FEC framing and delineation
Frank Effenberger
Huawei Technologies, US
Dec. 5, 2006
Outline
• Basic framing concept
• Continuous framing method
• Burst framing method
Basic concepts
• We want to maintain 64b66b line code
structure, to maximal degree
• We want to avoid excessive processing
requirements and time for delineation
Layer Diagram
MAC Control
MAC
66b PCS
FEC PCS
PMA
PMD
Continuous Transmission
FEC parity in 66b code
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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
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
Reception
FEC 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
Error tolerant 66b 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 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)
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)
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
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
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
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)
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
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
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)
Mean Time to Lost Burst
Mean Time to Lost Burst (sec)
1.00E+100
1.00E+80
BER
1.00E+60
1.00E-04
1.00E-03
1.00E+40
1.00E+20
Millennium
1.00E+00
16
32
64
Delimiter bit length
128
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
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
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