Embodiments of the invention provide an interleaver design and header fields for ITU-T G.hnem. The header may comprise two parts that are separately encoded. A common header segment is encoded alone, and an embedded header segment is encoded with payload data. The interleaver operates on blocks having a size based upon a total number of input bits in an FEC codeword block, a total number of bits loaded on symbols that span a half mains cycle, or a maximum fragment size of 3072 bits. The blocks may be repeated before interleaving. Each block and its repetitions may be interleaved together, such as for header data, or each block and repetition may be interleaved separately, such as for payload data. Cyclic padding may be used on each block to create an integer number of symbols for transmission.