A new protocol is proposed for transmission of data through lines such as telephone lines. The tones of a signal are grouped, and Trellis encoding is performed only of the members of a group. The computational cost of coding and decoding the data is reduced (compared to treating all the tones of a given direction equivalently), and the invention makes it possible to significantly reduce the computational and memory requirements of the encoder and decoder. Furthermore, Trellis decoding errors are not propagated between the groups.