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Re: [10GBT] Summary of issues with PAM12


Let's calculate the overhead due to the hole in the PAM12 constellation once
again. As you know, I've variously stated that the "optimum" PAM12 symbol
rate should have been in the neighborhood of 780Ms/s. In comparison with the
proposed symbol rate of 825Ms/s, the overhead is

(825-780)/780*10000 Mb/s = 577Mb/s

In contrast, the overhead due to the 64B/65B encoding is

1/64*10000 Mb/s = 156.25Mb/s

Therefore, the hole in the constellation is equivalent to doing a 64B/69B
encoding in PAM12. (No flames please, this is a mathematical coincidence and
no double entendre intended)

Sailesh Rao.

>From: Hugh Barrass <hbarrass@CISCO.COM>
>Reply-To: "IEEE P802.3an" <STDS-802-3-10GBT@LISTSERV.IEEE.ORG>
>Subject: Re: [10GBT] Summary of issues with PAM12
>Date: Wed, 18 Aug 2004 09:42:26 -0700
>I don't understand why the "hole in the constellation" is seen as an
>issue. It causes the PAM-12 to be less "efficient" than it could be,
>just like the padding bits and encapsulation overhead. The net result is
>that the proposal using PAM-12 needs a symbol rate of 825Mbaud where a
>lower clock rate might be used if the efficiency was better. However, if
>the comparison is made using that proposal and PAM-12 still comes out
>better then perhaps the "inefficiency" is acceptable. If, on the other
>hand and as Sailesh maintains, the comparison comes out in favor of
>PAM-8 then the PAM-12 proponents might want to look at ways of "trimming
>the fat."
>It would be equally valid to raise the "issue with PAM-8" of "only 12
>bits/baud" and require the PAM-8 fans to address that...
>Personally, I think 10GBASE-T would be best addressed by 4 pair, bonded,
>2BASE-TL on steroids :-)
>Roberts, Hal wrote:
>>Sailesh provides a nice compact list of (his) issues with regard to PAM12.
>>have seen responses to some of these but nothing addressing or summarizing
>>them all.
>>In addition it would be useful (at least to me) to see a similar summary
>>"Issues with PAM8" from a PAM12 proponent. (Unless based on Sailesh's
>>criticisms there are no longer any PAM12 proponents?   ;-)
>>Finally, Sailesh has a good point that a number of his issues have been
>>completely unanswered. I am surprised no one has addressed the 'hole in
>>constellation' issue.

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