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Sudeep, We definitely do not want 2 separate FECs for SR4, so the FEC needs to be selected to allow the full 100m reach. For shorter reaches (eg: 20m, or some another limit between 20m and 100m), we prefer the option to operate with the FEC turned off, which will benefit the low latency applications. --vineet From: Sudeep Bhoja [mailto:sbhoja@xxxxxxxxx]
Is the 100ns latency of the KR4 FEC an issue for your system? There were a number of presentations in the .bm task force regarding the 20m SR4 objective and
concerns raised that the 100ns latency of the KR4 is too large. A FEC like BCH(1452, 1430, 5) which can correct 2 bit errors might be sufficient for this application. It can operate on 22 blocks of 65 PCS bits (like 10G-KR
FEC). We don’t need aggressive transcoding to make this work. The total latency will be <20ns.
The input BER that can be supported by such a low weight FEC will be 1e-7 for 1e-15 output BER.
Regards, Sudeep From: Vineet Salunke (vineets) [mailto:vineets@xxxxxxxxx]
There will be large volume of 100G “optics only” ports (on large Switches and Routers) which do not need to support CR4 copper cables, and will not have the CR4/KR4 capability or its FEC (they will use CAUI-4 retimed interface to the module).
So any FEC needed for SR4 optics will be a new addition to these “optics only” ports, and there is not much benefit of staying with CR4 FEC. --vineet From: Ali Ghiasi [mailto:aghiasi@xxxxxxxxxxxx]
Chris 100Gbase-CR4 which is a front panel port based on CFP4/QSFP28 uses CL 91 RS FEC. Most switches and ASIC will have the CL91 FEC as result of gate count required on high port count devices it is very difficult to put more than on FEC. In 802.3bm we can define any FEC we like but then you can't claim it will be free as you may have to implemented externally to the large switch/ASIC/FPGA chip. Thanks, Ali On Nov 21, 2012, at 10:36 AM, Chris Cole wrote: Sudeep raises a very important point. We have been opportunistic in assuming that if we need FEC, it should be the KR4 FEC. While there is merit in reusing KR4 FEC because it already has to be implemented
to support CR4, that should not absolve us of the responsibility to carefully compare performance of FEC alternatives. There is no reason that 802.3bm can not specify a different FEC code than 802.3bj if warranted by performance. The requirements of the Cu
channel are different than MMF channel.
From: Sudeep
Bhoja [mailto:sbhoja@xxxxxxxxx] John, You mentioned. “7) In a optical link, assume bit errors are noise generated, independent and random. Further, since there will be
no DFEs, error multiplication is not expected.” Why reuse a RS FEC that was defined in Clause 91 with 10 bit symbols for the SR4/optical problem? The RS with a heavily shortened 10-bit symbols is meant for
correcting burst errors out of the DFE. A BCH code that corrects random errors is much more effective. BCH can give us a lower latency solution or alternatively higher coding gain for the same latency. Thanks, Sudeep From: John
Petrilla [mailto:john.petrilla@xxxxxxxxxxxxx] Hello Pete Thanks a) Regarding item 6) below, why would you not use binomial statistics? b) Further down, you write, “Because the errors with FEC
occur in groups of 8 or more per FEC frame, the BER in this case will be at least 8E-12.” I’m not sure what you mean. Could you clarify? Regards, John From: Anslow,
Peter [mailto:panslow@xxxxxxxxx] John, Your proposal with changes / comments in red. 1) For the 100 m MMF objective, definition will be based on the assumption that RS-FEC, RS(528, 514), defined in Clause 91 for 100GBASE-CR4 or
100GBASE-KR4 (hereinafter referred to as KR4 FEC) is available. 2) All of the error correction capability of KR4 FEC is allocated to the link supporting the 100 m MMF objective. 3) The incoming BER for the MMF PMD (including
any errors generated by CAUI-4 if present) will be equal to or better than 10^-12 and the target corrected BER for
