Editing U interface (section)

==Duplex transmission==
While in a four-wire interface such as the ISDN [[S interface|S]] and [[T interface|T]]-interfaces one wire pair is available for each direction of transmission, a two-wire interface needs to implement both directions on a single wire pair. To that end, ITU-T recommendation G.961 specifies two [[duplex (telecommunications)|duplex]] transmission technologies for the ISDN U interface, either of which shall be used: [[Echo cancellation]] (ECH) and Time Compression Multiplex (TCM).<ref name="G.961">{{cite web|url=http://www.itu.int/rec/T-REC-G.961|title=G.961: Digital transmission system on metallic local lines for ISDN basic rate access |publisher=ITU-T |access-date=2014-01-06 |date=March 1993}}</ref>

===Echo cancellation (ECH)===
When a transmitter applies a signal to the wire-pair, parts of the signal will be [[signal reflection|reflected]] as a result of imperfect balance of the [[telephone hybrid|hybrid]] and because of impedance discontinuities on the line.<ref name="G.961"/> These reflections return to the transmitter as an echo and are indistinguishable from a signal transmitted at the far end. In the echo cancellation (ECH) scheme, the transmitter locally simulates the echo it expects to receive, and subtracts it from the received signal.<ref name="G.961"/>

===Time Compression Multiplex (TCM)===
{{Distinguish|Trellis coded modulation}}
The Time Compression Multiplex (TCM) duplex method, also referred to as "burst mode", solves the echo problem indirectly.<ref name="G.961"/> The line is operated at a rate at least twice the signal rate and both ends of the line take turns transmitting, in a [[time-division duplex]] fashion.<ref name="G.961"/>