of the insulation. Vf = 1/(Er)^0.5

As a result, in for example a square wave (composed as you know of odd
harmonics of the fundamental), the different frequencies are propagated at
different speeds and the shape is distorted. (A trivial example but easy to
visualise.)



--
https://www.unitedway.org/our-impact...an-trafficking