Re: [BP] Question regarding Channels
Joe,
I wanted to touch on the following statement you
made:
Note that when I use the term "line," I'm using it figuratively to refer
to channels that fall within all aspects of the channel adhocs definition of a
compliant channel. I'm not solely referring to the SDD21
line.
This
is the area that I think is the heart of the problem. Where does that line
exist? What makes a channel compliant? Compliant with a signaling
approach? Compliant with an installed base? These questions are
rhetorical. I know that everyone is likely to have a different point of
view of what the line is for being compliant. How can the TF get to 75%
consensus while satisfying the Objectives and 5 Criteria? (Again, another
rhetorical question.) :-)
Thanks,
Brad
Let me finally add my views on the
subject. I haven't been purposefully silent, just swamped lately.
So we have this great debate about
channels above the line and those below. Assuming everyone agrees we
evaluate all the channels above the line, let's look at the reason for also
considering those below the line. The primary reason is to assess that
the signaling methodology has sufficient margin for the given channel
definition. We must do that, but I would claim the starting point for
that is not with use of channels below the line. The standard is likely
to be defined for a BER of e-12, but most system vendors have a backplane
requirement of e-17 or better. We should start by evaluating that the
signaling approach has this amount of margin or some other agreed to metric
(the performance criteria) for channels above the line . That's still
only a starting point. I'm much more concerned about potential channels
that have severe discontinuities that may all be fully above the defined line,
along wi! th xtalk, return loss and other characteristics that are also within
the compliant definition. Any of the signaling methodologies we're
looking at could well have a problem with some potential killer channels
before they have problems with some of the rather well behaved channels below
the line (like several of John's). We need participants to bring in
these potential killer channels as part of validating that we have a robust
signaling methodology as well as a comprehensive channel definition.
Anyone that has a view that looking at channels below the line is a
comprehensive way of evaluating margin is kidding themselves. The well
behaved ones somewhat below the line won't show problems, while the ugly ones
well below the line will simply be dismissed because they're well below the
line.
I don't have a
problem per se with including channels below the line in our simulation and
analysis, and certainly agree we need to do some of it. However I
believe the group has various views as to what it means to include channels
into the signaling adhoc evaluation. In my view, the group should first
evaluate the signaling methodology for channels above the line against it's
metrics. I expect many in the group believe we should look at some
channels below the line for margin purposes, a view I would agree with.
I also believe some in the group have the view that all channels which
are accepted into the signaling ad-hoc need to be solutioned. I don't
agree with this. Solutioning channels well below the line penalizes
those above the line in terms of power, area, complexity (time to market
risk), etc. Issues with definition of where the channel line is drawn
need to be dealt within the channel adho! c, not the signaling. So if
we're going to include these channels into the evaluation set, we need to get
the group in sync as to how they are used relative to the analysis of the
signaling methodology. Options I see are:
- Properly margin the analysis of
channels above the line, and don't consider any channels below the
line.
- Make the channels below the line
available but leave it optional as to whether a vendor includes them in
their simulation set or not. Results from these channels would only be
used if there is a tie-breaker need of results from channels above the line.
I think the group is self-policing enough that all vendors will bring
in results for a reasonable set of these channels, but no one would be
discounted for not doing all of them because the simulation and analysis
does take a considerable amount of time.
- Select a set of channels that are
within a reasonable deviation from the line and require they be used as part
of the evaluation set in order to show margin. The problem with this
approach is that it requires us to draw another line. Need I remind
anyone that we haven't succeeded in drawing a first line yet?
- Require that all channels be used
in the evaluation, but somehow weight the results of ones below the line
such that they won't overly influence the evaluation and lead to a
methodology that is way overdesigned to the objectives and therefore
penalizes systems with additional power, area, cost, and risk. To
propose a specific mechanism to do this we'd first need to complete the work
of defining the base evaluation metrics. I believe we'd also need to
do #3 because I expect some channels would be weighted differently than
others. Needless to say, this would be a difficult and cumbersome
approach.
- Require that all channels be used
in the evaluation, and require that a signaling methodology be identified
(or perhaps invented) that would adequately solution all channels.
My preference is option 2. I'm
deadset against option 5. Note that when I use the term "line," I'm
using it figuratively to refer to channels that fall within all aspects of
the channel adhocs definition of a compliant channel. I'm not solely
referring to the SDD21 line.
Thanks, Joe
Joe Abler
abler@us.ibm.com
IBM Microelectronics Division
919-254-0573
Technical Marketing & HSS Applications
919-254-9616 (fax)
3039 Cornwallis Road
Research Triangle Park, NC
27709