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[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.