Re: Unified PMD vs. Unified PHY
Roy
I guess at present the IP datagrams on internet is default maximum size of
576 octets. From this information, an average of might be 500 bytes ( in place
of 400) can be assumed for overhead recovery that can be achieved with frame
stuffing with IPG compression SONET overheads.
It is always appropriate to sent the small datagram to avoid unnecessary
fragmentation at intermediate gateways.
Pankaj
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
> > -----------------------------------------