# [e2e] Just a very quick remark on system theory Re: Why don't we talk about segments/objects instaead of layers? Re: Lost Layer?

Detlef Bosau detlef.bosau at web.de
Wed Feb 19 00:36:30 PST 2014

```because I'm out and about to see my dentist. (very adequate for a
networking guy: I will get a bridge.)

We all remember Ethernet. (This funny network with the yellow garden hose.)

And - jamming. Why was jamming necessary? Because of the systems step
response function applied to the first bit of the preamble.
And "Gibbs phenomenon".
So a sending Ethernet card ignores excess voltage on the line for the
first 40 (?) bit in order to ignore a "spurious collision".

Unfortunately, system theory does not really apply here, otherwise we
could eventually solve our energy problems because in the mathematical
abstraction, some voltages and currents in step- or impulse responses
grow beyond all limits.

In the formulae.

Actually, and luckily, some of the voltages and currents are restricted
by the power supply.

When you start with control theory on simple systems - and the literally
station wagon hurtling down the highway (Tanenbaum) runs with ten times
the speed of the light, you may perhaps discover, may, some people don't
ever, understand that models are only an approximation to reality and
perhaps some models are not always helpful.

And with particular respect to networks: State variables in difference
or differential equations are never bounded. In theory, they may grow
beyond all limits. Both for power (infinite storage capacity in bridges,
hopefully my mouth will be big enough in a few minutes because of the
dimensions of my new bridge) and "kinetic energy" on links (this damned
speed of the light, I will send a severe complaint to Albert afterwards).

That was the long explanation.

Executive summary for "control theory applied to computer networks":

Forget it.

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Detlef Bosau
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