Re: Please explain why an RJ45 splitter won't work

On Mar 28, 2:03 pm, Tomás Ó hÉilidhe <t...@xxxxxxxxxxx> wrote:
I understand that you can't use an RJ45 splitter as an ethernet
"double adapter".

But could someone please explain to me why? I'm not thick, I'll
understand. . .

I was under the impression that a very basic hub was a "dumb device"
that just connected certain wires together; I thought you'd be able to
make one using a wire stripplers, an ethernet cable, and a few RJ45's.

Or does a hub need some sort of processing?

The hub does do processing.

The simple answer to your question is that the types of Ethernet that
use RJ-45 plugs ARE NOT the types that allow paralleling of hosts
along a single bus. If they were, then your could do this sort of
splitting.

The flavors of Ethernet that use RJ-45 plugs depend on star
topologies. Only two devices connected on each link, one on each end.
Most of these have separate transmit and receive twisted pairs, for
one thing, which means that simply paralleling the Tx and Rx pairs
will not work.

Think about it. If you have multiple hosts sharing a single RJ-45
connection, with paralleled Tx and Rx, how will one host's
transmission reach another host's receiver? Is there any other way to
connect multiple hosts in parallel, when separate Trx and Rx twisted
pairs are in use?

In principle, you could use a single twisted pair, and connect all
hosts in parallel, using the same protocol as 10BASE2. Although this
is not easy with twisted pair, due to the less than reliable impedance
characteristics of twisted pair (which creates any number of
reflections along the line), Rich Seifert did explain on here some
time ago that Tut Systems developed just such a system. It was
developed for layering Ethernet over common household telephone
cabling. Analog telephones in fact are often paralleled that way. But
note: only one twisted pair was needed for the bus. This avoids the
problem of deciding how to parallel separate Tx and Rx twisted pairs.
And also, the distances involved are typically very short. Combined
with relatively low bit rate, which means that symbols along the
transmission line are not tightly packed, this can work well.

Bert
.

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