[802.3EEESG] The paradox of lower speed operation
All,
For those of you that missed the discussions in Atlanta (and some of you
who didn't :-) an interesting paradox emerged for PHY proposals that
offer very fast transition from a reduced speed mode back to the full speed.
If the transition time is less than the time to transmit a maximum
length frame at the lower speed, then it's better not to send data at
the lower speed.
This is because a PHY that is running at a lower speed must allow a
frame in progress to complete before changing to the higher speed
whereas a PHY that does not keep the lower speed channel active may
transition immediately. This effect clearly becomes more pronounced if
we consider compatibility with jumbo or super-jumbo frames.
In the examples that were presented to the TF we saw two almost
identical proposals for PHYs that could resume 10Gbps operation in
~10uS. One of them defined a gigabit "subset mode" the other defined a
deep sleep idle mode. It is interesting that the transition buffering
requirement of the PHY that keeps the gigabit channel alive is greater
than that of the PHY that goes into deep sleep as a max length frame
requires 16uS to complete at 1Gbps. Similarly, it is better to use a
resume time of less than 100uS than to drop back to 100Mbps.
I think that this analysis, along with the very fast transition
proposals, changes the nature of the task force away from the "PHY speed
changes" that we had been concentrating on up till now.
Hugh.