In summary, the USR and Lucent systems approach the problems of the analogue line in quite different ways. Lucent have chosen to use the full 256 levels - 8 bits/symbol - but shape their pulses to avoid ISI affecting their data. USR have accepted the penalty of a limited degree of ISI, but have taken steps to reduce the effect of natural noise. Both aim to reduce quantisation noise by using symbol amplitudes at the quantisation levels. Despite their differences, both are capable of the same data rate.
Examining the two coding schemes, it is possible to hypothesise on their weaknesses. Lucent's technique for handling ISI relies heavily on the modems' ability to measure the line characteristics during the training sequence. This must be done accurately to ensure that the pulse shaping keeps the data bits free from interference. The use of the full 256 levels will, however, make the system susceptible to both external and residual quantisation noise. Lucent's product may therefore perform better on relatively low-noise lines that have particularly bad frequency response. USR's limited ISI method will also require accurate knowledge about the line, or the interference may spread into more than one bit. Using only the top 128 levels confers a higher degree of noise immunity, with the result that the coding scheme may be superior in a higher noise environment. It is likely to have difficulty, however, with lines exhibiting poor frequency response.
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