Abstract: A mapper for a PAM encoder is provided and includes apparatus for generating desired code levels, and a constellation matrix memory which stores indications of a plurality of different N-dimensional constellations. The plurality of different constellations are used individually or together to support a plurality of different modem data (bit) rates. In order to maximize performance in the presence of robbed bit signalling, multidimensional constellations having different numbers of points are used together in the coding of a single group of bits.