OT: cat 5 cable, ethernet, connection jacks

[Lennart Sorensen]

On Thu, Mar 24, 2005 at 12:46:17PM -0500, Henry Spencer wrote:
> 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. 

Yeah I remember 10base2 thinwire ethernet.  What a pain.

> 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.)

Just remember, don't use a cross over cable for gigabit.  it won't work
and will drop you to 100TX instead.  Gigabit is bidirectional on 4 pairs
and doesn't need crossover ever.

Lennart Sorensen
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