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List:       linux-smp
Subject:    Re: Purchasing a Quad Processor machine. Any suggestion?
From:       jhpb () sarto ! gaithersburg ! md ! us (Joseph H !  Buehler)
Date:       1998-07-28 21:29:34
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Rainer Dorsch <rainer@rainer.informatik.uni-stuttgart.de> writes:

> > UPA - three-way crossbar switch (not bus)
> >
> 
> Sorry, but what is a crossbar? 

I believe it is a reference to some really old telephone switching
technology.  I think that there used to be telephone switches that
made use of switching components consisting of two sets of parallel
metal bars arranged with the input set perpendicular to the output
set, no bar touching any other bar:

        INPUT
       v v v v
       | | | |
    ---|-|-|-|--->
    ---|-|-|-|---> OUTPUT
    ---|-|-|-|--->
    ---|-|-|-|--->
       | | | |

The arrangement switched input circuits to output circuits by slamming
a selected input rod against a selected output rod.  The crossed-bar
arrangement became known as a "crossbar switch".

The same idea is around today, but implemented with electronics
instead of metal bars.

Joe Buehler

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