Jack and Paul
The question is not whether SFP+ can achieve 300 m SR reach similar to
XFP, but how do we get to 10G SFP+
at 2.5x the cost of 1G classic SFP for DCE (Data Center Ethernet) with
max reach of 100 m.
If we can get to 10G SFP+ at 2.5x the cost of 1G at 300 m then the 10G
PAR objective is complete, but how long
do we wait the need is know. But I do know the combination of lower
cost optics with EDC can deliver
the 2.5x cost objective for DCE applications near term. To get to
these cost the transmitter very likely
will not be fully SR compliant and in that case it does not matter if
the reach is 100 or 300 m.
The current assumption in the HSSG is that you can achieve SFP+
limiting performance with 4 or 10 channels without the
use of CDR in the module, with more crosstalk, less optimum layout,
SerDes having more jitter and less tolerance compare
to small port count PHYs, optics ??? You will get small benefit from
reducing fiber reach to 100m but not enough to close the
link budget. As Dan mentioned EDC is becoming an standard feature on
PHYs and we definitely need to leverage it for 40G/100G.
Use of linear interface is an approach that can close the link budget
without the use of CDR in the module, relax the optics
specifications, and the same interface can support passive copper
Twin-ax up to 10m.
Ali
Jack Jewell wrote:
Input
from a transceiver vendor with experience and interest in both serial
and parallel modules:
First, SFP+ achieves
the 300m SR objective with similar
ease as XFP. While a reduced-reach 10G PMD might reduce costs, it's a
retreating approach that requires the customer to purchase higher-cost
transceivers to fill in the reach gap, e.g. for reaches between 100m
and 300m.. A reduced operating temperature range offers immediate cost
savings for some customers without compromising reach and without any
new standards specifications. Use of a linear receiver and EDC and relaxed Tx specs can also
achieve the 300m reach at reduced cost.
For this application, the amount of compensation required is
far less than what is required for LRM.
EDC is becoming widely available, to the point of being a standard
feature of PHY ICs, so the cost of this
EDC is becoming insignificant. Forward-looking standards
efforts will achieve their full impact by making use of EDC.
For QSFP,. Here again, EDC offers great advantage, and
ignoring it would be a mistake. But crosstalk introduces module-level
limitations that might motivate reach reduction or other modification
to the PMD.
Comparing costs, the SFP+ will be lower than QSFP for the
forseeable future (per lane for the
same spec). The only advantage of QSFP over SFP+ is density and possibly simpler cabling. However,
a 12-channel parallel module pair,
e.g. SNAP12, offers greater density than QSFP and 2.5-3X the bandwidth
at <2X the cost.
Jack
Paul,
Regarding SFP+, I am very
familiar with the technology and have been tracking the SFF-8431
development. The architecture re-distribution of cost that SFP+ offers
will have a substantial impact on cost, especially when combined with
the higher density we can achieve with smaller geometry ASICs and
multiport PHYs that will come with it.
As for QSFP, I am less familiar
with whether or not it will provide a cost improvement over SFP+ or be
capable of meeting the existing SR spec. This is something for the QSFP
experts to consider, but like I said, a shorter 10G PMD might be the
avenue to take rather than an identity challenged 40G spec.
Regarding LAG, my conversation
with HP Server architects indicates there are a number of avenues for
improvement of LAG under development.
I cited these areas in my earlier message
and would appreciate them being addressed rather than ignored.
Dan
Dan,
I can't tell how successfully SFP+
and QSFP will be at meeting the existing 10GBASE-S spec. If they can,
a new shorter distance 10G PMD would not be of value. If they can't,
then a new PMD spec may be worth while. Those attempting to implement
these lower cost platforms need to weigh in to provide guidance. In
the event that either the QSFP and/or SFP+ can meet 10GBASE-S specs in
multiple vendor's platforms, or that a new shorter distance spec is
developed that allows lower cost, the performance issues of LAG will
remain. I believe Howard's presentations on LAG have indicated that
improving LAG would not be without compromise, leading me to conclude
that, however improved, LAG performance could not become equivalent to
a 40G pipe. Developing a 40G spec would ensure a solution that
simultaneously addresses these cost and performance issues.
Regards,
Paul Kolesar
CommScope Inc.
Enterprise® Solutions
1300 East Lookout Drive
Richardson, TX 75082
Phone: 972.792.3155
Fax: 972.792.3111
eMail: pkolesar@commscope.com
Hi Paul,
Good points. I was not really expecting to see a significant cost
differential at the PMD although its a good argument that a 100m PMD
would be less expensive. If this is the case, why not do another 10G
PMD focused on lowering the cost of server interconnect? I believe that
would be a smaller project and have a much less significant impact on
100G development.
Thanks,
Dan
From: Paul Kolesar
[mailto:PKOLESAR@SYSTIMAX.COM]
Sent: Tuesday, June 26, 2007 6:16 PM
To: STDS-802-3-HSSG@listserv.ieee.org
Subject: Re: [HSSG] The List
Dan,
thanks for your detailed thoughts and proposals. I appreciate the
points you made regarding the volume effect of 10G components on the
cost comparison. The presentation I submitted for the May interim
looked at the intrinsic cost factors and did not attempt to include
volume in the equation. But volume certainly can be a significant
factor. Your suggestion to look into its impact when comparing 4x10G
LAG to 40G is reasonable, but complicated at the PMD level. As my May
presentation shows there are a few ways to implement LAG on MMF. One
uses the XFP, another the SFP+, still another the QSFP. Today the XFP
is shipping to the 10GBASE-S spec, and supports 300m transmission.
