[Beowulf]Infrastruture planning for small HPC 40/100 gigabyet eyhernet or Infiniband?

Robert G. Brown rgb at phy.duke.edu
Tue Jul 29 07:33:01 PDT 2008

On Tue, 29 Jul 2008, Jim Lux wrote:

> But, RS232 was never intended for distances over, say, 10 meters. That's what 
> short haul modems were all about... (basically line drivers/receivers with 
> galvanic isolation)

Although it was used, extensively to universally, over distances ten
times that.  Terminal servers were expensive, wire cheap, terminals
were all there was to connect, so one ran UTP bundles all over the
place.  (I spent way too much time pulling said bundles and sorting out
color coded pairs on both ends and crimping on the little poke-pins for
an RS-232 shell, either size...).

> Just this sort of thing is why the very first Ethernet (before it was even 
> called that) called for galvanic isolation in the AUI.
> This is also why there are grounding rules in the National Electrical Code, 
> especially dealing with connections between buildings/structures.

Sure.  And if the people who installed the network wiring and built the
AUIs actually have read the spec, and if the manufacturing process
actually didn't actually create a short, and if the wires themselves
didn't get partially stripped by pulling them too hard through too small
or too full or too sharply bent a grounded conduit, and if neither the
computer or switch(es) have been hit by a lightning spike or heated to
90C in an AC fault so that their innards cooks, then the spec probably
is enforced and works.  Believably 99.9% of the time.  Or maybe 99.99%
of the time.  It doesn't matter.  Murphy is merciless, and to paraphrase
Adams, one in ten thousand chances are practically a sure thing...;-)

The point being that I'd never disbelieve somebody that saw arcing when
making an ethernet connection just because it is oxymoronically "almost
impossible" (which parses to mean "possible") -- I've seen it myself.
Not even over long distances.  Sometimes switches themselves are just
plain faulty in their wiring, or have been blown by a nearby lightning

   Things fall apart; the centre cannot hold;
   Mere anarchy is loosed upon the world,

You have to love Yeats; he clearly understood Murphy.



Robert G. Brown                            Phone(cell): 1-919-280-8443
Duke University Physics Dept, Box 90305
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Web: http://www.phy.duke.edu/~rgb
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