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[BP] Question regarding Channels


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:
  1. Properly margin the analysis of channels above the line, and don't consider any channels below the line.
  2. 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.
  3. 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?
  4. 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.  
  5. 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
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