An FDM communication system in which encoding/decoding settings for different sets of tones are specified using index values assigned to different sets of tones by selecting suitable respective values from a fixed set of index values. Each of the specified index values causes the corresponding digital signal processor to use a respective predefined pair of encoding or decoding settings that includes a respective predefined constellation and a respective predefined shaping code. In some embodiments, the used shaping codes are configured to operate as block codes, with the block sizes being selected such that a multi-code frame generated using multiple shaping codes can be matched to exactly one DMT symbol or to a desired number of whole DMT symbols.