Re: 9.584640
Drew,
The complexity I mentioned is relative. For example, an 1/10 GbE product such as
a switch may be implemetable with a single clock if the MAC/PLS data rate is set
to 10 Gbps since the MAC Tx (local) clock may well be 125 MHz. The same would
not be possible for the same 1/9.584640 product. The point being that a MAC/PLS
data rate of 9.584640 simply complicates the life of an Ethernet product
designer, especially one that needs to develop products which support multiple
Ethernet rates.
--
"Perkins, Drew" wrote:
> Rich,
> I find your assertions about the complexity of clocks with a
> 9.584640 rate untrue. In fact I assert that a 9.584640 rate does NOT make
> this more difficult. I don't think that having this clock, or having the
> rate not be a multiple of 1.00000, will be a problem. This was not a problem
> in any of the many switches that I have worked on.
>
> In the end though, argument by emphatic assertion is a waste of time. Real
> proof with credible data is required.
>
> Drew
> ---------------------------------------------------------
> CIENA Corporation Email: ddp@ciena.com
> Core Switching Division Tel: 408-865-6202
> 10201 Bubb Road Fax: 408-865-6291
> Cupertino, CA 95014 Cell/Pager: 408-829-8298
>
> -----Original Message-----
> From: owner-stds-802-3-hssg@majordomo.ieee.org
> [mailto:owner-stds-802-3-hssg@majordomo.ieee.org]On Behalf Of Rich
> Taborek
> Sent: Monday, July 19, 1999 10:17 AM
> To: rabynum@airmail.net; HSSG
> Subject: Re: 9.584640
>
> Roy,
>
> This one's pretty simple. The MAC/PLS clock for 10 Mbps Ethernet is based on
> the
> transport of bits. At 100 Mbps, it's nibbles (4 bits). At 1 Gbps, it's
> octets. At 10
> Gbps, several proposals suggest 8, 16 and 32 bit chunks. The local clock for
> Ethernet
> equipment which performs speed matching is easily derived from MAC/PLS clock
> of the
> slowest interface supported. Deriving a 9.584640 is not so straightforward.
> Reverse
> deriving lower speed clocks is also not so straightforward. The simplest
> solution is
> to use two clocks. This solution increases implementation cost and will
> significantly
> complicate clocking design.
>
> Best Regards,
> Rich
>
> --
>
> Roy Bynum wrote:
>
> > Rich,
> >
> > I hear much about the single system clock issue. In the past, the data
> transport
> > signal clock and the data system bus and processing clock were separate.
> For many
> > systems the data processing would exceed the transport signal in order to
> maintain
> > control. If that is the case, then your argument about a single clock
> does not
> > hold. I could be wrong. Would one of the system implementation people
> please
> > define how a data signal clock is derived in today's systems. Is internal
> 802.3
> > frame processing done at a higher rate than the transport signal rates in
> today's
> > GbE L2 switches? Is there a real issue with a separate transport signal
> rate or
> > even a separate transport data rate?
> >
> > Thank you,
> > Roy Bynum
> > MCI WorldCom
> >
> > Rich Taborek wrote:
> >
> > > Hon Wah,
> > >
> > > Thanks for kicking this issue off again!
> > >
> > > Hon Wah Chin wrote:
> > >
> > > > Perhaps a difficult number to remember, but with the +- 100ppm
> tolerance
> > > > and a bit rate that needs only to fit within about 200ppm of the
> nominal
> > > > SONET number we should be able to choose a round number with 4 digits
> in it.
> > > >
> > > > ---
> > > >
> > > > As I understand the presentations in Montreal on speed,
> > > > a strong advantage of choosing this OC-192 payload rate is
> > > > to transport the signal over SONET OC-192 equipment. This would
> > > > be from a "10Gb/s Ethernet" port out to SONET gear, which is really
> > > > a PMD external interface rather than a definition for the MAC/PLS
> interface
> > > > and data rate.
> > >
> > > In my conversations with several folks on both sides of the issue during
> the
> > > Montreal meetings, I've come to the conclusion that the root reasons to
> select
> > > either a 10 or 9.584640 Gbps are purely ease-of implementation based and
> have no
> > > architectural basis whatsoever. I believe this to be true on both sides
> of the
> > > argument with the choice of one over the other, rendering the
> implementation
> > > (i.e. product cost) of the losing side only slightly more difficult.
> Please
> > > allow me to explain the basis of this contention:
> > >
> > > 1) SONET, and specifically synchronous transport, is legacy in the MAN
> and WAN,
> > > will never be replaced by Ethernet completely or even quickly. Ethernet
> will
> > > make inroads into "green-field" applications, but SONET will be king for
> some
> > > time to come;
> > >
> > > 2) Ethernet, and specifically packet-based transport, is legacy in the
> LAN, is
> > > growing in its dominance in the LAN, and will likely gain market share
> in the
> > > LAN as well as encroach on other non-traditional Ethernet transports
> including
> > > MAN, SAN, and some WAN. I don't include WAN access in WAN. Instead I
> include WAN
> > > access in LAN or MAN;
> > >
> > > 3) The existing WAN infrastructure does a great job of transporting
> Ethernet
> > > packets end-to-end today. However, much protocol conversion and
> equipment to map
> > > between packets and TDM bits exists in mapping Ethernet to the WAN at
> each end.
