Re: 9.584640
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
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