The presentation highlighted the idea of forward vs. backward compatibility. I’m not sure I understood the distinction between these. At the moment my assumption is
Backward compatibility = compatibility with 1000BaseT1 autoneg (Clause 73)
Forward compatibility = compatibility with any future single-pair 802.3 PHYs.
For me, Backward compatibility is problematic for at least two reasons:
(i) The
signaling frequency is very ill-suited to the 1000m channel response. Slide 7 of the above presentation acknowledges the Clause 73 signalling ‘Will be severely attenuated by 1000 m link segment (>25dB)’ but
I think even this understates the problem, Slide 5 ofhttp://www.ieee802.org/3/cg/public/Jan2017/McClellan_3cg_01a_0117%20xBASE-T1%20Autoneg.pdf shows
that roughly 90% of the signal power is above 6.7MHz (0.4fBaud) but our current Link Segment proposal shows an Insertion Loss of 33.8dB at 6.7MHz.
(ii) Assuming
we can achieve reliable reception of Clause 73 signaling over 1000m what happens if two chips which implement both 802.3bp and 802.3cg are connected via 1000m? The autoneg process will resolve to a HCD technology of 1000BaseT1 and then promptly fail to bring
up a link as 1000BaseT1 only reaches to 40m.
Forward compatibility, as I have interpreted it above, seems far easier. We could still re-use a lot of the work in Clause 73 but simply shift the clock frequency down to a
range more appropriate to a 1000m cable. As to what we are negotiating at the moment that would simply be Master/Slave resolution.
The above assumes that the same autonegotiation will be used for the 15m PHY and the 1000m PHY and this may be a mistake. I can certainly see how Clause 73 autoneg would make
a lot of sense for a .cg PHY intended to meet the 15m objective where two other PHYs (100/1000BaseT1) already exist. I guess this leads us back to the question of whether the 15m and 1000m reach objectives would be met with a single PHY or two different PHYs.