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Re: [BP] Relationship to cabling standards



Brian,
 
Interesting points.  One big difference to also remember is that 802.3 doesn't write cabling standards.  I've been under the impression that .3ap is not going to write a backplane specification (materials, connectors, etc.) either.  If that's true, then it is fair for the Task Force to assume that someone well-versed in backplane technologies can use the channel models in the standard to create any implementation that they desire.  For example, a specified channel model with a set of parameters for IL, RL, NEXT, FEXT, etc. permits implementers to design backplanes with different materials and layouts to achieve various reaches and cost requirements.  A PHY designed to operate over the defined channel should work on each implementation that satisfies the channel model.
 
I think the question comes back to what should that channel model be.  If there is a channel model for 1G, then why doesn't the Task Force use that model as a starting point?  It appears that the reason some do not wish to use that model is that it constrains some of the choices of a PHY device.  Does the selection of a channel model impact the broad market potential?  That will be question that the Task Force will have to be able to answer when they get to Working Group and Sponsor Ballot.  If the channel model for 10G prevents the use of any 1G or XAUI backplanes, then some may believe that limits the market potential.
 
Cheers,
Brad


From: owner-stds-802-3-blade@listserv.ieee.org [mailto:owner-stds-802-3-blade@listserv.ieee.org] On Behalf Of Brian Seemann
Sent: Friday, October 08, 2004 10:23 AM
To: STDS-802-3-BLADE@listserv.ieee.org
Subject: [BP] Relationship to cabling standards

I believe there are important differences between the emerging 802.3ap standard and existing cabling standards.
  1. Cable, by its physical nature, does not have the potential to have stubs.
  2. Cable manufacturers are further prevented from creating ill-behaving electrical channels via the existence of cabling standards.
  3. The variation in electrical characteristics of cables will be very small compared to the electrical variations in the universe of backplanes.  Consequently, the problem space for cable channels is very narrow while the problem space for backplane channels is very wide, and has more dimensions.
  4. A particular implementation is not difficult to design signaling for, it's the huge universe of bad and weird channels that causes the problem for signaling.  Each one may represent 1% of the market, but putting an operating envelope around ALL of them is what is going to cost the industry in terms of die area, power, time to market and widescale availability.
  5. We are expecting to see the number of 802.3ap transceivers-per-IC sometimes appoach 100 transceivers.  The "costs" mentioned above will be multiplied by this number.  What might have been done for something like 1000BaseT, where a couple transceivers/IC might be seen, is not appropriate here.
BrianS