FEC framing and delineation Frank Effenberger Huawei Technologies

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

Outline • Basic framing concept • Continuous framing method (in appendix) • Burst framing method 2

Basic concepts • We want to maintain 64 b 66 b line code structure, to maximal degree • We want to avoid excessive processing requirements and time for delineation 3

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

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 5

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 6

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 10 G EPON seems the likely approach – Need a way to form a burst leader 7

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) 8

FEC alignment • 66 b coding sublayer receives leader frame from MAC – Would align 66 b codeword to start of leader frame – Could hard reset the 66 b 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 66 b coding sublayer – FEC codeword could be aligned to the leader frame – Could hard reset the FEC coder, since previous data need not be protected 9

Leader payload format • Payload would consist of – X Preamble blocks • Maximal density pattern: 0 x 55 • 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 10

Hamming Distance • If a delimiter is 4 N bits long, then a sequence can be found that has a hamming distance of 2 N-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) = (4 N)! / N! / (3 N)! * BER ^ N – Assume 100 Kburst/sec (very fast) 11

Mean Time to Lost Burst BER Millennium 12

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. 6 E 17 delimiters that have DC balance and odd-even balance • Initial studies have revealed many blocks with distance 29, e. g. : 254 A C 91 F 0 FE 3 21 B 7 13

Receiver processing • Receiver would have bit-wise correlator – Received pattern XOR Golden Block – Count the number of different bits D • For delimiter with 2 X+1 distance, apply following rule – If D<=X, then delimiter found, synchronize 66 b 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 14

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 15

Appendix 16

Continuous Transmission FEC parity in 66 b code • • • 17 Input is ordinary 66 b 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

Example with RS(255, 239) • Input grouped into 28 blocks of 66 b each • RS(255, 239) generates 16 bytes of parity • Parity assembled into 2 66 b blocks – 66 b framing bits are set arbitrarily, most likely to preserve the standard rules • Resulting FEC codeword is 30 66 b blocks 18

Reception FEC enabled 66 b code • Incoming stream is first delineated into 66 b blocks – Using a search and locking mechanism very similar to that used today in 10 Gb. E • Resulting stream of blocks is then serially searched for FEC parity – This requires 66 times less searching than pure serial locking, a significant improvement 19

Error tolerant 66 b framing • Current algorithm looks for 64 consecutive successes to declare lock, and 16 failures out of 64 to declare loss-oflock • 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 1 e-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) 20

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) 21