OT: cat 5 cable, ethernet, connection jacks

[Henry Spencer]

On Thu, 24 Mar 2005, Matt Price wrote:
> ...Is it at all possible to use
> the RJ45 jack as a simple junction connecting two pieces of cat 5
> cable? ...
> I imagine this is impossible, or people would do it all the time
> instead of using hubs...  anyway, if someone can explain to me at
> least why it doesn't work, that'd be a help.

Alas, at the wire level, 10BaseT -- the modern twisted-pair/RJ45 flavor of
Ethernet -- is strictly point-to-point, not party-line.  The signaling
scheme assumes that that there is a hub on one end and one (1) computer on
the other.  There's no provision, none at all, for sharing access to the
wire between two computers.  For example, when the line is idle, each end
regularly emits "link test" pulses to continuously verify that the wiring
hasn't gone bad, and expects to receive such pulses from the other end...
so which of the two computers would emit them?  For another, each end
"terminates" each wire pair properly to avoid signal reflections, and a
third termination in the middle will just make a horrible mess. 

All the provisions for networking an arbitrary number of computers
together are in the hub, not in the wiring. 

The above also applies, even more strongly, to the higher-speed versions
like 100BaseT and beyond.

(The protocols *are* designed to be symmetrical, so the computer end and 
the hub end are just mirror images of each other, and a simple crossover
cable can give you a computer-to-computer connection.  But that's still
strictly point-to-point, one device on each end of the cable and none in
the middle.)

                                                          Henry Spencer
                                                       henry-lqW1N6Cllo0sV2N9l4h3zg at public.gmane.org

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