>PS: As an electronic/electrical engineer, I find it very hard to believe
>hubs are half-duplex.

Why?  If it helps, just think of an N-port hub as N indiviudal
UTP-to-coax repeaters, with the coax sides connected to a single
shared coax.  All squished into a tiny box via the magic of VLSI.

The internal shared bus does away with the transcievers,
but you get the point.