On Mon, 8 Mar 2004, Johnny Billquist wrote:
> > Paul's rule (which is the one I was told) is that you can use
> > *any* stripes and there is *no* requirement for one stripe
> > in between transceivers (never mind two).
That is what I was taught, too. And I installed a few hundred or
so taps.
> > Given that this stuff was often hidden in suspended ceilings
> > and suchlike, it would be quite tricky to check that adjacent
> > stripes (or two adjacent stripes) were unused (I assume the rule
> > applies in both directions. Surely it would have been easier
> > to miss out the unusuable stripes?
Correct, this cable was usually on those ceilings, with taps being
set up there, and then an AUI dropcable down the wall to the device,
or sometimes another converter that ran BNC to small devices.
> I'm pretty sure that the stripes are at 2.5 m.
They are, and that is the minimum distance between devices on the
cable. So there is NO requirement to leave one inbetween, or my
connections would never have worked ;)
> I'm also pretty sure that the standard says you should have 5 meters
> between transcievers.
Nope.
> But I have actually never really understood why. Maybe just to not damage
> the cable to much with vampire taps? There cannot be an electrical or
> signal reason for this, as far as I can tell.
For signal echo reasons, much like the requiement for terminators
on both ends of the cable. If the devices are too close to eachother,
there will be too *little* space (read: time) inbetween for them to
check for any echo (collision). The transceiver will send out the
first bit, and will then wait the minimal time (the time it takes to
reach the first possible device on the cable) before it checks for
collision. It then sends out the packet (if no coll.) and waits a
full timeframe (meaning, the time maximally needed to reach a
terminator, and then back) for a collision. If none, we have sent
a frame. Otherwise, we have a collision.
That is why they set length limits (minimal dist between devices,
max length of segment (== "collision domain"), max. number of devices
per segment, and so on.
--f
--
Fred N. van Kempen, DEC (Digital Equipment Corporation) Collector/Archivist
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Email: waltje_at_pdp11.nl BUSSUM, THE NETHERLANDS / Mountain View, CA, USA
Received on Tue Mar 09 2004 - 01:22:16 GMT