the link output will be equal to or better than 10^-12. 4) KR4 FEC uses 528 symbols of 10 bits/symbol yielding a frame size of 5280 bits. 5) There are 514 data symbols and 14 parity symbols providing the ability to correct (528-514)/2 = 7 corrupted symbols. 6) The Frame Error Ratio,
FER, for operation without FEC for a BER = 1E-12 using P_frame_error = 1-(1-P_bit_error)^5280 is 5.28E-9. 7) In a optical link, assume bit errors are noise generated, independent and random. Further, since there will be no DFEs, error multiplication
is not expected before the descrambler. 8) The worst case that can be corrected is 7
10-bit symbols in error. Now, I get stuck because I think we need to agree on what the error criterion at the FEC output is. I think that John has assumed that the FEC Frame error ratio (FER) is 5.28E-9 i.e. the same FER as you would get if you had randomly distributed
errors and a BER of 1E-12 or in other words, the same FER as you would expect if you were operating without FEC. Because the errors with FEC occur in groups of 8 or more per FEC frame, the BER in this case will be at least 8E-12. Before going further and calculating what input BER corresponds to a FEC FER of 5.28E-9, I think we need to get agreement that this is the criterion
that we are going to use as it is not strictly in agreement with the objective: “Support a BER better than or equal to 10-12 at the MAC/PLS service interface” Regards, Pete Anslow | Senior
Standards Advisor From: John
Petrilla [mailto:john.petrilla@xxxxxxxxxxxxx] Hello Jonathan Thanks for proposing the meeting and especially for the 8:30 AM Pacific start time. Perhaps we can make some progress on FEC details prior to the meeting. Along that line I’ll propose the following strawman, so that we may have common values
to use for Q and BER in our various analyses. 1) For the 100 m MMF objective, definition will be based on the assumption that RS-FEC, RS(528, 514), defined in Clause 91 for 100GBASE-CR4
or 100GBASE-KR4 (hereinafter referred to as KR4 FEC) is available. 2) All of the error correction capability of KR4 FEC is allocated to the link supporting the 100 m MMF objective. 3) The incoming BER for the MMF PMD will be equal to or better than 10^-12 and the target corrected BER for the link output will
be equal to or better than 10^-12. 4) KR4 FEC uses 528 symbols of 10 bits/symbol yielding a frame size of 5280 bits. 5) There are 514 data symbols and 14 parity symbols providing the ability to correct (528-514)/2 = 7 corrupted symbols. 6) The Frame Error Rate, FER, for operation without FEC for a BER = 1E-12 using binomial statistics (probability density function)
is 5.30E-9. 7) In a optical link, assume bit errors are noise generated, independent and random. Further, since there will be no DFEs, error
multiplication is not expected. 8) The worst case that can be corrected is 7 bit errors for 7 symbols with 1 bit error/symbol. 9) The case equivalent to operation without KR4 FEC is where 8 symbols are corrupted, since for only 7, all errors are corrected. 10) For operation with KR4 FEC, a BER = 6.90E-5 yields an FER of 5.30E-9 (Qi = 3.8119) to match the FER for a BER = 1E-12 (Q = 7.034)
without FEC and 10Log(Qo/Qi) = 2.66 dB. 11) The link model supporting the 100 m MMF objective will use a value of 3.8119 for Q. If there are objection or counter proposals, let’s try to resolve them before the ad hoc meeting. Regards, John From: Jonathan
King [mailto:jonathan.king@xxxxxxxxxxx] Dear all, I’d like to propose we meet for an
MMF Ad Hoc conference call (via Webex) on 29th November, 8.30am Pacific Standard time (4.30pm GMT) . Call duration will be up to 1.5 hours, but can end earlier if meaningful discussion ends. If there aren’t too many objections I’ll confirm the date/time in a couple of days, and send out call details. Our main goal is to progress development of baseline proposals for the 100m reach and 20m reach MMF objectives, and
hopefully remove a few more TBDs from the proposed Tx and
Rx spec tables. Please send presentation requests to me by close of business on 28th November-
(I’ll send out the agenda that day). Best wishes jonathan |