> > > Considerable savings can be realized by architecting a more seamless
> Ethernet to
> > > SONET connection. This issue seems to be at the root of the 10 vs.
> 9.584640 Gbps
> > > issue.
> > >
> > > 4) There seems to be no intent by either side to consider any other
> changes but
> > > speed as a HSSG objective. Therefore, Ethernet will remain a simple,
> general
> > > purpose, packet-based transport, and SONET will remain a specific
> purpose
> > > (MAN/WAN), synchronous transport no matter which way the decision goes.
> > >
> > > 5) Consider a Ethernet to OC-192 line card (feeding a fiber or
> wavelength) in
> > > operation. Assume that receive and transmit paths are separate on the
> SONET side
> > > and related (i.e. full duplex) on the Ethernet side:
> > > a) Ethernet -> SONET @ 9.584640 Gbps: The Ethernet side can
> continuously feed
> > > the SONET link with no flow control required.
> > > b) Ethernet -> SONET @ 10 Gbps: The Ethernet side must be flow
> controlled to
> > > prevent over-feeding the SONET link
> > > c) SONET -> Ethernet @ 9.584640 or 10 Gbps: The Ethernet side can
> continuously
> > > source SONET data but will flow control or drop packets downstream
> whenever the
> > > network is congested.
> > >
> > > Therefore, the issue boils down to one of implementation of existing
> Ethernet
> > > mechanisms such as 802.3x flow control or a reasonable facsimile on the
> line
> > > card versus complicating the implementation of all Ethernet products
> which must
> > > support a MAC/PLS rate which is not a multiple of 10. These
> implementation
> > > difficulties include multiple clocks which may "beat" against each
> other, not
> > > being able to easily feed 10 slower links into one faster one, and
> numerous
> > > other difficulties which are best listed by Ethernet product
> implementers.
> > >
> > > My intention is not to make light of the problem but rather to agree
> with a
> > > solution direction along the line proposed by Dan Dove of HP at the
> Montreal
> > > meeting. I believe that Dan's general direction was to tradeoff a simple
> > > architectural change with respect to MAC operation to enable cost
> effective 10
> > > Gbps to SONET implementations. I don't particularly agree with resolving
> > > implementation cost issues between two dominant legacy protocols by
> tweaking
> > > with the underlying architecture, but I'll raise my hand in support of
> this
> > > solution to the problem.
> > >
> > > Such a solution would enable the implementation of a 10 Gbps Ethernet to
> SONET
> > > OC-192 line card without requiring a full MAC.
> > >
> > > I'll let Dan fill in the details of his proposal so I don't get it wrong
> if it
> > > is still applicable.
> > >
> > > Best Regards,
> > > Rich
> > >
> > > --
> > >
> > > > Given a raw continuous bit stream at the PMD, some scheme for
> > > > framing packets would be needed. 10M used a carrier, 100M used
> coding,
> > > > 1000M used coding. Using coding where the PMD speed is fixed at
> 9.58Gb/s
> > > > would mean a further speed reduction (probably 10-20%) at the MAC/PLS
> > > > interface. The discussion at the meeting has already started to
> consider ways
> > > > of
> > > > reducing the useful throughput at the MAC/PLS below the data clocking
> rate.
> > > > An
> > > > alternative framing scheme presented to HSSG, which has a smaller
> throughput
> > > > reduction, requires a packet length header -- a departure from
> previous 802
> > > > practice.
> > > >
> > > > In considering the advantage of leveraging SONET OC-192 transport
> > > > we should also consider the issues which come up in actually getting
> > > > the hoped-for benefits. It would also be worthwhile to carefully
> consider
> > > > what volume forecasts for the OC-192 components can be documented, in
> > > > evaluating the advantage to be gained. Counting IEEE802.3 10Gb/s data
> > > > ports (however the definition works out) to get 2 million ports sounds
> > > > good, but I found the forecast of 2,000,000 OC-192 ports in 2000
> rather
> > > > surprising.
> > > >
> > > > -hwc
>
> -------------------------------------------------------------
> Richard Taborek Sr. Tel: 650 210 8800 x101 or 408 370 9233
> Principal Architect Fax: 650 940 1898 or 408 374 3645
> Transcendata, Inc. Email: rtaborek@transcendata.com
> 1029 Corporation Way http://www.transcendata.com
> Palo Alto, CA 94303-4305 Alt email: rtaborek@earthlink.net
--
Best Regards,
Rich
-------------------------------------------------------------
Richard Taborek Sr. Tel: 650 210 8800 x101 or 408 370 9233
Principal Architect Fax: 650 940 1898 or 408 374 3645
Transcendata, Inc. Email: rtaborek@transcendata.com
1029 Corporation Way http://www.transcendata.com
Palo Alto, CA 94303-4305 Alt email: rtaborek@earthlink.net