Re: [802.3EEESG] The paradox of lower speed operation
All,
I agree with Hugh.
His observation of the presented matter substantiates my initial concern
that we can get focused on a means of "lowering speed" a bit too much
and not on the objective "lowering power". I would emphasize that its
important to look at the overall system (aka: Network) with the
objective of lowering power consumption and then work to the means for
doing that in a methodical way.
It seems to me that we have three key areas to concentrate.
A) A system-wide means for communicating the ability, the level of need,
and the timeframes required for assuming lower/higher power states.
B) A link-wide (point-to-point) means for communicating the power state
and providing a "heads-up" that higher capacity is required.
C) A PHY specification for lowering power during low-utilization
periods. This may include specification of B at the PHY level.
Some of these items (A?) may be outside the scope of 802.3 but not
outside of 802 and we should consider how to address that.
I would also note that some PHYs may not need a "lower power" mode of
operation because their active power utilization is low enough that the
effort to reduce it would not be justified by the savings offered.
Dan
-----Original Message-----
From: Hugh Barrass [mailto:hbarrass@CISCO.COM]
Sent: Tuesday, November 20, 2007 12:13 PM
To: STDS-802-3-EEE@LISTSERV.IEEE.ORG
Subject: [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.