# Re: [10GBT] Summary of issues with PAM12

```Sailesh,

Not sure what you had in mind, since this is not a mathematical
coincidence ...

If you check your math more carefully, at 825MHz the optimum PAM12 could
carry at most 64B/67B. 64/69 would require 840MHz (actually higher if
you include some LDPC framing and PHY control overhead such as THP

Cheers :)
Jose

-----Original Message-----
From: stds-802-3-10gbt@IEEE.ORG [mailto:stds-802-3-10gbt@IEEE.ORG] On
Behalf Of sailesh rao
Sent: Wednesday, August 18, 2004 3:43 PM
To: STDS-802-3-10GBT@listserv.ieee.org
Subject: Re: [10GBT] Summary of issues with PAM12

Hugh,

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)

Regards,
Sailesh Rao.
srao@phyten.com

>From: Hugh Barrass <hbarrass@CISCO.COM>
>To: STDS-802-3-10GBT@LISTSERV.IEEE.ORG
>Subject: Re: [10GBT] Summary of issues with PAM12
>Date: Wed, 18 Aug 2004 09:42:26 -0700
>
>Hal,
>
>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,
>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 :-)
>
>Hugh.
>
>
>
>Roberts, Hal wrote:
>
>>All,
>>
>>Sailesh provides a nice compact list of (his) issues with regard to
PAM12.
>>I
>>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 of "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