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Re: Unified PMD vs. Unified PHY


I realize you asked your question to Jonathan, but if you don't
mind I'll try an answer to this.

In support of the WAN, the serial PMDs (and PMAs) must support
a 9.95328 Gbaud rate. I think it was fairly clear from early
on that using an 8b10b encoding for the LAN would require a
12.5 Gbaud rate and that the PMA/PMD for LAN & WAN could not
be identical (as the WAN PMA/PMD doesn't simply scale up in
baud rate).

I believe that is the idea behind the 64b/66b and SLP proposals
as these encodings require 10.3125 and 10.000 Gbaud rates,
respectively. These baud rates are within the range of current
WAN PMA/PMDs to achieve. This means for the serial PMA/PMDs,
a single solution can be generated (or perhaps 2 - longwave
and shortwave) and dialed with an appropriate oscillator to
support the WAN rate (9.95328 Gbaud) or the LAN rate (10.3125
or 10.000 Gbaud).

The PMA/PMD cares little about the content of the data going
onto or coming off of the fiber. The encoding affects the baud
rate in order to account for overhead.

BTW: What is a Gb-Mtr?


Roy Bynum wrote:
> Johnathan,
> I was intending to ask you why you did not ask about unified PMDs
> separate from a unified PHY as part of your survey but did not get a
> chance.  At the 10GEA technical meeting you were very adamant about
> getting consensus for a small set of PMDs.  I agree that having a small
> group of PMDs is preferable.  Having a unified PHY in order to have a
> small set of PMDs may not be preferable.
> The cost of the unified PHY, as presented, so far has been very high in
> the form of lost transfer rate.  As it is, the unified PHY, as
> presented, does not meet the objective to have a 10.000 Gigabit MAC
> data transfer rate (Gb-Mtr).  Separate PHYs, LAN and WAN do meet the
> objectives.  Additionally, one of the scramble encoded WAN PHY
> presentations was able to achieve an average 10.000 Gb-Mtr transfer rate
> by using IPG compression, which can be inferred to meet the 10.000
> Gb-Mtr objective in addition to the 9.548 Gb-Mtr objective.
> A unified PMD set can support the block encoded LAN PHY and the scramble
> encoded WAN PHY, allowing both to meet the 10.000 Gb-Mtr objective.
> This will allow the PMD people to concentrate on the technologies of the
> PMDs with the consideration of a signaling range to support both PHYs.
> It will also simplify the marketing of 10GbE by reducing the confusion
> about distances and fiber types.
> As was demonstrated in some of the previous presentations (SUPI and OIF
> SERDES), it is possible to have unified PMDs without having a unified
> PHY.  If the question had been asked, would it have made a difference to
> separate the issues?  If they are separate issues, as a I believe they
> are, then should the survey be redone with that segregation?  Would this
> have put less pressure on group to have a unified PHY and changed the
> scaling of the responses?
> Thank you,
> Roy Bynum

Benjamin Brown
Router Products Division
Nortel Networks
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