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Transport aspects 2

FC POS CB TYPE

G.709-Y.1331(12)_FB.4

Figure B.4 – 513B block's control block header

The byte indicating the control block type (one of 15 legal values) is translated into a 4-bit code according
to the rightmost column of Figure B.2. The 3-bit POS field is used to encode the position in which this
control block was received in the sequence of eight 66B blocks. The flag continuation bit "FC" will be set to
a 0 if this is the final 66B control block or PCS lane alignment marker encoded in this 513B block, or to a 1 if
one or more 66B control blocks or PCS lane alignment markers follow this one. At the decoder, the flag bit
for the 513B block as a whole, plus the flag continuation bits in each row containing the mapping of a 66B
control block or PCS lane alignment marker will allow identification of those rows, which can then be
restored to their original position amongst any all-data 66B blocks at the egress according to the POS field.
The remaining 7 bytes of the row are filled with the last 7 bytes of the 66B control block.

An all-data 66B block is encoded into a row of the 513B block by dropping the sync header and copying the
remaining eight bytes into the row. If all eight rows of the 513B block are placements of 66B all-data blocks,
the flag bit "F" will be 0. If fewer than eight rows of the 513B block are placements of 66B all-data blocks,
they will appear at the end, and the row containing the placement of the final 66B control block will have a
flag continuation bit "FC" value of 0.

The decoder operates in the reverse of the encoder to reconstruct the original sequence of 66B blocks. If
flag bit "F" is 1, then 66B control blocks starting from the first row of the block are reconstructed and
placed in the position indicated by the POS field. This process continues through all of the control blocks
working downward from the top row. The final 66B control block placed within the 513B block will be
identified when the flag continuation bit "FC" is zero.
The structure of the 512B/513B code block is shown in Figure B.5. For example, if there is a single 64B/66B
control block CB1 in a 512B/513B code block and it was originally located between 64B/66B data blocks
DB2 and DB3, the first octet of the 64B character will contain 0.010.1101.CB1; the leading bit in the control
octet of 0 indicates the flag continuation "FC" that this 64B control block is the last one in the 512B/513B
code block, the value of 010 indicates CB1's position "POS" between DB2 and DB3, and the value of 1101 is
a four-bit representation of the control code's block type "CB TYPE" (of which the eight-bit original block
type is 0x55).

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