Bi-level framing structure for improved efficiency DSL over noisy lines

This invention relates to Digital-Subscriber Lines (DSL) systems, and more particularly to framing structures for lower line rates. A bi-level framing structure for DSL phone systems uses 4 KHz physical frames and mux data frames. The mux data frames each start with a sync byte and contain user payload bytes. A group of mux data frames are appended with forward-error-correction FEC bytes to make a codeword. The codeword is then partitioned into physical 4-KHz frames. The physical frames are transformed by an inverse fast-Fourier transform (IFFT) outputting symbols at 4 KHz for transmission. For high line rates, each codeword has S mux data frames and S physical frames. There are also at least S FEC bytes if error correction is enabled. However, for lower line rates, there are S physical frames but only S/M mux data frames in each codeword. The efficiency factor M is 1 for high line rates, but 4 for lower line rates. Reducing the number of mux data frames reduces the number of sync bytes in a codeword, decreasing overhead. The FEC bytes are spread among more physical frames, reducing error-correction overhead. The symbol rate and the rate of physical frames remains at 4 KHz, even at the lower line rate. The smaller number of bytes per symbol at the lower line rates is compensated for by reducing the number of mux data frames per 4 KHz frame, allowing larger, more efficient mux data frames and error correction to be used. Thus bandwidth efficiency is increased for lower line rates.