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'MoRe: [OT] Re: Network cat5 cabling question'
1998\05\14@014542 by paulb

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Eric Smith wrote:

> If it is Category 5 Unshielded Twisted Pair (UTP), it most certainly
> does consist of twisted pairs.

 Well, that's OK then!

> The silver satin flat modular line cord you are probably thinking of
> is not twisted.

 Except as a matter of how it's run.

> I'm not sure what Category it officialy requires, but any normal
> twisted pair telephone cable will work.

 Within limitations.  "Normal" phone cable (at least over here) is
twisted quad, i.e., four wires in a square comprising two pairs mutually
balanced out, with a slow common twist (spiral).  If however you run two
lengths closely parallel and the "twist" matches, they can couple.  How
does this compare to "Cat 5"?

> Note that flat modular telephone cable is often wired with a flip
> (i.e., pin 1 at one end is wired to pin 'n' at the other end.

 It is, as I gather, *supposed* to be.  That is, the wiring of the
equipment (at the jacks) expects it.

> 10BaseT and 100BaseT cable has to be "straight-through", unless you're
> using a crossover cable between two nodes, in which case two pairs get
> swapped.

 As has been posted, the pairs are not adjacent.  They are calculated
to be as near to two balanced pairs as can be practically implemented
with ribbon cable.

 Cheers,
       Paul B.

1998\05\14@151744 by Eric Smith

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I wrote:
> The silver satin flat modular line cord you are probably thinking of
> is not twisted.

"Paul B. Webster VK2BZC" <spam_OUTpaulbTakeThisOuTspamMIDCOAST.COM.AU> replied:
>   Except as a matter of how it's run.

I should clarify what I said.  Flat modular line cord does not consist of
individually twisted pairs, no matter how you run it.

I wrote:
> 10BaseT and 100BaseT cable has to be "straight-through", unless you're
> using a crossover cable between two nodes, in which case two pairs get
> swapped.

Paul replied:
>  As has been posted, the pairs are not adjacent.

I don't think I even implied that they were.  I deliberately didn't go into
detail on which pairs were used.

> They are calculated to be as near to two balanced pairs as can be
> practically implemented with ribbon cable.

No, they aren't.  They were chosen mostly arbitrarily.  The arrangement of
the twisted pairs within the outer jacket of the cable is not specified,
so there are no two pairs that can be known a priori to be better balanced.

Ribbon cable doesn't even enter into it.  You can't run 100BaseT over ribbon
cable.  But if that was a concern, the actual pair assignment used for
Ethernet would be suboptimal.  Using the 1/2 and 7/8 pairs would be better
than the standard 1/2 and 3/6 pairs.

ObPIC^H^H^HScenix:  If you don't mind overclocking the SX, it can run fast
enough to send Ethernet frames on a 10BaseT link in software.  However,
unless you operate the link as full duplex (typically on a port on an
Ethernet switch), you have to add external support for collision detection.
Unfortunately the Scenix can't be overclocked to a high enough rate to make
Ethernet reception practical without external hardware assist.

Cheers,
Eric

1998\05\16@070616 by paulb

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Eric Smith wrote:

> I should clarify what I said.  Flat modular line cord does not consist
> of individually twisted pairs, no matter how you run it.

 Sorry about this!  I have been working under the misapprehension that
the pin allocation on the "modular" 8-pin connectors and ribbon was
allocated as I as an engineer might have done, but find on closer
inspection that it *isn«t*.

 I have always *assumed* when looking at it, that one pair was pins, or
wires, 1 and 3 while the other pair was 2 and 6.  Putting two unused
wires between the pairs would have allowed *fair* balance.  As it is,
it«s rubbish, no contest!

 Cheers,
       Paul B.

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