----- Original Message -----
Sent: Wednesday, January 28, 2004 6:27
PM
Subject: RE: [bp] BER Objective for
BPE
Jeff,
Although you addressed this
directly to me, I assume that you put this topic to the reflector for general
discussion.
As you know, this proposal needs to
be brought forward to the study group and agreed to by at least a 75% majority
before it become an objective. I encourage you to make such a motion at
our next meeting if you feel that this is a necessary change.
Since you directed the message to me,
I'll also make some remarks as a citizen of 802.3 and not SG
chair:
My personal opinion is that it
would help if you elaborated on what this objective would mean in practice
(especially for those who do not participate in the OIF).
I am personally aware of system
requirements for 1E-15 and below, but as you imply in the phrasing of the
proposed objective, it is prohibitive to measure such low error rates.
The specific language you use implies that we will define a system that is
guaranteed to work at a BER of 1E-15 but we will only verify its performance
by measuring the BER to 1E-12. Giving this topic only limited thought, I see a couple
of ways to approach this:
1. Define the system (transmitter, channel,
receiver) such that simulated performance is 1E-15, but in compliance test
performance is only verified to 1E-12. Presumably, parametric values
specified in the standard include margin to account for the difference in what
was simulated at 1E-15 and what can actually be measured 1E-12. For a
quick example, consider random jitter. If the link is defined such the
peak-peak random jitter at 1E-15 is 0.15UI, then the specification would
presumably incorporate a specification for peak-peak random jitter at 1E-12 of
0.133UI.
2. Define a system for
performance to 1E-15, and as part of compliance test rely on extrapolation of
measured data (for example, bathtub curves along the vertical or horizontal
axis) to derive values for 1E-15 that can be compared with the specified
values.
One interesting observation regarding (1) is that, if we
were to have a closed eye system (i.e. the solution relies heavily on
receiver-based equalization, in that the eye at
the receiver input is closed), I would expect receiver compliance to be
based on "operation at a given BER when driven by compliant driver through a
compliant channel." This is the model that has been employed in
100/1000M twisted pair links and in 10GBASE-CX4. Using the model defined
in (1), we would drive a compliant channel with a compliant driver and ensure
that receiver BER performance was better than 1E-12. I would argue that
this says nothing about the receiver's ability to operate at 1E-15. Some
additional impairment (worse than
worst-case transmitter or channel) needs to be included to ensure that the
appropriate margin is in the receiver design.
With regards to (2), we would run
the risk of having a hole in the specification (solutions that are both perceived to be compliant but
are not interoperable) since error floors below 1E-12 would be undetected and
extrapolation would provide misleading results.
In summary, I do not debate that some applications would
like to see BER exceed 1E-12 (and in some cases, exceed 1E-15). With
regards to your proposed objective, I think it would be useful to get a better
understanding of how a Backplane Ethernet standard could be judged to have met
such an objective or not.
I think members of the OIF
community who also participate in the SG could share some insight here. I
welcome them to do so.
Thank
you,
-Adam
Adam,
During the recent IEEE Backplane Ethernet
(BPE) study group (SG) interim meeting in Vancouver, BC. Canada a
BER objective of 10EE-12 was voted into the initial set of BPE objectives by
a vote of 32-yes / 3-no / 1-abstain.
I was one of the three negative votes. My
reason was simple; because these future 10G backplane links in a
modular chassis are critical network links and the environment (inside
the box) is much more noisy than external links.
I felt the same approach as the OIF CEI
implementers agreement makes sense; they require a BER of 10EE-15 per lane
with the test requirement of 10EE-12.
I suspect that system vendors will have
the same concern with this 10EE-12 BER objective for
backplane applications and voice their concerns when this
objective reaches the 802.3 WG so it might be better to fix this
objective now rather than later.
I would propose a new BER objective, here's
some suggested text:
"Support a BER of 10EE-15 or better with the test requirement set at
10EE-12".
Cheers,
- Jeff Warren