Imagine two computers A and B. A has two NICs, A1 and A2. B has B1 and B2. So 4 NICs total. You connect directly A1 to B1 and A2 to B2 with crossover cables. You then route all the traffic from A to B over A1 to B1 and all the traffic from B to A over B2 to A2.
replyWhy do you do all this? To avoid collisions and the loss of effective bandwidth from back-offs.
It only really works with 2 computers because if you add a 3rd, now you need 12 NICs instead of 4 for unidirectional point-to-point connections.
That's not how modern ethernet works at all. A single NIC talking directly to an another one has no collisions ever. Depending on what your channel is, either you have separate wires for the directions, or you are using a hybrid circuit (as in telegraphs, the term is so overloaded it's hard to google). Either way, packets going in one direction never wait for packets going in the other.
replyBut why would you? You don't have collisions since the introduction of full duplex ethernet on both copper/fiber. Kinda sounds like you're confusing half duplex with simplex, or maybe bidi? As a network engineer I've never seen someone ever refer to "simplex ethernet".
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