Designs using SFP+ and QSFP will be more challenged to meet this spec
due to jitter, so it remains to be seen how successfully these lower
cost form factors can substitute for the XFP in 10GBASE-S compliant
LAG. However, a reduced distance requirement, such as that stated in
the HSSG objectives, would greatly improve the chances that QSFP will
suffice for "40GBASE-S". So while volume is important, these
unanswered questions on suitability make it impossible from my vantage
point to determine how the volumes for 10GBASE-S will be divided among
XFP, SFP+, and QSFP. And the effects of volume on production costs are
better left to those who manufacture the devices. Perhaps individuals
with such insights will offer some scenarios.
Regards,
Paul Kolesar
CommScope Inc.
Enterprise® Solutions
1300 East Lookout Drive
Richardson, TX 75082
Phone: 972.792.3155
Fax: 972.792.3111
eMail: pkolesar@commscope.com
My fellow colleagues ,
Last week I sent out a list of items that I felt need to be addressed
to ensure that a 40G PAR would be justified. At a subsequent EA
teleconference intended to build concensus in the HSSG, I offered to
review the presentations made in support of 40G Economic Feasibility
and comparing 40G vs 4x10 LAG performance to ensure that I was not
being too harsh in my consideration of the material that was presented.
Over the weekend, I reviewed every presentation I could find on these
subjects so that I could be comfortable that I was not being unfair in
my concerns. Fortunately, it was not a huge task as there are not that
many to review.
After doing so, I found myself less convinced in the validity
of some presentations that were made. This statement is not made to
criticize my colleagues, but to honor the concept of peer review which
requires that we review and criticize, otherwise we might as well just
upload them to a server and forget about them.
Specifically, I disagreed with cost arguments made on the assumption
that 10G cost remains a constant, when in fact I anticipate substantial
reductions in 10G cost over the next few years at a rate much faster
than today due to a few factors;
1) Higher density/lower cost optical form factors (SFP+) allowing
better utilization of switch infrastructural cost and QSFP for NICs.
2) Smaller geometry CMOS allowing higher port densities to work in
synergy with PMD cost reductions.
3) Integration of XFI / SFI interfaces directly into ASICs or
multi-port PHYs driving 10G cost further downward.
4) Higher volumes / commoditization of 10G driving cost down much
faster than the current trajectory.
While 40G can leverage some of these elements, it cannot leverage the
volume that feeds the downward cost spiral. So in 4 years, a 40G switch
port cost is going to be based on low-volume, freshly designed and
un-amortized silicon used primarily for server interconnect, whereas a
10G port cost will be based on amortized, high-volume silicon being
used in a huge array of applications. Having different trajectories,
the relative cost for 40G will be higher than presented. This is true
for 100G as well, but who is arguing a need for 100G based on cost? It
is bandwidth that drives 100G demand.
In addition, I found presentations claiming that LAG was insufficient
to address server I/O bandwidth needs, yet those presentations failed
to address upcoming technology enhancements like TRILL and its impact
combined with I/O Virtualization, perhaps with a physical manifestation
of QSFP and MPO optics which I believe can lead to graceful performance
scaling for servers that does not demand an intermediate IEEE standard.
In other words, activities and technologies are advancing which will
parse server network access into multiple conversations that can then
be put onto a LAG group with much higher than presented performance
levels.
Now, I realize that I am swimming upstream here by asking that the
proponents for "40G now" to complete a task that took the 100G
proponents almost a year to accomplish, in less than 6 months, but then
I am not asking them to do that. My first choice, the one I
proposed in Geneva, was that we move 100G forward (because it is DONE)
and that we continue to work on 40G (until it is done).
This appears to be a minority position because apparently some people
will accept an unproven 40G proposal rather than risk 100G. Others
think that 40G is proven sufficiently and are demanding "40G now" or
they will not allow a 100G PAR to go forward. Those in the latter camp
must either be unconvinced of my concerns, or they think my concerns
are insufficient to justify any further work being done to justify a
40G project.
I can accept differences of opinion.
What I cannot do, however, is pretend that these issues do not exist,
or that the work we would have to spend getting a 40G standard done is
not going to delay the much needed 100G aggregation solution our
customers demand. I cannot ignore what I perceive as holes in the 40G
presentations.
So, to provide a little more direction to my colleagues in the "40G now
or the HSSG stalls" crowd, I am asking you to include relative cost
trajectories in your analysis of 40G vs 10G cost models, and to include
technology enhancements to LAG (TRILL, I/O Virtualization, QSFP, MPO)
in your performance analysis.
If you feel that this is unnecessary, I am requesting that you
communicate this position to me as soon as possible so that I can
prepare a presentation on these areas of concern for the July meeting.
Respectfully,
Dan Dove
Dove Networking Solutions - Serving ProCurve Networking by HP
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