FEC framing and delineation

1. FEC framing and delineation Frank Effenberger Huawei Technologies, US Dec. 5, 2006

2. Outline • Basic framing concept • Continuous framing method • Burst framing method

3. Basic concepts • We want to maintain 64b66b line code structure, to maximal degree • We want to avoid excessive processing requirements and time for delineation

4. Layer Diagram MAC Control MAC 66b PCS FEC PCS PMA PMD

5. Continuous TransmissionFEC parity in 66b code • Input is ordinary 66b coded stream • Stream is broken into groups of X blocks • Parity is generated for each group • Parity is assembled into Y blocks • Codeword is then X+Y blocks long

6. Example with RS(255,239) • Input grouped into 28 blocks of 66b each • RS(255,239) generates 16 bytes of parity • Parity assembled into 2 66b blocks • 66b framing bits are set arbitrarily, most likely to preserve the standard rules • Resulting FEC codeword is 30 66b blocks

7. ReceptionFEC enabled 66b code • Incoming stream is first delineated into 66b blocks • Using a search and locking mechanism very similar to that used today in 10GbE • Resulting stream of blocks is then serially searched for FEC parity • This requires 66 times less searching than pure serial locking, a significant improvement

8. Error tolerant 66b framing • Current algorithm looks for 64 consecutive successes to declare lock, and 16 failures out of 64 to declare loss-of-lock • Extend this algorithm such that • X successes out of Y to declare lock • W failures out of Z to declare loss-of-lock • Exact setting of these parameters is for future study • However, it is clear that even strict locking (X=Y=64) is feasible at BER of even 1e-3 • Locking time on the order of 66*64 blocks (27 microseconds) with a serial technique (could be 66 times shorter with a parallel scheme)

9. Serial block searching • Receiver must calculate FEC parity on each block alignment • In the previous example, there are 30 alignments • This could be done serially • Would require 30*30 block to positively lock (5.8 microseconds)

10. Overall locking time • Proposed method of two stage locking is relatively fast • 33 (27+6) µs approximately worst case time • In comparison, bitwise serial FEC locking takes 380 µs (30*30*66 blocks to lock) • As good as this is, 33 µs is still too slow for burst mode • Time should be >>1 µs • It should also be more deterministic

11. Fast burst mode synchronization • To be efficient, burst mode transmissions need a leading pattern containing • A preamble to provide level and timing • Usually a pattern with a high transition density for easier clock recovery and perfect DC balance for level recovery • A delimiter to provide delineation • A special pattern that is searchable using a bitwise correlator • The pattern is chosen to have a large hamming distance from any other pattern likely to be seen on the line • The delineation part is what would give us the FEC codeword alignment

12. EPON burst preamble • EPON has no ‘special’ data patterns • Burst (from MAC control) just starts with any ordinary data frame • PHY is receiving idle codes all the time • Data detector turns on the laser with enough lead time to ensure good Tx • Extending this to 10G EPON seems the likely approach • Need a way to form a burst leader

13. The Leader frame • At the beginning of the burst, MAC control sends the Leader Frame • An Ethernet frame, with all the usual header and trailer parts • Payload is designed to provide an efficient delimiter, and perhaps the preamble • Signal on line would consist of • X garbled idle blocks (laser warming up) • Y clean idle blocks (should be minimized) • Leader frame (Z blocks long)

14. FEC alignment • 66b coding sublayer receives leader frame from MAC • Would align 66b codeword to start of leader frame • Could hard reset the 66b coder (this is before burst starts, after all, so we don’t care about detailed data pattern) • Could initialize the scrambler at end of the leader frame (important – leader frame pattern must not be scrambled!) • FEC coding sublayer receives leader frame from 66b coding sublayer • FEC codeword could be aligned to the leader frame • Could hard reset the FEC coder, since previous data need not be protected

15. Leader payload format • Payload would consist of • X Preamble blocks • Maximal density pattern: 0x55 • Decide how many blocks depending on PMD • Y Delimiter blocks • Pattern would be a special Barker-like sequence • Designed to be maximally distant from all shifts of the leader payload • Frame could be up to ~180 blocks (1.2 µs) • Should be plenty for our needs

16. Hamming Distance • If a delimiter is 4N bits long, then a sequence can be found that has a hamming distance of 2N-1 • Such a sequence can tolerate up to N-1 errors (in N bits) and still find burst • How many errors do we need to tolerate? • P (lost burst) = (4N)! / N! / (3N)! * BER ^ N • Assume 100Kburst/sec (very fast)

17. Mean Time to Lost Burst BER Millennium

18. Finding the Golden Block • The 64 bit block that has maximal distance 31 from every shift of itself and the preamble is the ‘Golden Block’ • Found empirically, using trial and error • For a 64 bit delimiter, there are 3.6E17 delimiters that have DC balance and odd-even balance • Initial studies have revealed many blocks with distance 29, e.g.: 254A C91F 0FE3 21B7

19. Receiver processing • Receiver would have bit-wise correlator • Received pattern XOR Golden Block • Count the number of different bits D • For delimiter with 2X+1 distance, apply following rule • If D<=X, then delimiter found, synchronize 66b and FEC coders at set position from this instant • If D>X, look at next position • Using the previous example, X=14 • 14 errors in 64 bits is tolerable! • 11 bits tolerance still gives MTLB ~ life of universe, and lower false-positive rate

20. Fast sync suppression • Looking for a delimiter in random data should be avoided • False positives are much more common • Fast locking (operation of the correlator) should be disabled once lock is achieved • Re-enabled once lock is lost hard (that is, estimated BER~0.5) • This should happen only between bursts