quartenary
In order to be able to transmit more data at the same data rate, multi- level coding( MLC) is used in the transmission processes.
One multi-level coding method is the bipolar method, which works with two levels, another is the ternary method, which works with three different levels, and a third is the quaternary method.
In quaternary coding, the code element has four different levels. The levels each represent a voltage value and can be labeled +3, +1, -1, and -3. Each level is assigned a discrete digital value. This can be a digital state, but it can also be a dibit, as in 2B1Q coding. A level change thus corresponds to a dibit in each case.