On Mar 22, 19:47, Eric Smith wrote:
> Pete Turnbull <pete_at_dunnington.u-net.com> wrote:
> > I'm not sure these are repeaters, though. So a signal that's sent from
a
> > station on one of the AUI interfaces would go onto the wire, but I'm
not
> > sure that it would get back (directly) to the other AUI interface(s).
>
> It had better, or you don't have a functional ethernet, because two of
> the nodes can't talk to each other. Also, the nodes have to be in the
> same collision domain, so there's not really anything to be saved by
> using some wacky scheme where the nodes can't talk to each other.
I have seen such schemes. The small objects sold as "passive" 2-port or
3-port 10baseT hubs work like that, and two adjacent nodes can't talk
directly to each other. It rarely matters. Well, I guess that depends on
your network setup, actually! If you've not seen them, they look like a
little black (usually) box with two RJ45 sockets on one end (into which you
connect, say, two workstations or PCs) and a short RJ45 cable at the other
end (which you connect to a real hub, switch, or a server). They're
intended to let you connect more clients than you have hub ports.
It's not a question of using some wacky scheme; rather a question of
simplicity: if you want the adjacent nodes to talk to each other, you need
to have the Tx from each connected to the Rx of the other. That's easy to
arrange with just two 10baseT ports (that's all a crossed cable does, after
all), but with three (as in the so-called passive hubs) you would end up
with *everything* connected together -- or you need some electronics to
isolate and filter (as in a proper hub/repeater). The passive units don't
have that, so the two RJ45 sockets can each talk/listen to the RJ45 cable,
but not to each other.
There's a similar problem connecting three or more AUI ports; it's further
complicated by the collision detect, which is normally done in the
transceiver. Now if it were a multiport repeater, I'd have expected all
the AUI connectors to be the same gender (since they'd be functionally
equivalent). But the fanout units have one of a different gender, so I
suspect that either they use a similar scheme to the passive hub I
described, or they actually have more electronics than a normal repeater
(or at least arranged rather differently), in order to get the signals in
the right arrangement to drive a transceiver instead of a drop cable to
another AUI.
--
Pete Peter Turnbull
Dept. of Computer Science
University of York
Received on Wed Mar 22 2000 - 18:37:35 GMT