MMS43 coding - MMS stands for Modified Monitored Sum - is also known as 4B3T coding. In this line coding, which is used in ISDN and telecommunications, four binary values (4B) are assigned to three ternary values (3T).
The four bits correspond to 16 different bit combinations, and with the three ternary values, each of which has three levels, 27 signal values can be mapped. The MMS43 code is constructed in such a way that there is no or only a small DC component that could affect the average signal level. It is used, among other things, in NTBAs where an MMS43 code is converted into an AMI code.