in terms of maintaining wave shape, it is, but in terms of attenuation it's the actual (skin) resistance that counts.
I remember doing that ghastly calculus of an infinite number of Ls in series and Cs in p[parallel and showing that in the limit it looked just like a resistor...
Once upon a time I knew all that. 35 years have pretty much left me with what I posted :)
So the guy who actually made the decision on the cable type for use on ethernet, world renowned IEEE expert member, who on several occasions the Task Force chairman, and editor of 802.3x standards document[1], explains why the DC resistance is an important parameter for the CD mechanism:
What a dilemma, who should we believe?
Well I am sure he will be pleased to have your validation.
[1]
Well, I think I see the problem. And I'm astonished it worked at all - Ethernet should not be configured as a ring...
Elsewhere in the thread - Manchester encoding is used because it has no net DC component.
And a story. I once lost complete faith in my engineering manager when he plugged a bit of coax into a T-piece, took the entire office network down, and didn't even connect him doing this with the way the entire office erupted in expletives.
Perhaps that's why he made me redundant a year or so later!
Andy
How does the collision detect work ?
Paul
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.
No. The impedance is real, although brought about by Ls and Cs. But the real energy passed into this impedance is not dissipatet in the (hypothetically and practically) negligible resistance but propagated away until it gets to the matched resistive load at the end of the cable.[1]
[1] If the cable has not got a matched load at the end then you *don't* see the characteristic impedance at the beginning. But the impedance you see with a matched load *isn't* the load itself but the cable impedance.
theoretically dispersion doesn't happen :-)
AFAICR its down to the effective dielectric constant not being constant with frequency. Bit like chromatic aberration in glass.
Oh, you do...until the reflection comes back! And unterminated but infinitely long transmission line ...looks like a terminated one
But the impedance
Which is the same value....
Did you ever look at the signal on the central coax cable with an oscilloscope ? What did you see ? Were you surprised at what you saw ?
"the transition on TXO proceeds monotonically to zero current." ------------
Notice how they talk about current and not voltage.
The coax is 50 ohm doubly terminated, with one ground connection per cable segment.
You pump a current into it. The terminations convert that current into a voltage.
Once you've seen an oscilloscope picture of what's on the cable, you won't forget it.
Since V=R*I , the resistance of the cable is important to proper function. Errors in resistance will lead to errors in developed voltage, causing problems with collision detection and so on.
The transceivers I used at work, claimed a certain operating distance, but the manufacturer claimed they would do double that distance (with the proper coax in usage). Really quite amazing tech, compared to the home-made transceivers some of the engineers at work used when prototyping. The first day I got on the job, my first task was to repair enough coaxial sections so I could have a network :-( Our home made transceivers were voltage sources, at a guess, and I soon discarded them. The commercial replacements were *expensive*, but, the difference was, they worked, and we could easily build max-length networks with them.
There's one other thing you should have learned about that network, as a "manager". Did you notice something ? It only took a couple years before our first disaster. When I saw it, a f****ng light bulb went on...
Paul
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.)
I need something that can produce electric power, when l have power cut . For over 20 hours.
:))))
You also need the modem/router to run, etc.
Called a UPS Un-interruptible Power Supply. Only practical for relatively low power devices like a computer.
you shouldn't use them at all .....work of the Devil
Ho old was this post on Home owners club. Was this originated by Fred flintstone? I mean really I would have thought that a powerline adaptor, or rf jammer as I call them would be the least of your worries if you have 20 hour power cuts on a regular basis. More likely you need a satalight phone and a means to keep it charged and a large battery to allow one to run other stuff, and some heating and cooking facilities.
Strange question. Brian
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