Editing U interface (section)

===SU32===
Appendix IV of G.961 specifies a line system based on echo cancellation and a substitutional 3B2T (SU32) line code, which maps three bits into 2 ternary symbols.<ref name="G.961"/> As with MMS43 and AMI, the ternary symbol can either be a positive (+), zero (0), or negative (-) voltage.<ref name="G.961"/> The mapping from 2<sup>3</sup>=8 to 3<sup>2</sup>=9 symbols leaves one unused symbol.<ref name="G.961"/> When two subsequent input (binary) information words are identical, the (ternary) code word is substituted by the unused code word.<ref name="G.961"/> A 0.75&nbsp;ms frame carrying 108 bits of 2B+D data is mapped to 72 ternary symbols.<ref name="G.961"/> To this frame, a 6-symbol preamble, one CRC symbol, and 2 symbols from the C<sub>L</sub> channel are added, yielding a frame size of 81 ternary symbols and a symbol rate of 108&nbsp;kilobaud.<ref name="G.961"/> The C<sub>L</sub> channel is used for supervisory and maintenance functions between the LT and NT1.<ref name="G.961"/> The 15-bit CRC covers 16 frames.<ref name="G.961"/>