Re: On two clock issue
Rich,
Just a note of clarification. If the receive clock is separate from the system clock,
what controls the transmit clock. Does each link transmit bit rate provide the clock
to the other system's receiver? Would this tend to cause free running and separate
transmit and receive data rates? How do existing OC48C POS interfaces use line timing
from the carrier LTE to control receive as well as transmit speeds? If 802.3 were to
be used on anything other than enclosed LANs, what would be available to provide line
timing? Are there alternatives?
Thank you,
Roy Bynum
MCI WorldCom
Rich Taborek wrote:
> Devendra,
>
> We're taking a few too many tangents here. To bring everyone else up to speed, the
> root issue is a MAC/PLS rate of 10.0 Gbps vs. 9.584640 Gbps. I asserted that a
> 9.584640 Gbps would complicate the clock design of Ethernet equipment due to the
> requirement to support a rate which is not a multiple of slower Ethernet MAC/PLS
> local clocks.
>
> Receive clocks are separate from local clocks. I believe that most Ethernet
> equipment supporting multiple interfaces DOES NOT use the receive clock from any
> interface as its local clock.
>
> Devendra Tripathi wrote:
>
> > Rich,
> >
> > One clarification here. The use of two clocks is almost always there.
> > Ofcourse
> > some designs carry the retreived clock all the way to host fifo interface
> > and some
> > terminate it early. I am doubtful, though, if anyone uses that clock for
> > transmit
> > purpose. Because in that case, the reference clock (which is taken as transmit
> > clock by PHYs) also has to be retrieved one, thus making a loop in the PLL.
> >
> > Thanks,
> > Tripathi.
> >
> > At 10:17 AM 7/19/99 -0700, you wrote:
> > >
> > >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@xxxxxxxxxxxxxxxx
> > >1029 Corporation Way http://www.transcendata.com
> > >Palo Alto, CA 94303-4305 Alt email: rtaborek@xxxxxxxxxxxxx
> > >
>
> --
>
> 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@xxxxxxxxxxxxxxxx
> 1029 Corporation Way http://www.transcendata.com
> Palo Alto, CA 94303-4305 Alt email: rtaborek@xxxxxxxxxxxxx