Source : Free On-Line Dictionary of Computing

two-binary, one-quaternary
     
         (2B1Q) A {physical layer} encoding used for
        {Integrated Services Digital Network} {basic rate interface}.
        2B1Q represents two {bit}s (2B - a "dibit") using one of four
        signal levels (1Q - a "quadratude").  The first bit of the
        dibit is indicated by polarity: positive indicates a binary 1
        and negative indicates a 0.  The second half of the dibit is
        indicated by voltage magnitude: 1 Volt indicates a binary 1
        and 3 Volts indicates binary 0.
     
        (2003-01-10)