Re: [802.3AZ] EEE - Negotiation clarification
Sujay,
The first part of your question covers the PHY level Tq/Tr negotiation.
The details of this are still being settled for each PHY type so you
need to look at the definitions for the specific PHYs. The general
principle is established that PHYs may be developed that require a
relatively short refresh cycle (Tq > Tr) that reduces the energy when in
the low power idle state. This meets the objective of the task force.
However, it has been suggested that some PHYs may be able to support
much longer refresh cycles (Tq >> Tr) and therefore save more energy.
When two PHYs negotiate, if they both support the longer refresh cycle
then it is clearly a good thing that they can both save more energy. If
one of the PHYs needs a shorter refresh period then they must both use
the shorter refresh period as that PHY will need to refresh its transmit
and receive side circuits more frequently. This is the general
principle, as the PHY details are settled the details will become clear.
The second part of your question covers the system level wake up time. I
think you have misunderstood the definition in the draft.
Each host sends two numbers to its link partner (via LLDP).
The transmit side wake time (transmit Tw) is the longest time that the
host can wait after deasserting LPI, before sending data. This is
typically governed by the transmit side buffering.
The receive side wake time (receive Tw) is the receiver would like to
use for the wake-up process. In other words it is the time that he
receiver is requesting its link partner to wait after deasserting LPI
before sending data.
As you can see, this makes a total of 4 numbers exchanged. These numbers
resolve to 2 resolved transmit wake times (resolved transmit Tw) - one
for each host. For each host, its resolved transmit wake time is the
smaller of its own transmit Tw and its link partner's receive Tw.
Each host is now bound by its own resolved transmit Tw: it must wait for
that amount of time after deasserting SPI before sending data.
Additionally, each host will know how its link partner has resolved and
so it will know how much wake-up time it will have between when the link
partner deasserts LPI and when it can expect to receive data; it should
choose its sleep mode accordingly.
In your example
A - Transmit Tw - 10 ms, Receive Tw - 20 ms B - Transmit Tw - 10 ms,
Receive Tw - 20 ms
(these cannot be the default value for the PHY type as the PHY has only
1 default Tw)
Host A resolves (own transmit Tw = 10ms) vs (partner receive Tw = 20ms)
- resolved transmit Tw = 10ms
Host B resolves (own transmit Tw = 10ms) vs (partner receive Tw = 20ms)
- resolved transmit Tw = 10ms
Another,, more interesting example
A - Transmit Tw - 20 ms, Receive Tw - 20 ms B - Transmit Tw - 10 ms,
Receive Tw - 40 ms
Host A resolves (own transmit Tw = 20ms) vs (partner receive Tw = 40ms)
- resolved transmit Tw = 40ms
Host B resolves (own transmit Tw = 10ms) vs (partner receive Tw = 20ms)
- resolved transmit Tw = 10ms
I hope that is clear.
Hugh.
-----Original Message-----
From: sujay gupta [mailto:sujay.ietf@xxxxxxxxx]
Sent: Friday, October 03, 2008 9:43 AM
To: STDS-802-3-EEE@xxxxxxxxxxxxxxxxx
Subject: [802.3AZ] EEE - Negotiation clarification
Hi,
I have a few doubt's when it comes to the Negotiation part as in the
802.3az 0.9 draft.
Would be grateful for any clarifications provided.
1.) It is proposed that the systems exchange Tq/Tr supported in AN, and
the lowest is settled for;
For ex;
A ------ B
Tq/Tr(A) best case advertised is 'lowest energy' -> i.e. (Tq >> Tr)
Tq/Tr(B) best case advertised is 'reduced energy' -> i.e. (Tq > Tr)
And they both settle for Tq/Tr(B) , "to ensure lowest common value to
ensure robust and quality link" .
Now as the Tq/Tr parameters are useful only for the peer link partner,
what is the benefit in making my peer choose a ratio which is not as
energy efficient as what I may best support, just on the basis that my
own Tq/Tr ratio cannot be as good as my peers.
How does that effect "robustness" & "link quality"??
In my opinion considering the asymettric nature of EEE ( ie only the TX
triggers it not depending on the peer TX), it only effects the timers
running on my side for the benefit of the peer. And gives an overall
more efficient system rather than what would come by ensuring "lowest
common value"
2.) In the LLDP negotiation, we resolve the Transmit Tw ( section
93.4.2.3 ) , to be the "minimum" of local Transmit Tw and the
received(from the link partner) Receive Tw. And the local device shall
wait for the time indicated by the Resolved time after deasserting from
LPI and sending data packets.
For ex:
A - Transmit Tw - 10 ms, Receive Tw - 20 ms B - Transmit Tw - 10 ms,
Receive Tw - 20 ms
( These values exchanged are default values for the given PHY)
Now post LLDP exchange;
On A;
Resolved Tw, cannot be 10ms, as the peer Tw is at least 20ms, which is
contrary to the statement of being minimum.
Thanks,
-Sujay