Editing U interface (section)

===2B1Q===
{{Main|2B1Q}}
[[2B1Q]] coding is the standard used in North America, Italy, and Switzerland. 2B1Q means that two bits are combined to form a single Quaternary line state ([[symbol (data)|symbol]]). 2B1Q combines two bits at a time to be represented by one of four signal levels on the line. [[Echo cancellation]] techniques allow full-duplex operation on the line.

2B1Q coding is defined in Appendix II of G.961,<ref name="G.961"/> ANSI T1.601,<ref name="ansi"/> and Annex A of ETR 080.<ref name="etsi"/> It can operate at distances up to about 18,000 feet ({{val|5.5|u=km}}) with loss up to {{gaps|42|dB}}. An internal termination impedance of 135 ohms is presented to the line at each end of the U-interface.

A 1.5&nbsp;ms frame carrying 216 scrambled bits of 2B+D data is mapped to 108 quaternary symbols.<ref name="G.961"/> To this frame, a 9-symbol preamble and 3 symbols from the C<sub>L</sub> channel are added, yielding a frame size of 120 quaternary symbols and a symbol rate of 80&nbsp;kilobaud.<ref name="G.961"/> The C<sub>L</sub> channel is used for communication between LT and NT1, a 12-bit [[cyclic redundancy check]] (CRC), and various other physical layer functions.<ref name="G.961"/> The CRC covers one 12&nbsp;ms multiframe (8×1.5&nbsp;ms frames).<ref name="G.961"/>