Editing U interface (section)

===MMMS43 (4B3T)===
{{Main|4B3T}}
The Modified Monitoring State Code mapping 4 bits into 3 ternary symbols (MMS43), which is also referred to as 4B3T (four binary, three ternary) is a line system used in Europe and elsewhere in the world. 4B3T is a "block code" that uses Return-to-Zero states on the line. 4B3T converts each group of 4 data bits into 3 "ternary" line signal states (3 symbols).  Echo cancellation techniques allow full-duplex operation on the line.

MMS43 is defined in Appendix I of G.961,<ref name="G.961"/> Annex B of ETR 080,<ref name="etsi"/> and other national standards, like Germany's 1TR220. 4B3T can be transmitted reliably at up to {{val|4.2|u=km}} over {{val|0.4|u=mm}} cable or up to {{val|8.2|u=km}} over {{val|0.6|u=mm}} cable. An internal termination impedance of {{gaps|150|ohms}} is presented to the line at each end of the U-interface.

A 1&nbsp;ms frame carrying 144&nbsp;bits of 2B+D data is mapped to 108 ternary symbols.<ref name="G.961"/> These symbols are scrambled, with different scrambling codes for the two transmission directions, in order reduce correlation between transmitted and received signal.<ref name="G.961"/> To this frame, an 11-symbol preamble and a symbol from the C<sub>L</sub> channel are added, yielding a frame size of 120 ternary symbols and a symbol rate of 120&nbsp;[[Baud|kilobaud]].<ref name="G.961"/> The C<sub>L</sub> channel is used to request activation or deactivation of a loopback in either the NT1 or a line regenerator.<ref name="G.961"/>

In 4B3T coding, there are three states presented to line: a positive pulse (+), a negative pulse (-), or a zero-state (no pulse: 0). An analogy here is that operation is similar to [[B8ZS]] or [[HDB3]] in T1/E1 systems, except that there is an actual gain in the information rate by coding 2<sup>4</sup>=16 possible binary states to one of 3<sup>3</sup>=27 ternary states. This added redundancy is used to generate a zero DC-bias signal.<ref name="G.961"/>

One requirement for line transmission is that there should be no [[DC bias|DC build-up]] on the line, so the accumulated DC build-up is monitored and the codewords are chosen accordingly. Of the 16 binary information words, some are always mapped to a DC-component free (ternary) code word, while others can be mapped to either one of two code words, one with a positive and the other with a negative DC-component.<ref name="G.961"/> In the latter case, the transmitter chooses whether to send the code-word with negative or positive DC-component based on the accumulated DC-offset.<ref name="G.961"/>