| Thread Links | Date Links | ||||
|---|---|---|---|---|---|
| Thread Prev | Thread Next | Thread Index | Date Prev | Date Next | Date Index |
|
… or to be more explicit – we have to consider whether any error propagation due to DFE/EDC etc. will impact the FEC performance or whether crosstalk and similar
effects will produce correlated errors that can appear at different points in the data stream. The .3bj FEC approach was taken specifically because of these considerations. Hugh. From: Mark Gustlin [mailto:mark.gustlin@xxxxxxxxxx]
Another consideration is: will the ‘optical’ FEC be used to cover for errors on any electrical interfaces to the optical module in the future (CPPI-4, CAUI-2
etc); and if yes, what are the error characteristics on that electrical interface (independent or correlated). And therefore how will it impact the FEC choice.
Thanks, Mark From: Mike Dudek [mailto:mike.dudek@xxxxxxxxxx]
While I agree that from a pure standards perspective “There is no reason that 802.3bm can not specify a different FEC code than 802.3bj if warranted by performance”.
In order to consider a different code however I’d want to see a very significant performance enhancement (eg 200m reach instead of 120m reach or No CDR’s needed rather than CDR’s needed). The extra complexity and silicon area to include yet a different FEC
in a common port chip certainly would need to be justified by this sort of improvement. FYI. From a Frame-error rate perspective 802.3bj draft is using a target of 1.7e-10 for 64 Octet frames with minimum inter-packet gap, which I believe is somewhat
better than what is achieved by a BER of 1e-12. I’m not saying that this is the correct number but think that if 802.3bm uses a different target then the groups should have a reason for the difference. Mike Dudek
QLogic Corporation Senior Manager Signal Integrity 26650 Aliso Viejo Parkway Aliso Viejo CA 92656 949 389 6269 - office. From: Chris Cole [mailto:chris.cole@xxxxxxxxxxx]
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
|