Robert11 wrote:
Can someone please explain the major differences between simplex, half
duplex, full duplex, etc. ?

See
http://eserver.bell.ac.uk/mirrors/dc100www/dc_014.htm


What is there inherent in the theory that prevents two way conversation,
like on a phone line, in the simplex mode (such as in maritime VHF/fm ship
to shore conversations) ?

There is nothing inherent in the theory that prevents this. It's just
part of the communication system design.

RK