10 Gb/s PONFEC-Framing
Contributors names
Sept 2006
Introduction
•Presentations in July seemed todemonstrate general consensus on:
–FEC is definitely needed for 10G
–FEC should be at the lowest layer
•There are two parts to the FEC puzzle
–‘Framing,’ or how to arrange the bits
–‘Algorithm,’ or the actual math of FEC
•This set of slides concentrates on framing
FEC framing
•FEC will be applied at the lowest layer
–Below the 64b66b sub-layer
–Right before the PMA
•FEC sub-layer will be responsible for obtainingcodeword lock, because without it, FEC isimpossible
–Frame lock must work with extensive errors
–In the upstream, lock must work very fast
•64b66b sub-layer will be handed aligned data,so there is no need for its own framing system
FEC framing structure issues
•There are several differently sized data objectsin the 10G EPON technology that we shouldconsider:
–64b66b blocks, 6.4 ns long
–MPCP time quanta, 16 ns long
–FEC codeword, (yet to be determined)
•The simplest and most efficient system will
–Arrange objects so sizes are related by ratios of smallintegers
–Result in a final line-rate that is a small integer ratio ofthe input MAC rate
64b66b and time quanta
•The least common denominator of timequanta and 64b66b blocks is 32 ns
–5 blocks
–2 time quanta
•Regardless of FEC code choice, if wewant to keep things neat, then time-quantashould always be specified in evennumbers
RS code as an example
•For this presentation, we will consider the triedand true RS(239,255) code (and shortenedvariants) as a example code
–This gives us a concrete set of code constraints towork out the method of solution
–This is not meant to favor RS over other codes
•As the PMD analysis moves forward, the choiceof FEC algorithm will get clearer
•However, the basic ideas presented here willremain the same
Form of FEC codeword
•A FEC codeword will contain three important items
–Framing pattern
–User data
–FEC parity
•In continuous mode systems, framing pattern is typicallyshort, and state machine with long memory is used tolock onto codewords
•In burst-mode systems, framing pattern is longer, toprovide instant lock-on
–This can occur once at the beginning of the frame, with nofurther framing structure required
Good codeword arrangementsfor 66b blocks
•Maximum number of 66b blocks that fit is 28
–1848 bits payload
–40 bits synchronization
–128 bits parity
–252 total bytes: 9/8 line rate
•With an even number of quanta, 25 blocks fit
–1650 bits payload
–22 bits synchronization
–128 bits parity
–225 total bytes: 9/8 line rate
Choice of 64b66b encoding
•The 2 bit header in 64b66b is redundant, sinceFEC sub-layer will be aligning the data
–Can reduce to 1 bit (the T-bit) to increase effciency
•Sounds good, but redundant bits in the payloadcould be used for auxilliary alignment purpose,so sending 66b blocks is not useless
Good codeword arrangementsfor 65b blocks
•Maximum number of 65b blocks that fit is 29
–1885 bits payload
–17 bits synchronization
–128 bits parity
–2030 total bits: 35/32 line rate
•With an even number of quanta, 25 blocks fit
–1625 bits payload
–22 bits synchronization
–128 bits parity
–1775 total bits: 71/64 line rate
Downstream FEC synchronization
•In the downstream, any of the abovementioned framing lengths would work
–We would adjust the state machineparameters to obtain whatever lockprobabilities we wanted
–For reference, 2^64 was considered a ‘goodlock’ in the 66b system
–2~4 sync patterns will produce similar results
Upstream FEC synchronization
•Two phases of synchronization
•Initial lock requires a larger and error-resistantsequence that can reliably produce a uniqueautocorrelation signal
–For reference, merely 20 bits is recommended for G-PON operating at 1e-4 raw BER
•Maintenance is nearly redundant (protectsagainst clock slips – how frequent are they?) butprobably will be included to retain clockfrequency harmonization