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10BASE-T does not do collision detection by detecting a DC level as you assert below.
10BASE-T is a link segment based system with separate media for transmit and receive.
All you have to do to detect collision is AND receive_signal_detect with transmit_enable.
Both of those are based on well formed single source signals so the detection is trivial.
Doing collision detection on a mixing segment (i.e. a multi-station bus) is what is done on 10BASE5, 10BASE2 and 10BASE-FP.
That is what is required of 10BASE-T1.
10BASE5 and 10BASE2 do it on a current basis since the AC signal characteristics/waveform cannot be depended on when multiple asynchronous sources are involved.
We tried very hard to do AC signal based collision detect in early versions of 10BASE-FP and we were never able to prove that it fully worked and were able t o prove cases where it didn't work pretty easily.
We finally allowed a version of FP to go forward with a unique code in each stations preamble but since standards based 10BASE-FP didn't make it into production I wouldn't be willing to bet it was reliable either.
I don't think we have rigorously proved technical feasibility for collision detect of 10BASE-T1. When we had a methodology in the draft it generated enough ballot comments that it was removed and left as "implementation dependent". I don't think we have had any proof that giving each implementer free rein can produce a reliable signal.
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