Team,
I would like
echo Jeff’s concern about market acceptance of FEC latency,
for server to server (East-West) traffic in datacenters.
·
Majority of the traffic in the access
and aggregation switches is E-W traffic, and increased
latency directly affects the “response time”
seen by the applications and users.
·
For 100m fiber links, the RS-FEC
latency will add more than 20% to the fiber delay, which
will be a noticeable impact. (and worse for shorter links).
·
This increased latency will be
noticeable when 100G SR4 / PSM4 modules are used to replace
existing 40G SR4 or 100G SR10 modules.
·
Considering the industry trend and
investments in lowering the latency of datacenter switches,
if customers demand zero latency on PMDs,
it will promote non-standard optics
(or operating modes) that disable the FEC on SR4 / PSM4
modules. (and then provide extended reach using FEC).
Also the
assumption that FEC is already available on the host may not
be true.
·
We expect 100G “optical” ports to use
retimed modules, and support for CR4 passive copper cables
may not be required or may be lower priority,
so the FEC is not “free” for these
hosts. There will be definite cost and power increase going
from CAUI-4 / VSR port to CR4 capable port.
·
The use of FEC for .3bm optics (SR4,
PSM4) should be treated as additional component being added
to the host design, or inside the module.
·
We need to compare the cost / power of
“adding” FEC logic into VSR type ports, versus other
solutions (eg: more equalization, better optics).
--vineet
John,
The assumption
of the use of FEC has to be explicit so all know that it
must be implemented and so that they know that the latency
of FEC will occur. Is FEC really so important to the
technical proposal so at to justify it? Cost reduction of
FEC would be one metric for its justification.
The issue then
turns to market acceptance of FEC latency. For 500 meters,
one might argue 802.3bj FEC latency is not an issue. As the
reach goes down, the relative contribution of the FEC
latency increases in terms of the overall link latency (time
of flight of light in glass). Please keep in mind also that
absolute link latency is a market differentiating variable.
I’d like to see market acceptance of FEC latency
presented as a function of reach for optical links.
(Just because the 802.3bj task force adopted an FEC with a
certain amount of latency does not mean we get to stop
thinking in the 802.3bm task force about our own market
acceptance of FEC.)
Jeff
Hello Jeff
Unfortunately,
justification of the 500m SMF objective for the project is
explicitly cost, size and power. While I can sympathize
with other with other issues, I would need a lot of help
explaining why cost, power or sized was sacrificed.
Regards,
John
All,
We have to make
sure that the assumption of the use of FEC is explicit.
Although we might argue not to count the cost of the FEC
encoder/decoder itself, we do need to understand the impact
on cost that the use or lack of use of FEC poses. It is
good thus to make analysis of the impact on cost of designs
presuming the use of FEC versus designs presumed NOT to use
FEC. I see the potential lack of need of FEC as one of the
technical advantages of parallel single mode as well as
market-acceptance advantages.
Jeff
Hi,
As
previously announced, there is an SMF Ad Hoc meeting
starting at 8:00 am Pacific today Tuesday 18 December.
I
have currently received two requests for presentations, so
the draft agenda is:
·
IEEE patent
policy reminder
o
http://www.ieee802.org/3/patent.html
·
Approval of
the draft minutes from 4 December call
·
Presentation
o
PSM4
Technology & Relative Cost Analysis
Update
Jon Anderson, Oclaro
o
Basic Study on
Receiver Bandwidth Requirement for Discrete Multi-tone
Modulation Masato Nishihara, Fujitsu
·
Discussion
·
Future
meetings (next opportunity - 8 Jan)
I
hope to post both presentations on the SMF Ad Hoc
web
page just prior to the meeting.
Peter Anslow from Ciena has invited you to join
a meeting on the Web, using WebEx. Please join the meeting
5-10 minutes early so we may begin on time.
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Topic: "SMF Ad Hoc"
Date & Time: Tuesday, 18 December 2012 at 16:00, GMT
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