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Re: [802.3_10SPE] Feasibility of 147.3.5 Collision detection


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.

Best regards,

Geoff Thompson

On Feb 7, 2019, at 12:36 PMPST, Hongming An <Hongming.An@xxxxxxxxxxxxx> wrote:

Let me elaborate a little bit more on this:
10SPE uses DME for ac balance and dc zero (PoDL) which transmits 40ns and 80ns pulses in the bus with specified swing, when physical collision happens, two or more nodes transmit and the pulse will added/subtracted. By detecting the pulse height of exceeding the specified swing one can reliably determine if there is a collision. Current standard specifications indeed assure the physical collision detection. Whether detecting with DC level (as 10Base-T) or AC level (as 10Base-T1) is implementation dependent and I do not believe we have reliable detection issue here, at least at PHY level.
Best regards,

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