Re: Unified PMD vs. Unified PHY
Roy,
Let's please keep this on the reflector so everyone can follow
along with the discussion. This way, others with similar concerns
or questions won't be kept in the dark.
A question has been raised regarding how tightly coupled the
XAUI and 64b/66b encodings are or need to be. Several people,
including me, have voiced the opinion that there shouldn't
be any requirement that 64b/66b uses the encoding of XAUI.
As for the expense in transfer rate, I'm a little confused. I
believe Howard Frazier pointed out that over WAN, the 64b/66b
encoding scheme is somewhat less efficient (3%?) than a
scrambled encoding. I agree this is an issue worth discussing
but it is a PCS issue, not a PMD one.
Look at a serial PHY. From the MAC to the PCS is an XGMII.
Some implementations may choose to extend this XGMII using
XAUI but this interconnect is optional. The PCS should not
require any features of the XAUI. The PCS encodes the MAC
data from the XGMII then this data is serialized and driven
onto the fiber. The encoding scheme within the PCS is the
factor which determines the required baud rate on the fiber.
Because we chose to make as an objective the support of a
WAN compatible PHY, we chose a baud rate of 9.95328 G for
the PMA/PMD. To share this PMA/PMD with serial LAN solutions
(in order to reduce the number of discreet PMA/PMDs in the
standard), we'd like to choose an encoding scheme for the
LAN which shares this baud rate (or something close enough
that works). We're kind of working this problem backwards.
We'd also like to have a common encoding scheme (or as
common as possible) between the WAN and the LAN. For both
of these reasons, we're looking at 64b/66b and scrambling.
Both of these can support a common baud rate necessary to
reduce the number of PMA/PMDs and a common encoding scheme
necessary to support the results of Jonathan's survey.
Ben
Roy Bynum wrote:
>
> Ben,
>
> Gb-Mtr is an acronym that I created because I quickly got tired of
> repeatedly spelling out "Gigbit MAC transfer rate". The real question was
> not relative to the baud rate of a LAN PMD vs a WAN PMD, but the confusion
> that has been introduced by the effort to "unify" the PHY. XAUI/64B66B
> encoding makes XAUI a requirement, and efforts to reduce the PMD rate to a
> single common is going to be very expensive in transfer rate. By abandoning
> the "Hari" based 8B10B block encoding, the frame stuffing proposals by
> Nortel and Lucent give the ability recover much if not all of the MAC
> transfer rate.
>
> Johnathan has been using the object of having common PMDs as the reason for
> supporting a PHY that provides a specific vendor the ability to maintain the
> 8B10B to be required at the MAC chip. The issue is to segregate the issue
> of common PMDs from that of a common PHY, so that the requirement for 8B10B
> can be released.
>
> Thank you,
> Roy Bynum
>
> ----- Original Message -----
> From: Benjamin J. Brown <bebrown@xxxxxxxxxxxxxxxxxx>
> To: 802.3ae <stds-802-3-hssg@xxxxxxxx>
> Sent: Sunday, March 12, 2000 3:27 PM
> Subject: Re: Unified PMD vs. Unified PHY
>
> >
> >
> > Roy,
> >
> > 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?
> >
> > Ben
> >
> > 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
> > 1 Bedford Farms,
> > Kilton Road
> > Bedford, NH 03110
> > 603-629-3027 - Work
> > 603-629-3070 - Fax
> > 603-798-4115 - Home
> > bebrown@xxxxxxxxxxxxxxxxxx
> > -----------------------------------------
--
-----------------------------------------
Benjamin Brown
Router Products Division
Nortel Networks
1 Bedford Farms,
Kilton Road
Bedford, NH 03110
603-629-3027 - Work
603-629-3070 - Fax
603-798-4115 - Home
bebrown@xxxxxxxxxxxxxxxxxx
-----------------------------------------