1. An optical transmission system, comprising: a modulation subsystem for launching an optical signal having intensity levels responsive to encoded levels, said intensity levels having unequal spacings for substantially compensating for a combination of impairments including a signal dependent noise having variances proportional to said intensity levels, respectively, and a transmitter extinction ratio; and an encoder for using impairment information indicative of said combination of said impairments for converting representations of data into said encoded levels.