2B1Q
Two- binary, one- quaternary (2B1Q) is a physical layer encoding used for Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) implementations. 2B1Q uses four signal levels, which are −450 mV, −150 mV, 150 mV and 450 mV, each (1Q) equivalent to two bits (2b). A competing encoding technique, also used for ISDN basic rate interfaces, is 4B3T.
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