RE: Unified PMD vs. Unified PHY

["Rogers, Shawn" <s-rogers@xxxxxx>]

Title: RE: Unified PMD vs. Unified PHY

Well stated Ben.

Shawn

-----Original Message-----
From: Benjamin J. Brown [mailto:bebrown@xxxxxxxxxxxxxxxxxx]
Sent: Tuesday, March 14, 2000 8:04 PM
To: Fred Weniger; 802.3ae
Subject: Re: Unified PMD vs. Unified PHY

Fred,

I believe you are right. A maximum length packet (VLAN=1522)
takes up 98.7% of the bandwidth when transferred at the
maximum rate (1522/1542). At 10.0 Gbps MAC transfer rate,
this would require 9.87 Gbps raw data rate (assuming you
could compress both the IPG and preamble - I don't recall
what the plan was). Since the WAN compatible PHY can provide
no more than 9.58464 Gbps, you are correct in stating that
rate adaption is still required.

I think Roy's intent was that by using IPG compression and
scrambling, you can achieve a higher throughput rate using
a WAN compatible PHY than you can using 64b/66b encoding.
I agree with this statment. However, it doesn't solve all
the problems. I think his concern is that the WAN compatible
PHY is already at a 4% disadvantage with respect to the
LAN PHY and another 3.125% is less than desireable.

LAN has the advantage of being able to increase the baud
rate in order to maintain the bit rate. The WAN compatible
PHY has a fixed baud rate so the higher the encoding
penalty, the worse the overall throughput.

Ben

Fred Weniger wrote:
>
> Hello Ben,'
>
> It sounds great if this could work, but I thought that even if you
> completely remove IPGs, a succession of maximum length packets will exceed
> the 9.58464 Gb/s rate required for existing WAN compatibility.  You still
> need a throttle mechanism for the MAC, don't you?  (even if so, this is not
> a deal-breaker as far as I'm concerned)
>
> Regards,
>
> At 09:43 AM 3/14/00 -0500, you wrote:
>
> >Roy,
> >
> >I agree with your numbers. However, this is a PCS issue (encoding)
> >and is not a PMA/PMD issue. 64b/66b is one example of a unified
> >PHY, it is not the only one on the table. Why couldn't a unified
> >PHY support a scrambled scheme over the serial LAN (Lucent's SLP?)
> >then also do IPG compression when sent across the WAN. I don't
> >see anything wrong with this being pursued. I have no idea if
> >there is sufficient support for such a thing in the standard but
> >the idea is not out of the question (or shouldn't be until July).
> >
> >Ben
> >
> >Roy Bynum wrote:
> > >
> > > Ben,
> > >
> > > The expense in transfer rate is an addtional 3% above the ~4% of the SONET
> > > framing.  This makes the total bandwidth expense of the Unified PHY
> > close to
> > > 7%.  This is almost half of the overhead cost of ATM.
> > >
> > > With the proposal of IPG compression in the PHY, most of the ~4%
> > overhead of
> > > the SONET framing can be recovered.  The overhead recovery will be more
> > > effective with small frames than with large frames, but I believe that it
> > > will average out.  At present, I have been told that the average IP
> > datagram
> > > on the Internet is 380 bytes.  This is the same as it was two years ago, so
> > > it does not seem to be shifting very much.  From this information, an
> > > average of 400 bytes can be somewhat safely used to determine the average
> > > overhead recovery that can be achieved with frame stuffing as proposed by
> > > Nortel and Lucent.  With a reduction of the IPG by 10 bytes, using an
> > > average 400 byte frame (with current IPG, 420bytes), 2.3% average overhead
> > > recovery can be added to the MAC transfer rate.
> > >
> > > With IPG recovery using frame stuffing, the overhead cost of the WAN phy
> > > becomes ~1.7%. Compared to the ~7% overhead of the 64B/66B proposal,
> > that is
> > > a difference of 6.3%.   This makes the cost of the unifed PHY at least 6.3%
> > > greater than the seperate WAN PHY.  I think that the original
> > compromise and
> > > the objectives as stated are correct, there needs to be seperate LAN
> > and WAN
> > > PHYs.
> > >
> > > Thank you,
> > > Roy Bynum
> > >
> > > ----- Original Message -----
> > > From: Benjamin J. Brown <bebrown@xxxxxxxxxxxxxxxxxx>
> > > To: 802.3ae <stds-802-3-hssg@xxxxxxxx>
> > > Sent: Monday, March 13, 2000 8:50 AM
> > > Subject: 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
> > > > -----------------------------------------
> >
> >
> >--
> >-----------------------------------------
> >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
> >-----------------------------------------
>
> Fred Weniger

--
-----------------------------------------
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
-----------------------------------------