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2                                                 Transport aspects



                                                 Transmission order within FEC code word

                             1   2   3    4                      238 239 240 241                  254 255

               FEC sub-row D 254  D 253  D 252  D 251  D j       D 17  D 16  R 15  R 14  R j      R 1  R 0



                                            1  2 3 4   5 6 7 8                 1  2 3  4 5   6  7  8
                                           d 7j  d 6j  d 5j  d 4j  d 3j  d 2j  d 1j  d 0j  r 7j  r 6j  r 5j  r 4j  r 3j  r 2j  r 1j  r 0j

                                                                                          G.709/Y.1331_FA.2


                                                Figure A.2  FEC code word


            Information bytes are represented by:

                                       I (z )   D 254   . z 254   D 253   . z  253   ... D  16   . z 16

            Where Dj (j = 16 to 254) is the information byte represented by an element out of GF(256) and:

                                       D   d  j 7    7    d  j 6    6   ...  d  j 1    1   d  j 0
                                         j
            Bit d  j 7   is the MSB and d  j 0   the LSB of the information byte.

             D 254  corresponds to byte 1 in the FEC sub-row and  D 16   to byte 239.

            Parity bytes are represented by:

                                                                            1
                                       R (z ) R   15  z 15    R 14  z   14   ... R   1  z   R 0
            Where Rj (j = 0 to 15) is the parity byte represented by an element out of GF(256) and:

                                        R    r  j 7    7   r   6   ... r   1    r  j 0
                                          j
                                                         j 6
                                                                       j 1
            Bit  r 7  j   is the MSB and  r 0  j   the LSB of the parity byte.
             R 15  corresponds to the byte 240 in the FEC sub-row and R0 to byte 255.

            R(z) is calculated by:

                                                    R(z) = I(z) mod G(z)
            where "mod" is the modulo calculation over the code generator polynomial G(z) with elements out of the
            GF(256). Each element in GF(256) is defined by the binary primitive polynomial    x 8 x 4 x 3 x 2   .  1

            The Hamming distance of the RS(255,239) code is dmin = 17. The code can correct up to 8 symbol errors in
            the FEC code word when it is used for error correction. The FEC can detect up to 16 symbol errors in the
            FEC code word when it is used for error detection capability only.















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